Maintenance Circle

DG Set - Detailed Information - Part One

2010-03-25T07:43:00.000+05:30

Introduction
Diesel engine is the prime mover, which drives an alternator to produce electrical energy. In the diesel engine, air is drawn into the cylinder and is compressed to a high ratio (14:1 to 25:1). During this compression, the air is heated to temperature of 700–900°C. A metered quantity of diesel fuel is injected, at high pressure, into the cylinder, which ignites spontaneously because of the high air temperature. Hence, diesel engine is also known as compression ignition (CI) engine. DG set can be classified according to cycle type two stroke and four strokes. However, majority of engines use four stroke cycle. Let us look at the principle of operation of the four-stroke diesel engine.

Four Stroke Diesel Engine
The 4 stroke operation in a diesel engine are: induction, compression, ignition, power and exhaust strokes.

1st: Induction (Suction) stroke - while the inlet valve is open, the descending piston draws fresh air.
2nd: Compression stroke - while the valves are closed, the air is compressed to a pressure of up to 25 bar.
3rd: Ignition and power stroke - fuel is injected, while the valves are closed (fuel injection actually starts at the end of the previous stroke), the fuel ignites spontaneously and the piston is forced downwards by the combustion gases.
4th: Exhaust stroke - the exhaust valve is open and the rising piston discharges the spent gases from the cylinder.

Since power is developed during only one stroke, the single cylinder four-stroke engine has a low degree of uniformity. Smoother running is obtained with multi cylinder engines because the cranks are staggered in relation to one another on the crankshaft. There are many variations of engine configuration, for example. 4 or 6 cylinder, in-line, horizontally opposed, V or radial configurations.

DG Set as a System
A diesel generating set should be considered as a system since its successful operation depends on the well-matched performance of the components, namely:

• Diesel engine and its accessories
• AC Generator (Alternator)
• Control systems and switchgear
• Foundation and power house civil works
• Connected load with its own components like heating, motor drives, lighting and others

It is necessary to select the components with highest efficiency and operate them at their efficient levels to conserve energy in this system.

Selection Considerations
To make decision on the type of engine, which is most suitable for a specific application, several factors need to be considered. The two most important factors are: power and speed of the engine. The power requirement is determined by the maximum load. The engine power rating should be 10 – 20 % more than the power demand by the end use. This prevents overloading the machine by absorbing extra load during starting of motors or switching of some types of lighting systems or when wear and tear on the equipment pushes up its power consumption. Speed is measured at the output shaft and given in revolutions per minute (RPM). An engine will operate over a range of speeds, with diesel engines typically running at lower speeds (1300 – 3000 RPM). There will be an optimum speed at which fuel efficiency will be greatest. Engines should be run as closely as possible to their rated speed to avoid poor efficiency and to prevent build up of engine deposits due to incomplete combustion – which will lead to higher maintenance and running costs. To determine the speed requirement of an engine, one has to again look at the requirement of the load.

For some applications, the speed of the engine is not critical, but for other applications such as a generator, it is important to get a good speed match. If a good match can be obtained, direct coupling of engine and generator is possible; if not, then some form of gearing will be necessary - a gearbox or belt system, which will add to the cost and reduce the efficiency.

There are various other factors that have to be considered, when choosing an engine for a given application. These include the following: cooling system, abnormal environmental conditions (dust, dirt, etc.), fuel quality, speed governing (fixed or variable speed), poor maintenance, control system, starting equipment, drive type, ambient temperature, altitude, humidity, etc.

Suppliers or manufacturers literature will specify the required information when purchasing an engine. The efficiency of an engine depends on various factors, for example, load factor (percentage of full load), engine size, and engine type.

Diesel Generator Captive Power Plants
Diesel engine power plants are most frequently used in small power (captive non-utility) systems. The main reason for their extensive use is the higher efficiency of the diesel engines compared with gas turbines and small steam turbines in the output range considered. In applications requiring low captive power, without much requirement of process steam, the ideal method of power generation would be by installing diesel generator plants. The fuels burnt in diesel engines range from light distillates to residual fuel oils. Most frequently used diesel engine sizes are between the range 4 to 15 MW. For continuous operation, low speed diesel engine is more cost-effective than high speed diesel engine.

Advantages of adopting Diesel Power Plants are:
• Low installation cost
• Short delivery periods and installation period
• Higher efficiency (as high as 43 – 45 %)
• More efficient plant performance under part loads
• Suitable for different type of fuels such as low sulfur heavy stock and heavy fuel oil in case of large capacities
• Minimum cooling water requirements,
• Adopted with air cooled heat exchanger in areas where water is not available
• Short start up time

Diesel Engine Power Plant Developments
The diesel engine developments have been steady and impressive. The specific fuel consumption has come down from a value of 220grams/KWh in the 1970s to around 160grams/KWh in recent times. Slow speed diesel engine, with its flat fuel consumption curve over a wide load range (50%–100%), compares very favorably over other prime movers such as medium speed diesel engine, steam turbines and gas turbines. With the use of modern, high efficiency turbochargers, it is possible to use an exhaust gas driven turbine generator to further increase the engine rated output. The net result is lower fuel consumption per KWh and further increase in overall thermal efficiency. The diesel engine is able to burn the poorest quality fuel oils, unlike gas turbine, which is able to do so with only costly fuel treatment equipment. Slow speed dual fuel engines are now available using high-pressure gas injection, which gives the same thermal efficiency and power output as a regular fuel oil engine.

Selection and Installation Factors Sizing of a DG set:

a) If the DG set is required for 100% standby, then the entire connected load in HP / KVA should be added. After finding out the diversity factor, the correct capacity of a DG set can be found out.

Example:
Connected Load = 650 KW
Diversity Factor = 0.54
(Demand / connected load)
Max. Demand = 650 x 0.54 = 350 KW
% Loading = 70
Set rating = 350/0.7 = 500 KW
At 0.8 PF, rating = 625 KVA

b) For an existing installation, record the current, voltage and power factors (KWh / KVAh) reading at the main bus-bar of the system at every half-an-hour interval for a period of 2–3 days and during this period the factory should be having its normal operations. The non-essential loads should be switched off to find the realistic current taken for running essential equipment. This will give a fair idea about the current taken from which the rating of the set can be calculated.

For existing installation:
KVA = √3 V I KVA Rating = KVA / Load Factor where Load factor = Average KVA / Maximum KVA

c) For a new installation, an approximate method of estimating the capacity of a DG set is to add full load currents of all the proposed loads to be run in DG set. Then, applying a diversity factor depending on the industry, process involved and guidelines obtained from other similar units, correct capacity can be arrived at.
High Speed Engine or Slow/Medium Speed Engine
The normal accepted definition of high speed engine is 1500 rpm. The high speed sets have been developed in India, whereas the slow speed engines of higher capacities are often imported. The other features and comparison between high and medium / slow speed engines are shown in the table.

Keeping the above factors and available capacities of DG set in mind, the cost of economics for both the engines should be worked out before arriving at a decision.

Capacity Combinations
From the point of view of space, operation, maintenance and initial capital investment, it is certainly economical to go in for one large DG set than two or more DG sets in parallel. Two or more DG sets running in parallel can be a advantage as only the short-fall in power–depending upon the extent of power cut prevailing - needs to filled up. Also, flexibility of operation is increased since one DG set can be stopped, while the other DG set is generating at least 50% of the power requirement. Another advantage is that one DG set can become 100% standby during lean and low power-cut periods.

Air Cooling Vs. Water Cooling
The general feeling has been that a water cooled DG set is better than an air cooled set, as most users are worried about the overheating of engines during summer months. This is to some extent is true and precautions have to be taken to ensure that the cooling water temperature does not exceed the prescribed limits. However, from performance and maintenance point of view, water and air cooled sets are equally good except that proper care should be taken to ensure cross ventilation so that as much cool air as possible is circulated through the radiator to keep its cooling water temperature within limits. While, it may be possible to have air cooled engines in the lower capacities, it will be necessary to go in for water cooled engines in larger capacities to ensure that the engine does not get over-heated during summer months.

Safety Features
It is advisable to have short circuit, over load and earth fault protection on all the DG sets. However, in case of smaller capacity DG sets, this may become uneconomical. Hence, it is strongly recommended to install a circuit protection. Other safety equipment like high temperature, low lube oil pressure cut-outs should be provided, so that in the event of any of these abnormalities, the engine would stop and prevent damage. It is also essential to provide reverse power relay when DG sets are to run in parallel to avoid back feeding from one alternator to another.

Parallel Operation with Grid
Running the DG set in parallel with the mains from the supply undertakings can be done in consultation with concerned electricity authorities. However, some supply undertakings ask the consumer to give an undertaking that the DG set will not be run in parallel with their supply. The reasons stated are that the grid is an infinite bus and paralleling a small capacity DG set would involve operational risks despite normal protections like reverse power relay, voltage and frequency relays.

2009-07-20T10:17:00.000+05:30

Word for the day: Grounding Practices

Often, we are advised by our seniors, parents and teachers not to ignore "ground" realities when making a decision. Philosophers assert that people who have their foot firm on "ground" are the ones who become successful in life. It is no magical co-incidence that any electrical or electronic equipment that is not grounded will not work properly and can pose electrical hazard to the users. Be it simple hair dryer in our wardrobe or a complex process line running in a set-up, they must be grounded. Modern electronic equipments does not work without proper grounding.

What is grounding?

Grounding, also called earthing, is a method of providing return path for an electrical potential from an equipment that might otherwise be left floating. If some other component or person touches such ungrounded equipment, the floating potential will immediately find a return path and damage the equipment. Not to mention the electrical shock it imposes on the person touching it.

Grounding is a vast subject and some of the concepts will be exclusive to the type of system to be grounded. The grounding requirement for a railway line can be completely different from the grounding requirement for a coal mine. In this newsletter, we intend to explore the very basics of grounding in a typical manufacturing set up.

The grounding must be separate for body (equipment) and neutral in a typical electrical system. They are generally called NEUTRAL EARTH (NE) and BODY EARTH (BE). Body grounding protects personnel against any electrical signals passing thru the "non-electrical" parts of any equipment like machine frames, bearings and panels. Neutral earthing protects against any insulation failure or floating voltage occurring in neutral line.

Apart from exposing personnel to electrical hazard, poor grounding also produce errors in various instruments, measuring devices and communication equipments. Grounding also eliminates effect from any static charges generated in the electrical system, thus preventing damage of expensive equipments.

Grounding is accomplished by inserting highly conductive metal conductor, called GROUNDING ELECTRODE, deep into the earth. The electrode, in different shapes, will be usually made from high-quality steel or copper or copper-clad steel.

The number of ground electrodes to be fixed will be generally decided after measuring one important attribute, called GROUND RESISTANCE. It is similar to resistance of any electrical component but varies and hence is difficult to calculate. It can be measured by using GROUND or EARTH RESISTANCE METERS. General purpose multi-meters and meggers should not be used for measuring this resistance value. But, multi-meters can be used for checking the continuity among various ground points.

Ground resistance will be affected by quality and length of grounding electrode used, its contact area with the soil, surrounding soil condition and number of electrodes in a particular location. The condition of soil plays a vital role in reducing ground resistance, and attributes of other components can be externally controlled. Soil with high moisture content will have LESS ground resistance than dry soil. Following is a list of different types of soil in the increasing order of its resistance (Item 1 will have least resistance compared to item 6).

Wet marshy lands, or lands containing ashes, cinders or brine waste
Clay, loamy soil, arable land clay
Clay & loam mixed with varying proportion of gravel & sand
Damp & wet sands
Dry sand
Gravel & Stones

The quantity and electrical quality of soil surrounding electrodes determines its ability to discharge the ground current, as quickly as possible. This property of soil to provide least resistance and quick discharge of ground current is measured by its RESISTIVITY. Referring again to the list above, item 1 has less resistivity compared to item 6. A comprehensive resistivity table is provided on page 6 for ready reference.

REMEMBER: To reduce resistivity and hence ground resistance, longer electrode or multiple electrodes must be selected instead of increasing the diameter alone. For non-circular electrodes, it is suggested to install thinner sheets or wires than thick ones.

The grounding electrodes can be made in different shapes, although round ones are very popular. One of the most important requirement of the ground electrode should be to INCREASE contact area with surrounding soil and REDUCE resistance. Therefore, the electrodes must be free of paint, coatings and other insulating materials. Following shapes are generally used for electrodes:

Round rods or pipes – Most commonly used in factories, commercial establishments
Horizontal wires – Most commonly used in receiving stations, distribution stations and transformer yards
Four Pointed stars – Most commonly used in RF tower stations (mobile towers for example)
Conductive Plates – Can be used effectively as an alternative to round rods or pipes – Increases contact area
Water pipes – One of the most common medium for grounding in residential townships and layouts. If they run close to low tension cables (either underground or overhead), they will be used as common grounding electrode.

Water pipes which run in almost all townships and residential layouts are more suitable for AC voltage sources. If DC circuits are grounded, any ground current can cause severe electrolysis which result in scale formation inside the pipes, thus blocking water flow.

The resistance offered by soil surrounding a grounding electrode has its influence within certain region, generally called "Circle or Sphere of Influence." The diameter of this circle of influence will be usually equal to twice the length of grounding electrode. So, if more than one electrode is being installed, distance between them should be equal to or greater than twice the electrode length. If the grounding electrode is non-circular, the spacing should be equal to or greater than the electrode's width. Normally round rod (or pipe) and square plate electrodes are commonly used, if electrodes are fitted inside.

It is quite difficult to get ideal soil condition in all locations for maintaining minimum ground resistance. Under such circumstances, by addition of certain chemicals, almost ideal condition can be obtained. The most commonly used material for reducing ground resistance are:

Charcoal or Coke power
Common Salt (Sodium Chloride, NaCl) or Calcium Chloride (CaCl) or Sodium Nitrate (NaNO₃)or Magnesium Sulphate (MgSO₄)
Construction grade sand

Items 1 and 2 are usually laid in layer form around the grounding electrode. Electrode will be firmly retained inside the sand. The sand retains moisture level for long time thus keeping ground resistance always low. The installation pictures at end of this article explain civil work involved in creating the "EARTH PIT" for circular and flat plate electrodes. When locating the spot for earth pits, pay close attention to following points:

There are no underground cables or pipe lines – These might be damaged when excavating
There is no huge stagnation of water – Too much water will dilute the salt content, which increases ground resistance
The location is easily accessible for regular inspection and maintenance
No future construction work is planned in the earth pit area
Prevent growth of marsh around the earth pit – Otherwise, it becomes difficult – after few months – even to locate the earth pit

Generally, galvanized iron or MS (Mild Steel) pipes are used as grounding electrodes in many places to reduce initial cost. But apart from high ground resistance, they are also prone to corrosion from the chemical treatment provided. If the set-up has critical and expensive machines, it is always suggested to install copper based electrodes for proper grounding and safety of equipments.

Some merits and demerits of copper / steel electrodes are listed below:

Steel electrode should be 5 to 6 times more than copper in terms of size to achieve same ground resistance (Example: If copper electrode is one meter, steel electrode should be between 5 to 6 meters)
If copper electrodes are installed, they must be kept away from any underground steel pipes. Otherwise, they create a "cathodic" reaction cell which will corrode the steel pipes.
Steel is mechanically stronger than copper and hence needs less support in an earth pit.

REMEMBER: Earth-pits are NOT FILL & FORGET type. Their regular inspection should be a part of preventive maintenance and inspection procedure. If earth pits are located in dry area, pour water to maintain sufficient moisture level. For a 3-meter deep earth pit, three to four buckets (15 to 20 liters) of water once in a month is sufficient.

Types of Grounding installations:

Based on the soil condition and type of installation to be grounded, different methods of grounding set-up are practiced. Adjacent pictures show some of the common methods of grounding practices. Independent of the type of design used, increasing the contact area of ground electrode with surrounding soil should be one of the primary objectives.

Once the grounding electrodes are installed, they must be connected properly to the equipments in the facility. It is a common practice to use continuous strip of galvanized iron or mild steel plate on the walls, closer to which machines and equipments are installed.

Copper strips are generally used inside distribution panels for easier installation.

Then, each equipment should be separately connected using another strip or multi-strand copper cable or GI wire to this ground bus. Even if a machine is located slightly away from the nearest ground bus, it must not be connected to another machine which is closer. Each grounding must be separately connected. This will prevent ground fault current of one machine affecting the other. Also, resistance of individual ground circuit adds thus increasing total ground resistance of last equipment.

Neutral from each machine will not be usually grounded, although it is suggested for special purpose and expensive machines. But, grounding must be done for neutral point (star point) of the alternator. Grounding of neutral from main isolation step-down transformer must also be done, in the transformer yard itself. It is also advisable to create a neutral ground pit at the main distribution panel as well.

Once the earth pits are fixed, it is important to introduce the process of measuring earth resistance into predictive maintenance schedule. Disconnect the strip or cable from main plant and isolate earth pit. Using a reference probe, fixed at a particular distance from the ground electrode, measure the resistance value (in ohms). Record the value at different times of day and calculate average resistance value. Repeat the exercise once or twice a year and monitor deviations. Proper corrective actions must be taken if values are found above acceptable limit.

Soil Resistivity & Earth Resistance Table (Approximate Values)

Type of Soil	Soil Resistivity	Earthing Resistance (OHMS)
	Soil Resistivity	Ground Electrode Depth (meters)			Earthing Strip (meters)
	OHM-METER	3	6	10	5	10	20
Very moist soil (swamp like)	30	10	5	3	12	6	3
Farming soil and clay soil	100	33	17	10	40	20	10
Sandy clay soil	150	50	25	15	60	30	15
Moist sandy soil	300	66	33	20	80	40	20
Concrete	400	-	-	-	160	80	40
Moist gravel	500	160	80	48	200	100	50
Dry sandy soil	1000	330	165	100	400	200	100
Dry gravel	1000	330	165	100	400	200	100
Stoney oil	30000	1000	500	300	1200	600	300
Rock	107	-	-	-	-	-	-

Grounding Symbols – Adjacent picture show three of the commonly used symbols for grounding. It is important to differentiate between types for proper grounding design. None of these circuits should be combined when connecting to a grounding electrode.

How grounding electrode size will be determined?

One of the most important functions of a grounding electrode is to withstand sudden inrush current which could occur due to short-circuit or overloading in an equipment. Therefore, the electrode must be able to withstand this short-circuit current. Of all the equipments installed in the set-up, consider highest short-circuit current (usually printed on the circuit breakers or on machine name plate or in the installation manuals). For size calculation you can follow a thumb rule. Steel can withstand 0.124 kA (124 amperes) per square millimeter of conductor area and Copper can withstand 0.282 kA (282 amperes) per square millimeter.

Despite having a good grounding system, it is important to install ground circuit sensing and protection devices that will isolate power at the next highest upstream level to prevent further damage and accident. Earth Leakage Relay (ELR) is the most commonly used safety sensing device fitted in the incoming panel of a 3-phase 4-wire system. A CBCT (Core Balanced Current Transformer) fitted around the 4-wire system checks the imbalanced current flow in neutral or return line and if it exceeds a present value for a present time, it will close a contact. This auxiliary contact will in turn usually trip the incoming ACB or MCCB or any other circuit breaker.

Remember: If ELR trips, do not immediately increase the trip current or time limit. Reset the breaker and re-check. If ELR trips again, check all the downstream equipments before increasing the limits. ELR is a very sensitive device and does not trip without reason.

In a single-phase system, RCCB (Residual Current Circuit Breaker) must be fitted to prevent electrical hazard to the personnel. This will be especially important in domestic areas where people of all age groups could be dealing with one or other type of electrical equipment. The equipment we "assume" to be most safest can become a electrical hazard without proper protection devices. Also, it is not advisable to use any equipment – computers, electric heaters, ovens, etc – with two pin socket system. This will completely isolate the grounding protection posing great danger to the users.

NOTE: Even while constructing a house, it is highly recommended to follow the grounding rules and build a small earth pit, similar to manufacturing set-up. This will greatly help in isolating all ground faults to maximum extent. Drawing a simple GI wire into ground alone is not sufficient.

Standard Pipe Earth Pit System Description

Plate Type Earth Pit System Description

2009-07-03T20:20:00.001+05:30

Word for the day: Electrical Protection Components - Part Two

Poets, artists and many "non-technical" people have always advised the mankind to pursue what the heart feels and not what the mind says. Feeling of heart has always overridden the logics of mind. By doing or enjoying something that we love most, our heart will miss a beat, for sure. But if same heart misses more beats, it needs medical attention. And, one of the culprit causing this is ELECTRICITY. Malfunctioning of heart is one of the most instant effects of electric shock, apart from other bodily injuries. This week let us explore how electricity affects our body and understand its seriousness, by heart!!!

A common phrase makes frequent appearance in electrical field: "It is the current, not the voltage that kills" While there is an element of truth to this statement, we can understand more about the electric shock and its effects on human body. If voltage presented no danger, there would not have been a board that read:

"DANGER – HIGH VOLTAGE !!"

It is the electric current that burns tissues, freezes muscles and disturbs heart beat. However, electric current doesn't just occur on its own. The voltage is responsible for creating current which flow thru body and damages the organs. To understand how electricity affects our body, we may have to re-visit Ohm's Law:

The amount of current thru body is, therefore, directly proportional to voltage applied and inversely proportional to body's resistance. The amount of resistance body offers varies from person to person and which parts get touched and also the surrounding weather conditions. Therefore, for a given resistance, if the voltage applied is more, the magnitude of current flowing will also be more, indicating more injury to the body.

The resistance offered by body changes depending on where the contact is being made. The contact can be between hand to hand or hand to foot or hand to chest or foot to foot. Also, a wet hand – due to sweat, for example – offers less resistance compared to a dry hand. Interestingly, body size, amount of fat in the body also alter the resistance. Remember also that blood, being a mixture of many minerals, is a good conductor of electricity.

So how much current is generally harmful for the body? Amount of current that can be considered be hazardous is listed in following table along with its impact. From the table, it is quite evident that AC voltage is more harmful than DC voltage.

All currents are in milli Amperes (One milli Ampere = 1/1000^th of an ampere = 0.001 Ampere)

NOTE: Most of the readings were obtained from quite a few practical experiences and tests done on various animals. It is quite possible that the geographical location can also affect these current limits for a body. But, this table can serve as a good starting point to incorporate necessary safety and precautions.

Current Limit, DC		Current Limit, AC 50 / 60 Hz		Current Limit, AC 10000 Hz		Effect on the body
Men	Women	Men	Women	Men	Women
1.0	0.6	0.4	0.3	7	5	Slight sensation, like a shivering felt in hands
5.2	3.5	1.1	0.7	12	8	Can feel the shock, but does not have any direct effect
62	41	9	6	55	37	Painful, but the body can be immediately retracted from source to avoid further injuries
76	51	16	10.5	75	50	Muscles contract and the body cannot let go of the conductor
90	60	23	15	94	63	Severe pain with extreme difficulty in breathing and burn may occur
500		100				Possible heart fibrillation with severe burns. May result in death also

On average, a dry human body has a resistance value of 100,000 Ohms ( 1 MΩ ). With this resistance, let us calculate how much voltage is necessary to produce a current of 20 milli amperes, which is sufficient to cause muscle contraction. The resistance is measured between two hands holding two probes of a multi-meter tightly.

V = I x R

V = 20 mA x 100,000 Ω

V = 20,000 volts or 20 kV

Consider the second scenario whether the hands are wet and resistance measures only 1000 ( 1 KΩ ) Ohms. Under such condition, the voltage required to produce same current will be much less

V = I x R

V = 20 mA x 1000

V = 20 volts

Therefore, depending on body resistance under different circumstances, even low voltage system which we all "assume" to be safe can prove fatal.

Remember also that the body resistance DECREASES with increased duration of exposure to the electricity. So, it is extremely essential to disconnect the power source or isolate the person as soon as possible, without creating panic. The body resistance value is also influenced by what conductor we are holding and HOW we are holding it. Research in this area have obtained certain approximate values for different conditions, as shown in the list below:

Wire touched by a single finger – 40,000 (40 kΩ)to 1000000 (1 MΩ) ohms when dry and 4000 (4kΩ) to 15000 (15kΩ) ohms when wet
Wire totally held by hand – 15,000 (15kΩ) to 50,000 (50kΩ) ohms when dry and 3000 (3kΩ) to 5000 (5kΩ) ohms when wet
Non-insulated cutting (or any metal) plier by hand – 5000 (5kΩ) to 10,000 (10kΩ) ohms when dry and 1000 (1kΩ) to 3000 (3kΩ) ohms when wet
Metal contact with palm (instead of fingers) – 3000 (3kΩ) to 8000 (8kΩ) when dry and 1000 (1kΩ) to 2000 (2kΩ) ohms when wet
1 ½" metal pipe firmly held by ONE hand – 1000 (1kΩ) to 3000 (3kΩ) ohms when dry and 500 to 1500 (1.5kΩ) ohms when wet
1 ½" metal pipe firmly held by BOTH hands – 500 to 1500 (1.5kΩ) ohms when dry and 250 to 750 ohms when wet
Hand immersed in conductive liquid – 200 to 500 ohms
Foot immersed in conductive liquid – 100 to 300 ohms

It is interesting to note that resistance value when holding pipe by two hands is exactly half the value, when held by one hand. This agrees with the principle of RESISTANCE IN PARALLEL. When two hands are holding the conduct, it will DOUBLE the contact area and reduces the resistance.

From practical work point of view, it is not possible to always shut down power before working. Sometimes, we may be troubleshooting the working of a machine or checking the trip conditions of a breaker or just measuring the voltage in a conductor. 30 or less volts is considered to be safe limit for not suffering any electrical shocks. But having said that, it is important to take all precautions before attempting to work on "live" circuit. It is quite common to have mix of different voltage levels in an electrical panel. Following is a small list of precautions to prevent electrical hazards

Before attempting to go near any "live" circuit, ensure your hands and legs are dry
Remove all metal items on your body including rings and other jewelries. Avoid keeping too many items like tester, mobile phones in your pockets. They may accidentally touch some "live" terminal
Do not stand on loose points from where your chances of toppling or slipping are high. Due to such instability, the person may not be able to balance himself and will fall on high voltage conductors. This will be specially important when working outdoors
As much as possible use only one hand while working – Keep other hand away
Do not work alone – Have one or more personnel either assisting you or just watching your activity. A stitch in time saves nine!!
Communicate to everyone around you, including non-technical personnel, that you are working on some part of electrical system and nobody should turn on any equipment without confirming
Wear necessary protective elements – Eye-glasses, Gloves, Rubber boots – to prevent any untoward incident. The resistance of all these add to protect body from electric shocks
Personnel, specifically attending "live" electrical work must use proper TOOLING and should have undergone necessary & proper TRAINING. They should be THINKING about what they are doing instead of getting distracted by other mental pressures - With a perfect balance of three T's, virtually zero electrical accidents can be achieved
Never use wrong tools for adjusting something - Use right tools and right measuring instruments – Sometimes when tightening a nut-bolt assembly that is "invisible," you could get a feeling of "tightening" but it might not have happened – Pay close attention under such circumstances
Avoid using left hand as much as possible, since the current flow path is close to heart
When working on electronic equipments, independent of its size, wear antistatic wrist wrap. This will not only help prevent small shocks to your body but will equally prevent electrostatic charge from our body damaging the electronic components
Even if the main circuit breaker is disconnected, the conductor(s) of load side on which you will work, must be temporarily connected to ground using a piece of wire – This is an usual practice in high-tension transmission systems
Wherever you are walking, especially on rainy days, be cautious. Look "above" and "below" for any loose cables fallen from a pole. Never be curious to play with electrical wires without confirming voltage levels

From the prevention stand point, few points are listed that you can follow to avoid (possibly eliminate) all electricity related hazards

Create a "grounding-check audit" schedule and make it part of preventive maintenance activity. The frequency of audit can be determined based on complexity of electrical system in your set-up. It is essential to ground the BODY and NEUTRAL, separately. If neutral is left ungrounded, "floating" voltage from other sources may pass thru this and create electric shock
- There is no limit for number of "pits" that can be created for grounding. Based on the location and convenience, it is better to have as many "pits" as possible to ensure 100% grounding under all circumstances. This will be extremely helpful if set-up is new and possibility of adding more machines exist in future
Discharge capacitors before attempting to work on them or connected device. Measuring with a multi-meter itself is a good method of discharging capacitor
- Avoid discharging capacitor using a short piece of wire. The sudden discharge will exert lot of pressure on dielectric material which will eventually reduce capacitor's life. Using resistors of suitable rating is recommended
DO NOT CONNECT GROUNDING WIRES OF EQUIPMENTS IN SERIES, just to save few meter of cable. This is called daisy chaining and will increase the magnitude of ground current. You might get electric shock from grounding line without knowing where it originated – The best practice is to run a continuous strip of conductor bar (Aluminum, GI or Copper) around the set-up and INDIVIDUALLY hook-up grounding to this bar
- And, never "twist" some wire piece for grounding purpose. The grounding wire must be firmly tightened at both ends to increase conductivity and reduce ground related faults
Sometimes, when you use a "tester" on a conductor, its light will feebly turn on, indicating some "floating" voltage somewhere. With multiple electrical equipments connected, it is pretty difficult to explain this phenomenon. But it clearly points to some grounding fault somewhere. Perform a grounding-check audit
- The floating voltage which "appears" on tester could be an indication of some "internal leakage" somewhere. Poorly insulated windings or motor, windings of transformers are two examples that can cause "internal leakage"
Never force loose cables into sockets / receptacles to "save" time. And, never force more than acceptable cable into a socket. All electrical wiring must be done with patience and in proper manner. Short-cut & Short-term methods will lead to accidents
Never run cables across ways where there is movement of people or vehicles – If that be necessary, guide the cable thru strong pipe and provide sufficient ramp for vehicle to pass over smoothly (or if permissible, use armored cables)
Cables should never be allowed to suspend by itself or be used as support ropes for holding something else – Always provide sufficient support for cables and they should be free of all "mechanical" weights
Each cable has "minimum bending radius" limit – Do not force them beyond it. This will be especially applicable to armored cables, which if bent beyond limit will develop severe mechanical stress resulting in insulation failure
In humid and wet locations provide sufficient protection for electrical items from moisture – Distribute plenty of "silica gel" packets inside panels to absorb moisture and replace them from time to time
Install necessary grade – DRY POWDER TYPE – fire extinguishers near important electrical panels and also across the plant
The electrical panels / distribution systems should have sufficient clearance all around, with good ventilation (natural or forced)

As much as possible eliminate power from the equipment on which you will perform some electrical work. The number of equipments / machines to be isolated depends on location of electrical work. For performing some work on main incoming panel, obviously the incoming HT-LT distribution system itself must be isolated. On the other hand, Based on the level of power isolation possible, it can be classified into three states

ZERO ENERGY STATE (ZES)
When all the power to an equipment or a system is completely eliminated or isolated and any component can be safely touched, removed and worked upon, it is said to be in zero energy state. This state also includes other "power" sources like pressurized air in a receiver, pressurized hydraulic fluid, heavy weight suspending from certain height and inflammable products. All of these must be isolated before concluding that the equipment or system is in total zero energy state.

Specifically from electrical perspective, if the equipment or system is connected to multiple power sources, all of them must be isolated. For example, a machine can have main incoming power for electrical components and "conditioned" power from UPS for electronic components. Both of them must be isolated for attaining zero energy state. If capacitors are installed, they must also be fully discharged for achieving zero energy state.

Although zero energy state eliminates risk in handling components, it does not compromise on safety requirements necessary for accomplishing the desired task.
INTERMEDIATE ENERGY STATE (IES)
When certain power exists in an equipment or a system, even after partial isolation, it is said to be in intermediate energy stage. An air-receiver could be storing pressurized air while the electrical power alone is isolated. Similarly, when it is "interfaced" to other machines or computer for exchange of certain signals, voltage could exist on those wires which cannot be isolated for various reasons.

From electrical perspective, if some work is being carried out on control system with specific isolation, main incoming power could still be on feeding other components on the machine.

Equal precaution and care should be taken while working on equipments placed in intermediate energy state.
COMPLETE OR FULL ENERGY STATE (CES or FES)
When all the necessary power is available to an equipment or a system and can be fully functional, it is said to be in complete or full energy state. This is the normal state of equipments, when they are operational.

CPR – Cardio Pulmonary Resuscitation – One of the oldest known and best methods of restoring heart beat when it gets "fibrillated." (Besides, CPR is also administered on persons suffering heart attack due to other reasons – mental stress, high BP, drowning, sudden mental or physical shock etc.,) CPR may not guarantee complete restoration but will definitely help keep the heart functioning to some extent, at least till primary medical assistance is provided.

The adjacent picture shows three most elementary steps of CPR.

CPR training must be made compulsory for people involved in working with machines and equipments. Of course, there is no harm in providing CPR training to all the personnel in any set-up. It will be helpful under any circumstances.

Apart from typical first-aid kit that is fixed at various locations, it is also essential – especially for maintenance personnel – to be familiar with using doctor's stethoscope and thermometer. These two devices are very effective in checking body condition when any such incidents occur and are very easy to use.

Special note on shock from AC versus DC From the current limit table somewhere in this newsletter, it is quite evident that less magnitude of alternating current causes same damage compared to direct current. Alternating current, as the name implies, fluctuates between a positive and negative value repeatedly over a period of time, compared to direct current which flows in one direction. To understand why this happens, just recall how we break a piece of small metal rod.

The metal rod is usually held in hand (or fixed in vice) and repeatedly bent up & down (hit both sides) which breaks it faster than just bending in one direction or hitting with hammer. Also, observe that after repeated bending, the metal would become little soft and very hot, especially in the broken region. Exactly similar will be the effect of alternating current on muscles and tissues. It causes repeated contraction and expansion causes accelerated rupture and more damage.

Follow 3-P principle for saving a life that otherwise can be lost.

Avoid P – PANIC – Do not panic and create a commotion or confusion., while witnessing and attending the victim.

Have P – PATIENCE – Nothing happens by magic. Have patience and recollect all actions that you must do to save life. Nobody should try to prove "super-hero" actions under such circumstances.

Practice P – PREVENTION – Once the situation is under control and the victim is out of danger, perform a thorough, in-depth analysis of what all led to the incident. Take all steps to prevent the same incident from occurring. If necessary, use statistical tools like RCA, PDCA, 5W-1H, Cause-Effect Diagram, Spider Diagram to arrive at specific solutions.

After spending some time reading the contents and "by-hearting" them for a safe tomorrow, activate your brain's electrical impulse to solve the Techuzzle on this subject !!

Happy Solving!!!

Across

1. What is the opposite of resistance?

4. Insulating hand gloves are usually made from this material

6. Salt and which other material are commonly used in pit?

7. Voltage you "see" on tester, but usually cannot be measured

8. Another word for earthing?

Down

2. What does C stand for in CPR?

3. Lost of rhythmic beat of heart

5. Does body resistance increase or decrease with prolonged contact with electricity

2009-06-23T22:13:00.001+05:30

WORD FOR THE DAY: COMPUTER TERMINOLOGIES

When Blasé Pascal and many other scientists made a "computer" to perform basic addition and subtractions some fifty years back, the electronic components consisting of power supplies, valves, cooling fans occupied one big room !! And, it took quite a long time to perform simple calculations. And, there were many cooling fans running to keep the electronic components from burning themselves. How far we all have come today. Sitting somewhere on a beach side or from the comfort of bedroom, you can browse the internet and keep a check on what is happening anywhere in the world, within minutes. This technological revolution is not only limited to satisfy our "audio-visual" (entertainment) needs. It has changed the way in which even machines function and communicate. It is simply amazing just to know that a bunch of satellites, millions of kilometers of optic fiber cables, telephone cables, wireless towers, many computers transcend all national borders and lets to talk to your loved ones anywhere on this beautiful globe (if you have someone in Moon or Mars, you can still communicate !!!)

With so much of technological changes happening around us, independent of where and which department we work in, it is imperative to know a little about them which will help us understand, communicate and work better in this "electronic era." This week, let us just brush upon some of the "technical jargons" commonly used in day to day affairs. This newsletter does not intend to elaborate in detail since each will be a subject by itself.
Welcome aboard for a electronic journey !!

USB – Universal Series Bus – A common "port" available on modern computers, programmable logic controllers and many electronic devices. As the name implies, it is "universal" in connectivity. An external memory drive (pen drive), printer, or any device with USB cable can be connected. The system will be programmed to automatically detect hardware type and perform corresponding action. It has a high data transfer rate of up to 12 MBps. It communicates in serial mode using four pins.

Example i) Flash memory card
ii) Printers
iii) Advanced variable speed drives for downloading parameters
iv) Latest PLC machines for downloading machine programs

LAN – Local Area Network – A network of computers, peripheral devices like scanners, printers, projectors. The LAN system can be broadly divided into two sections: "CLIENT" & "SERVER." Client is a computer (sometimes also called terminal) that requests some information or a task to be accomplished. For example, a client can request for printing a document or can download some file. The "server" checks all requests by all clients and assigns priority to their requests. The task of assigning priorities, creating different client groups and access controls is broadly called as "Network Administration." A LAN system consists of computers, routers, servers, various network cables. Each computer will be identified on LAN system by a unique code called IP (Internet Protocol) address.

When the network of computer increases, it is called as WAN or Wide Area Network. INTERNET (INTERnational NETwork) is a huge network of thousands of LAN and WAN. When a network is used within an organization with limited access to a specific group of people, it is called INTRANET.

Example    i) Office networks
        ii) Internet café
        iii) Machine control systems

JPEG / PNG / WMF / TIFF / GIF / BMP / DWG / DXF - With the advent of digital cameras and scanners, capturing, storing, editing and sharing images became more simple and easier. E-mailing images as attachments or uploading to public network is an extremely simple task. Let us take a look at some commonly used image storing formats.

JPEG –Joint Photographic Experts Group – One of the most widely used formats of storing images. The image stored will have JPEG or JPG as file extensions. This image format needs less memory space and hence can be easily sent as attachments. One disadvantage of this file format is its lack of sharpness. An extremely rich colorful picture will lose clarify when viewing. This will be specially obvious when you try to zoom the image more than its original size. Will be very useful for storing scanning drawings, manuals and overview pictures.

Examples:     i) Machine Manuals
        ii) Work Instructions / Work Station photos
        iii) Prints / Overall pictures

PNG – Portable Network Graphics – This format is extremely useful when storing pictures for more clarity. Although this format requires more memory space than JPG format, it retains all the finer details of image. If picture of some critical component needs to be exchanged for communication, use this format. The image can be zoomed without losing clarity.

Examples:    i) Broken or damaged parts of machine
        ii) Electronic PCBs / Small components
        iii) Extremely colorful pictures

WMF – Windows Meta File – A format specifically created by Microsoft for their windows applications. This format is widely used for storing line diagrams, flow charts. The images having less colors but more size can be stored in this format. This image format also requires less disk space.

Examples: i) Line Diagrams
ii) Flow Charts
iii)Component Diagrams

TIFF - Tagged Image File Format
GIF – Graphic Interchange Format

GIF and TIFF formats are also enhanced versions of image storing. They are used more in creation of animated files which are widely used on web pages.

BMP – Bit Map is another image format with high quality. For common tasks, it is not advisable to use these formats since they require large memory space. They are used in creation of internet pages and color specific applications. MS Paintbrush is one of the most common applications used for creating bit map images.

DWG / DXF – Drawing or Drawing Exchange Format are the two most common methods of storing drawing files which are created using different drafting tools like AutoCAD and Solid Works. These drawing will be made to scale and illustrate actual dimensions of an object being drawn.

MP3 / MP4 / WMV - With everything becoming digital, audio is definitely not left behind. Audio need not necessarily mean only music or a song. The sound recording s of machine, generating set or observations of shock pulse monitoring are also considered as sound. MP3 and MP4 are the most common methods of storing music files in "compressed" form. MP3 is a short form for MPEG – 3 which when abbreviated means Motion Picture Experts Group – Layer 3. It is a type of compression technology used to reduce the size of sound file by almost 60%. Although MP4 stores the file with lesser memory requirement, it loses the clarity of original sound track. For most practical purposes, MP3 will satisfy the need.

Examples:    i) Music / Song files
        ii) Voice recording of doctors, experts talking on subject
        iii) Any audio conversation

CAN BUS – Controller Area Network – It is an industry standard and comes under the distributed control system category. It is a serial network type where more than one CAN-product (PLC or a Drive or a PC) are connected in series. Each CAN product is called a node and communicate with each other according to the priority. In CAN bus, all controllers will not be able to communicate simultaneously. After one CAN product completes the task, other CAN product will take over. This method of communication is called "token passing" and is used widely in latest industrial control systems. CAN bus was originally set-up and used by Robert Bosch for their automotive applications, but gained popularity later in all other industry segments.

Examples    i) Engine Control Units
        ii) Medium machine control systems
        iii) PLC Controlled machines
        iv) Special purpose machines

MODBUS – This is an RS-485 based peer-to-peer network protocol which uses the MODBUS data structure. It transmits at 1 Mbps and allows up to 32 nodes on a network. Most of industrial control system manufacturers when designing a total DCS use MODBUS as common standard. For example, a combination of many PLCs, drives, data acquisition system can be connected using a common MODBUS network. With the addition of "repeater", up to 64 nodes can be connected without compromising on the data transfer speed. The end nodes which are not further connected to any other subsequent nodes must be terminated with a resistor – usually 120 OHMS – to avoid noise created by open wires.

PROFIBUS : Process Field Bus is a standard for field bus communication in automation technology. It is somewhat similar to MODBUS architecture. Individual components like valves, sensors can also be PROFIBUS enabled and can be connected to the network.

RS-232C – One of most common Recommended Standards developed by Electronic Industries Association (EIA). It communicates in serial mode. Originally RS232C was designed to use DB25 (25 pin) connector but gradually changed to 9-pin DB9 connector. RS232C is an unbalanced system and hence cannot communicate over very long distance, usually not more than 15 meters (45 meters) at a baud rate of 19.5 Kbps. The most common six pins used on RS232C are listed below:

Protective Ground – Should be connected to an outer shield conductor and tied to the chassis on ONE END ONLY.
TD – Serial "Transmit Data," an output
RD – Serial "Receive Data," an input
RTS – Request To Send, an output
CTS – Clear To Send, an input
DSR – Data Set Ready, an input
DTR – Data Terminal Ready, an output
(DSR & DTR indicate that the equipment is powered and ready to communicate)
Signal Ground – Common signal reference. Should not be connected to any external ground

The standard circuitry uses voltage level between +3 to +25 V DC to represent "one" and between -3 to -25V to represent "zero" binary conditions. The information is passed in a packet of 8 bits at a time.

RS-422 – This communication standard is a more balanced system compared to RS232C and hence can communicate over long distances, at much faster baud rate. Two separate set of wires are used for "receiving" and "transmitting" data (compared to RS232 where only one common wire is referenced for both receiving and transmission). This cable standard can transmit data at almost 10Mbps up to 1200 meters (4000 feet). RS-422 is the most commonly used method of communication between PLCs, auxiliary equipments like drives, control board.

The latest energy monitoring devices are also available with RS422 interface. When downloading data to a central computer, usually a "RS-422 to RS-232C" or "RS-422 to USB" converters are used.

RS-485 – This communication standard is similar to RS422 EXCEPT that a device which is NOT communicating can also be connected without interfering with other ACTIVE devices. One of the best example of RS-485 network is DCS system of many programmable logic controllers. If for any reason, one PLC is disabled, it need not be disconnected from the network (The device which is NOT communicating on line goes to HIGH IMPEDANCE state in a RS-485 network). RS-485 allows transmission up to 100 Kbps for a distance of 1200 meters (4000 feet).

CLIENT / SERVER – One of the most common terminology used in a networking system. The multiple computers present on a network are broadly divided into two groups based on their functionality. Those which "request" for any information is called a CLIENT. They may not necessarily store the information with them. Those computers which "serve" and complete any request from client is called a SERVER. For example, you log-in to your bank account to check the balance. The computer from which you log-in is a CLIENT request balance information. The computer which belongs to the bank having your account balance information becomes a SERVER providing necessary information when requested.

SWF – Shock Wave File – It is also called a FLASH file and stores animated files. The animated files will be created using various latest applications like Adobe and are widely used in creating training modules. To view SWF file formats, you should install Adobe Flash Player® or Adobe Shock Wave Player® (or equivalent) on your computer. It is one of the most interactive methods of creating multimedia applications.

RJ11 – The cable connecting telephone line or MODEM (Modulation-DEModulation) to the computer. It is a single pair cable performing serial communication with the service provider. It is also the common cable connected to telephones.

PROTOCOL – Every activity needs certain guide lines. There will be certain rules and regulations that every object must follow in different circumstances. Any such commonly agreed and established method of standardized procedure is called PROTOCOL and will be equally applicable in electronic field also. All communications methods like LAN, WAN, RS232, RS485 are called protocols. The hardware including cables, connectors and necessary software follow certain set of protocols all over the world for easier exchange of information and interchangeability.

BAUD RATE – The most widely used method of indicating data transfer speed in a communication line. The baud rate is usually displayed in Kbps or Mbps which stands for KILO BITS (not BYTES) or MEGA BITS (not BYTES) per second. Based on the connection and traffic at any given point of time, the baud rate will vary and is not fixed.

LPT – Line Parallel Terminal – This is a communication method, usually used on printers and photo copying machines. As the name says, the communication is parallel and hence data transfer will be faster compared to a serial communication. LPT type of connections are also widely used in expansion modules of a PLC system. They are also used in connecting output from analog modules to individual field boards.

COMBO DRIVE – As the name suggest it is a COMBINATION drive which can read and write almost all types of CDs – Data, Music, Video, VCD, DVD and Blue Ray discs. It can also write (called "burning") to various types of CDs at different speeds. It is available in different types to suit desktop PCs, laptops. It is also available as a separate system which can be connected externally thru USB port. One of the important parameter for a CD drive is its speed. We all have heard of 8X, 24X or 48X speed drives. The "X" stands for data transfer rate and one X is equal to 150 KILO BYTES PER SECOND (KBps). The speed at which reading or recording is done is dependent on type of combo drive and information on the CD. There are non-combo drives available which either can read only CDs, DVDs or VCDs but are fast becoming obsolete with affordability of combo drives.

FLASH CARD - There are many technologies of storing digital information. It can be stored on floppy disks (which has become outdated), hard disk drives, cassettes, CDs. The latest technology using "nano electronics" is flash memory. A flash memory card can be tremendously small in size holding large volumes of data. Pen drives, SD cards commonly used on digital cameras, external hard disks are the example of flash technology based storage devices. Although they are extremely compact, their storage in ideal conditions is essential for prolonged life. They are not as rugged as a compact disc or hard-disk drive.

ETHERNET – One of the most common methods of connecting network computers. It was originally configured by Xerox Corporation (makers of photo copying machines) to connect their large computer network. It gradually became a world standard and started being used in typical LAN type of network systems. The Ethernet communication is accomplished using either a flat 8 pin cable or co-axial cable or optic fiber cable. The need for specific type of cable depends on distance, reliability of information and speed at which data has to be transmitted. Of course, it is possible to have a mix of all these types in one ETHERNET network. CAT-5 and CAT-6 type flat cables are the most commonly used methods found in typical factory. Lot of machine control systems also have adopted Ethernet as the standard communication protocol. There are two advantages of using Ethernet on machine control system. One, it will allow faster data transmission between machines located very far in a factory. Second, it can be accessed from internet anywhere in the world. For example, if machine manufacturer wants to look into any program details, they can log-in from anywhere to this machine and go thru every detail. It is even possible to operate entire machine system from remote connection. That is the power of Ethernet enabled machines !!

Following table gives a brief description of different communication standards used in Ethernet
(The most commonly used Ethernet cable is highlighted yellow. Twisted pair CAT cable is also called RJ-45 type)

Signal Name	AKA	Data Rate	Maximum Distance	Special Designation	Media
10Base2	Thinnet	10 Mbps	185 meters (616 feet)	Approximately 200 meters	50 OHM thin co-axial cable
10Base5	Thicknet	10 Mbps	500 meters (1665 feet)	Approximately 500 meters	50 OHM thick co-axial cable
10BaseT		10 Mbps	100 meters (330 feet)	Twisted pair cable	CAT 3 or 4 twisted pair cable
10BaseF		10 Mbps	2000 meters (6600 feet)	Fiber optic cable
100BaseT	Fast Ethernet	100 Mbps	100 meters (330 feet)	Twisted pair cable	CAT 5 twisted pair cable
100BaseF	Fast Ethernet	100 Mbps	400 meters (1332 feet)	Fiber optic cable
1000BaseT	Gigabit Ethernet	1000 Mbps	100 meters (330 feet)	Twisted pair cable	CAT 5e or 6 twisted pair cable
1000BaseF	Gigabit Ethernet	1000 Mbps	220 meters (732.6 feet)	Fiber optic cable

DCS - Distributed Control System is a method of connecting many machines which will have its own control system utilizing any of the protocols discussed above. CANBUS, PROFIBUS, MODBUS are some of the common methods of networking multiple controllers in a DCS. Each machine will have its own controller and program. They will be able to run independently but will be interconnected for exchanging various inputs, outputs and common commands. Packaging machineries, paper mills, chemical industries will have DCS for their machines.

SCADA – Supervisory Control And Data Acquisition is a centralized system of having an "overview" of entire process set-up. All machines can be controlled from a central point and real-time data can be obtained about the process and machine status. Large refineries, power stations, power transmission and distribution, air traffic control, railway signal control are some of the examples of vastness of SCADA.

Now, don't you think we should all let our brains DIGITIZE itself a little bit and solve the Techuzzle on whatever we were reading till now?

Across

3. A common LAN system in the world

5. The rate of data transfer

7. Vital device for communicating on telephone line

8. Process Field Bus?

Down

1. Who will satisfy the client?

2. A strict communication standard

4. This bus can connect many machines

6. Communication method of SINGLE ROAD

2009-05-24T17:47:00.001+05:30

rom time immemorial, man has spent and still spending quite a lot of time - and money - exploring the "invisible" forces that are controlling this earth and all happenings on it. Not to forget the deep "hidden" desire of knowing "future" by interpreting these "invisible" forces. Well, we are not starting to predict your future !! What we are really keen about is to learn how we can protect our "future" – whatever that turns out to be – from one invisible force which if not given proper care can result in a disaster. ELECTRICAL ENERGY. An invisible energy which when used safely and properly will make our life comfortable. If neglected, it sure will damage our "future."

Any end component that utilizes electricity for its functioning, like motors and heaters, are not "smart" enough to protect themselves against any eventualities. Be it short-circuit, overload or grounding fault, these end components "destroy" themselves. Consider a motor connected to some load for example. If the motor is "stalled" due to mechanical jam or is overloaded, the windings overheat gradually melting varnish insulation, short circuiting and eventually failing. Major downtime, production loss, rewinding and lot of man hours obviously follow this breakdown.

Similarly, consider an heating system used for heating the water. If the water has high salt content or is acidic, it will deposit scale on heater tube eventually damaging it. The exposed heater coil will "short-circuit" damaging the system, not to forget possible human fatalities. And the list can be endless.

A comprehensive protective system is therefore necessary in protecting expensive electrical equipments, apart from saving precious human lives. The electrical protective components can be broadly classified into two categories: FAULT SENSING and FAULT TRIPPING or ISOLATING. The devices which sense (measure) electrical parameters but does not directly isolate the power supply fall under fault sensing category. Overload Relays, Current Transformers, Thermocouples, Load cells are few examples. Even the "static" power carrying components like cables, bus bars can be grouped under fault sensing category. Heating of cable is an indication of some fault in the circuit, probably overloading or loose connection. The devices which sense AND isolate or only isolate the power when overload or short-circuit occurs fall under fault tripping or isolating category. Common fuses, MCBs, ACBs, Thermostats are few examples.

In this newsletter, let us understand a little more about the most common and widely used fault sensing and tripping electrical components. Elaborating on each in is not intended to be covered in this newsletter. Each component needs one full size article, which we will publish soon.

Fuses – The oldest, simple, reliable and widely used method of protecting any electrical system is a small piece of wire enclosed inside a compact insulator housing. Independent of installation of any number of sophisticated protection system, fuses will be installed somewhere in the circuit (Usually, it will be the starting point of a distribution circuit). As the name implies, fuse "fuses" or connects supply and load side of electrical system with a piece of wire, made from silver or silver-oxide (tinned) copper. This piece of wire gets heated up as the current passing thru it increases. As soon as the current exceeds carrying ability of wire, it melts and breaks the circuit. Depending on type of fuse, entire fuse assembly – called cartridge – or wire has to be replaced. Although ageing or environmental conditions reduce current carrying capacity of fuse, it will never fail to protect the load. Fuse wires are housed inside variety of insulators.

Ceramic Fuses –

Ceramic is one of the most widely used insulating materials in electrical distribution system and are used in fuses. The ceramic fuses are bulkier, but provide excellent safety during handling. If the wire melts, it can be replaced with equivalent wire and put back into operation. They are the least expensive of all types of fuses. Only requirement is to keep a stock of correct diameter wires. Refer to the following table for current carrying capacity of different wire diameters.

Rated current of fuse Amperes	Diameter of tinner copper fuse wire (millimeter)
6	0.2
10	0.35
16	0.5
20	0.63
25	0.75
32	0.85
40	1.25
63	1.5
80	1.8
100	2.0

Cartridge Fuses –
This is a replaceable type of fuse enclosed typically inside a cylindrical or rectangular ceramic housing filled with fine grain of sand or silica. Usually, a metal pin will pop out of the housing if the fuse blows. These type of fuses are generally used in low voltage applications, up to 600 amperes. The cartridge fuses are broadly classified as HRC (High Rupture Capacity) & HBC (High Breaking Capacity) types. For equipments demanding instant protection during short circuit, HRC type is preferred. Expensive CNC machines, drive systems must be protected by HRC type fuses. Equipments which are prone to certain normal overloading must be protected by HBC type of fuses, which has a slight time delay before breaking the contact.

Semiconductor Fuses – They are extremely fast acting current limiting fuses specifically designed for protecting semi-conductor based equipments like UPS, Drives, Inverters. They are sensitive to over-heating, voltage spikes and peak currents. They are broadly identified as gR & aR type.

Bottle (glass) Fuses –
Small fuses, usually rated up to 10 amperes which are widely used on PCB (Printed Circuit boards).

OLR – Over Load Relay (also called Thermal Relay) – A very simple safety tripping device, generally used to limit the current drawn by electric motors. The current drawn by motor flow thru a bi-metallic strip housed inside plastic case, usually made from phenol formaldehyde. When current flows thru the bi-metallic strip, it becomes hot and expands. This expansion of strip is used to push a lever which actuates two contacts. The opened contact will break continuity of an electrical circuit which usually de-energizes the contactor, thus stopping the motor. Once tripped, the OLR CANNOT be reset immediately. The bi-metallic strip has to "cool" down before it can be reset. Based on current rating, the reset time can vary from few seconds to several minutes. The current adjusting knob on OLR actually increases spring tension inside housing. This is the force that the load current should overcome before tripping. OLR does NOT directly disconnect the load from main circuit. It is only a FAULT SENSING device and not a FAULT TRIPPING device. Because of its electro-mechanical construction, it should only be mounted in horizontal position. The LOAD and LINE side should not be changed. Under no circumstances should the OLR be by-passed or knob be adjusted beyond its marked region.

The current setting on OLR can be set as either DIRECT rating or ( 1.732) of the current rating. Pay attention to the label for proper setting. OLR does NOT trip without a cause. If OLR repeatedly trips, eliminate the cause before adjusting current rating or replacing the OLR. Because of its working principle, OLR DOES NOT protect against SHORT CIRCUIT or INSTANT OVERLOADING. A back-up fuse assembly or Circuit Breaker must be installed for complete protection. Based on current rating, one or two OLRs can be fitted in a motor starting system. It is quite common to use two OLRs, one with main and other with delta contactor in motor systems beyond 30 HP

MCB – Miniature Circuit Breaker, is an electrical FAULT TRIPPING device that breaks the circuits and protects electrical loads. It is designed and selected to protect against a preset overload and short circuit. MCBs are used on low voltage

applications usually upto 63 amperes. The MCBs are available in single phase (single pole), single phase neutral (two pole), three phase (three pole) and three phase neutral (four pole) types. Since direction of current flow is critical for

proper operation of MCBs, the "load" and "line" side must not be interchanged. On a four pole type, the phase must not be connected to neutral contact, since it is not designed to trip beyond the rated current.

MPCB – Motor Protection Circuit Breaker is a combination of MCB and OLR taking advantage of both systems. So, MPCB as the name implies is both a fault sensing and tripping device. It eliminates the additional wiring required for an OLR and current can be adjusted on face plate. MPCBs do not trip on instant load and hence can be directly used in a motor protection circuit.

MCCB – Molded Case Circuit Breaker is the higher version of MCB, used on higher currents usually beyond 100 amperes. MCCBs are available in both fixed and variable current settings. As the option implies, in a variable current type, MCCBs trip current can be adjusted within certain range. Also MCCBs can be fitted with "shut trip coils" which will break the circuit whenever a fault is detected by a "remote" device. For example, an ELR or over current relay can close "shut trip coil" wiring and activate MCCB. On distribution panels, an emergency stop button with its normally open contact wired to the trip coil. In an emergency situation, any nearest person can depress the button to isolate main power, preventing accidents.

CBCT – Core Balance (also called Zero Sequence) Current Transformer is basically a current sensing device thru which the three phase and neutral conductors pass before being distributed to loads. Usually, CBCTs are installed for low voltage main incoming point and / or incoming of distribution panels. The CBCT senses differential current arising out of three phase system. Whenever there is an "imbalance" in current, which usually happens due to ground fault or more current flow in neutral conductor, there will be a proportional output across its terminals. The CBCT will be wired to a ELR (Earth Leakage Relay) which breaks the contact of a fault tripping device like ACB or MCCB whenever a fault is detected. To prevent unnecessary tripping which can happen due to high inrush currents or welding machines or solid state electronic components, two important settings are available on the ELR. One, Sense current limit which sets the upper or tripping limit for the circuit. Whenever the CBCT generates current equal to or greater than this value, ELR will trip the circuit. Two, sense time which ensures that the ELR does not trip when CBCT detects instant high currents. If the CBCT senses the "set" current for "set" duration, then ELR will trip the circuit. Most of the time, without proper understanding of the electrical system in a set-up, both these parameters are always set on higher side. If system does not respond to any ground faults and break the circuit, expensive electrical and electronic components can be damaged, apart from posing serious safety thread. Hence, it is essential to adjust both these parameters at the "minimum" level and then gradually increase till the most optimum setting points are reached. From then on, the circuit must be checked periodically to ensure that all components are working in perfect synchronization.

CT – Current Transformer is a very sensitive current sensing device which measure current flow in a conductor. Whenever current flow in a conductor, a proportional but less magnitude of current will be produced in current transformer winding. The conductor current is called "Primary Current" and current in the transformer winding is called "Secondary Current." So, the CT is usually identified by the ratio of primary to secondary current, called CT ratio.

A CT ratio of 400/5 means when 400 amperes of current flow in primary conductor, 5 amperes will be produced in the CT winding. So, if current thru primary conductor reduces to 200 amperes, secondary current will also reduce to 2.5 amperes and it is proportional. Current Transformer can be connected to various fault tripping devices like Air Circuit Breakers, Vacuum Circuit Breakers, Overload sensors. They are also used for measuring current, energy, power factor and demand ratios.

When connecting CT to a sensing or tripping device, pay attention to its polarity. Usually, CTs will have S1 & S2 as secondary wires. If the primary conductor direction changes, S1 and S2 must be interchanged. If three CTs are used on each phase of a three phase system, it is normal to connect S1 or S2 in common depending on direction of current flow.

RCCB – Residual Current Circuit Breaker, which is also known as Earth Leakage Circuit Breaker is a very sensitive residual current sensing component that detects leakage current in "neutral" or return line of the single or three phase system. Instead of neutral, ground line can also be connected for leakage current detection. RCCBs can detect leakage currents as low as 25 milli amperes and not only protect electrical equipments, but will also save precious human life. They are designed only to detect leakage currents. They do not provide protection against overload or short-circuit. A back-up MCB or MCCB is essential for completion of electrical safety system. RCCB is a fault sensing and tripping device. The RCCB has a small arcing chamber which prevents fire when a fault occurs.

ACB / VCB / OCB– Air Circuit Breaker / Vacuum Circuit Breaker / Oil Circuit Breaker - All these are meant to disconnect and connect high current loads. In air circuit breaker, the contacts are exposed to air. So, when the breaker makes and breaks the contact, there will be repeated sparking in "contact" area. Over a period of time, this might cause carbonizing resulting in reduced current carrying capacity. Period maintenance and proper servicing of ACB and its contact prolong the life and increase reliability.

In vacuum circuit breaker, the contacts are sealed inside air-tight vacuum. From fundamentals of science, we know that without oxygen there will be no fire. So, in vacuum enclosure, there will be virtually no sparking between contacts resulting in increased current carrying capacity and contacts does not carbonize. Difficulty of maintenance and bulky construction are some disadvantages of vacuum circuit breaker. In an oil circuit breaker, the contacts are immersed in an oil bath. This not only eliminates sparking, but also acts as a coolant in removing heat from current carrying area. Oil circuit breakers have high efficiency and longer life. Air or Vacuum or Oil circuit breakers are used both low and high voltage systems. In low voltage applications, there breakers are used usually beyond 600 amperes.

What should we keep in mind?

The preceding paragraphs gave a bird's eye view of few of common and widely used electrical protection system. There are many more very specific protection devices that should be selected for protecting specific application. What is chosen for protecting a fully-computerized CNC machine may not be suitable for an air compressor system. But, common sense prevails and plays an important role in protecting all electrical systems.

Remember, no electrical protection component fails without a cause. Eliminate the cause on war footing basis. Do not neglect. No reverse engineering is applicable for an accident
Never by-pass the protection system. Do not use over-rated components "just" to keep machine running. Never replace a fuse with solid piece of wire. It may cost you a lot in future.
Prepare an exhaustive plan and conduct regular audits of all electrical protection components in the set-up.
All "contact" areas must be kept clean, free of moisture, dust and inflammable materials. Do not use wood or plastic or card-board pieces to provide insulation. If space does not permit, re-design the system itself to provide ample space between conductors and neutral / ground wires.
Do not try to force more conductors into terminal slots of circuit breakers. Use proper crimping lugs, bus bars and good quality insulators for termination.
Although PVC insulation tape is the most commonly used "quick fix" insulator, it does NOT provide long term insulation for all conductors.
Use proper tools to tighten electrical fasteners. When crimping or joining conductors, use joining compound to reduce contact resistance which in turn prevents sparking and residual faults.
Cables should not be forced to twist below its "minimum bending radius" and never be used as a support "rope." Provide necessary cable clamps and ensure no "mechanical" load is exerted at the contact point.

Remember, SHORT-CIRCUIT IS NEVER A "CAUSE" OF FIRE. A loose connection, poor insulation or closely located conductors can be THE CAUSE and SHORT-CIRCUIT IS ONLY AN "AFTER-EFFECT" of this cause. Eliminate the cause at the earliest

Across

2.The effect of two conductors touching each other

5.Over load relay is also called ...... relay

6.CBCT senses this type of current in a circuit

8.ELR, when expanded means Earth ....... Relay

9.Air gap increases this resistance

Down

1.Most important part of a overload relay

3.These fuses are replaceable type. Has a pop up lever to indicate failure.

4.Sound does not travel in this medium

5.There are two types of safety: Fault Sensing & Fault ........

7.Chattering of a contact causes this type of fire

2009-05-19T08:53:00.001+05:30

Many of us would not have passed our childhood without visiting a circus, at least once. And, how happy we were laughing our heart out when the "joker team" made all sorts of pranks. Their colorful conical cap is one interesting part of their attire that gave them a distinct personality. Not to forget the big red cherry nose, of course. And no wonder that the cap became so popular that it was called "foolscap." It was made short or long either from a 216 mm (8.5 inch) x 343 mm (13.5 inch) or 432 mm (17 inch) x 343 mm (13.5 inch) paper. And, many of grew up to become engineers, entrepreneurs, artists so on and so forth. Whatever the path of journey may be, it is impossible to stay away from one thing: PAPER. And, paper of different sizes have helped us accomplish many things. Writing, painting, typing, printing and scribbling something. How much ever can be the popularity of latest technologies, paper remains an indispensable part of our daily activities.

This week, let us explore the paper from its dimensional perspective. Whether you are using an envelope to post a letter or printing some report or photocopying a journal. International Organization for Standardization (ISO) have setup certain guidelines for different paper sizes. This will help in working with paper anywhere in the world easier and common.

Standard paper sizes like ISO A4 are widely used all over the word today. This text explains the ISO 216 paper size system and the science behind its design.

The ISO paper size concept

In the ISO paper size system, the height to width ratio of all pages is the square root of two ( = 1.4142:1) called ASPECT RATIO. In other words, the width and height of a page relate to each other like the side and diagonal of a square. This aspect ratio is especially convenient for a paper size. If two "small" size pages are put next to each other, it will result in next "higher" size. For example, when two A4 sheets are placed next to each other matching "longer" side, it will result in A3 sheet.

The ISO paper sizes are based on metric system. Hence, square root of two will not permit the exact dimension. Therefore, there area of pages has been defined to have round metric values. As paper's quality is essentially mentioned in GSM (grams per square meter) the metric value makes it simple for calculation of weight of the document if the format and number of pages are known.

ISO 216 defines the A series of paper sizes based on these simple principles:

The height divided by width of all formats will have a ratio equal to square root of two, 1.4142

Format A0 has an area of one square meter

Format A1 is A0 cut exactly into two pieces. This means the height of A1 is width of A0 and width of A1 is half the height of A0

Although inches are also used for measurement, generally the paper dimensions are defined in millimeters. Refer to table-1 ( all numbers are in millimeters) for details. The inch dimensions are used in USA, Canada and few other countries. They are broadly categorized as "legal" or "letter" or "executive" types.

For applications where the ISO A series does not provide adequate format, the B series has been introduced to cover a wider range of paper sizes. The C series of formats has been defined for envelopes.

The width and height of a B format are the GEOMETRIC MEAN between those of a A size and its next HIGHER size. For instance, B1 is the geometric mean between A1 & A0, that means the same magnification factor that scales A1 to B1 also scales B1 to A0.

The Japanese JIS P 0138-61 standard defines the same A series as ISO 216, but slightly different B series of paper sizes, sometimes called as JIS B or JB series. JIS B0 has an area of 1.5 square meter such that the area of JIS B page is the arithmetic mean of the A series pages with the same and next higher number. The JIS B or JB paper standards are not very popular outside Japan since they do not conform to ISO standards.

All dimensions are in millimeters.

Table 1

A Series Formats		B Series Formats		C Series Formats
4A0	1682 x 2378	-	-	-	-
2A0	1189 x 1682	-	-	-	-
A0	841 x 1189	B0	1000 x 1414	C0	917 x 1297
A1	594 x 841	B1	707 x 1000	C1	648 x 917
A2	420 x 594	B2	500 x 707	C2	458 x 648
A3	297 x 420	B3	353 x 500	C3	324 x 458
A4	210 x 297	B4	250 x 353	C4	229 x 324
A5	148 x 210	B5	176 x 250	C5	162 x 229
A6	105 x 148	B6	125 x 176	C6	114 x 162
A7	74 x 105	B7	88 x 125	C7	81 x 114
A8	52 x 74	B8	62 x 88	C8	57 x 81
A9	37 x 52	B9	44 x 62	C9	40 x 57
A10	26 x 37	B10	31 x 44	C10	28 x 40

The allowed tolerances are ±1.5mm for dimensions up to 150mm, ±2mm for dimensions between 151 and 600 mm, and ±3 mm beyond 601mm. Some national equivalent of ISO 216 specify tighter tolerances. For instance, DIN 476 specifies ±1mm, ±1.5mm and ±2mm respectively.

The ISO standard paper covers a wide range of formats, but not all of them are widely used in practice. Among them, A4 is clearly the most important one for daily office use. Some main applications of the most popular formats are summarized in the following table-2.

Table 2

A0, A1	Engineering drawings, Posters
A1, A2	Flip Charts, Part drawings, Assembly procedures
A4	Letters, magazines, forms, catalogs, laser printer and copying machine output
A5	Note pads
A6	Post cards
B5, A5, B6, A6	Books
C4, C5, C6	Envelopes for A4 letter – Unfolded (C4), Folded Once (C5), Folded Twice (C6)
B4, A3	Newspapers, supported by most copying machines
B8, A8	Playing cards

The main advantage of ISO standard paper sizes becomes obvious for users of photo copying machines.

Example – 1

You are in a library and want to copy an article out of journal that has A4 format. In order to save paper, you want to copy two journal pages on two sheets of A4 paper. If you open the journal, the two A4 pages that you will now see together have A3 format. By setting the magnification factor on copying machine to 71% or by pressing A3 → A4 button that is usually available on most copying machines, both A4 pages of the journal article together will fit exactly the A4 produced by copying machine. One reproduced A4 page will now have A5 format. No wasted paper margins appear, no text has been cut off and no experiments for finding the appropriate magnification factor are necessary. The same principle works for books in B5 or A5 format.

Copying machines designed for ISO paper sizes usually provide following special short cut keys for easier magnification.

Table 3

71%	A3 → A4
84%	B4 → A4
119%	A4 → B4 (also B5 → A4)
141%	A4 → A3 (also A5 → A4)

The following table shows magnification factor for all A series ISO dimension papers. This will be helpful during photocopying or zooming a document for further applications.

Table 4

To → From ↓	A0	A1	A2	A3	A4	A5	A6	A7	A8	A9	A10
A0	100%	71%	50%	35%	25%	18%	12.5%	8.8%	6.2%	4.4%	3.1%
A1	141%	100%	71%	55%	35%	25%	18%	12.5%	8.8%	6.2%	4.4%
A2	200%	141%	100%	71%	50%	35%	25%	18%	12.5%	8.8%	6.2%
A3	283%	200%	141%	100%	71%	50%	35%	25%	18%	12.5%	8.8%
A4	400%	283%	200%	141%	100%	71%	50%	35%	25%	18%	12.5%
A6	566%	400%	283%	200%	141%	100%	71%	50%	35%	25%	18%
A6	800%	566%	400%	283%	200%	141%	100%	71%	50%	35%	25%
A7	1131%	800%	566%	400%	283%	200%	141%	100%	71%	50%	35%
A8	1600%	1131%	800%	566%	400%	283%	200%	141%	100%	71%	50%
A9	2263%	1600%	1131%	800%	566%	400%	283%	200%	141%	100%	71%
A10	3200%	2263%	1600%	1131%	800%	566%	400%	283%	200%	141%	100%

Not only the operation of copying machines in offices and libraries, but also photography, microfilming and printing are simplified by the aspect ratio (1: 1.4142) of the ISO standards.

Example – 2

If you are preparing a letter, you will have to know the weight of content in order to determine the postal fee. This can be conveniently calculated with the ISO A series paper sizes. Usual typewriter and laser printer paper weight 80 GSM (grams per square meter). An A0 sheet measures one square meter and next smaller measures half its area and so on. So an A4 sheet measures 1/16 of A0 weight, which is equal to 5 grams. So, if five A4 sheets are to be posted, it will measure 20 grams.

Calculation of weight of books, newspapers or packed paper is equally simple. You probably will not need such calculations often, but they nicely show the beauty of the concept of metric (ISO) paper sizes.

Using standard paper size saves money and make many applications simple. For example, if all scientific journals use only ISO formats, then libraries would have to buy only very few different sizes for the binders. Shelves can also be designed such that standard formats will fit exactly without too much wasted shelf volume.

ISO formats are widely used in applications other than paper. German citizen ID card uses A7 format and most of the passports are of B7 format. Interestingly, toilet paper in Germany are of A6 width size.

Sometimes, paper formats with a different aspect ratio are required for labels, tickets and other purposes. These should preferably be derived by cutting standard series sizes into 2, 3, 4 or 8 equal parts parallel with shorter side such that the ratio between longer and shorter side is greater than aspect ratio. Some example long formats are given in following table.

Table 5

(1/3) A4	99 x 210
(1/4) A4	74 x 210
(1/8) A4	37 x 210
(1/4) A3	105 x 297
(1/3) A5	70 x 148

Envelope formats

For postal purposes, ISO 269 and DIN 678 define the following envelope formats, although there could be few differences in some countries.

Table 6

Format	Size (mm)	Content Format
C6	114 x 162	A4 folded twice
DL	110 x 220	A4 folded twice
C6 / C5	114 x 229	A4 folded twice
C5	162 x 229	A4 folded once
C4	229 x 324	A4 unfolded
C3	324 x 458	A3 unfolded
B6	125 x 176	C6 envelope
B4	250 x 353	C5 envelope
E4	280 x 400	B4

Folding larger pages to A4 for filing

DIN 824 describes a method of folding A0, A1, A2 and A3 formats to A4 formats for filing. This folding technique ensures that there will be a 20 millimeter single-layer margin for filing holes and that the page can be folded / unfolded without removing from file. This will also ensure that the labels printed either in bottom-left or bottom-right corner of an engineering drawing will orient itself to the top for easy readability.

Filing Holes

ISO 838 specifies that, for filing purposes, two holes of 6±0.5mm diameter can be punched into sheet with a centre to centre distance of 80±0.5mm and offset by 12±1mm to the nearest edge. The holes must be located symmetrically in relation to the axis of the sheet or document. Any format that is at least as large as A7 can be filed using this system. If two more holes are to be punched, they will be 80mm above and below these holes giving it more stability inside a file.

Technical drawing pen sizes

Technical drawing pens follow the same size-ration principle. The standard sizes differ by a factor of square root of 2 (1.4142). The pen sizes will be 2, 1.4, 1, 0.7, 0.5, 0.35, 0.25, 0.18 and 0.13 mm. So after drawing with a 0.35mm pen on A3 paper and reducing it to A4, you can continue with the 0.25mm pen (ISO 9175-1)

Overhead projectors

When you prepare overhead projector slides for a conference, you might wonder, how large the picture are of the projector that you will have available is. ISO 7943-1 specifies two standard sizes of overhead projector picture areas. Type-A is 250 x 250 mm and Type B is 285 x 285 mm. Therefore if you use A4 transparencies, leave at least 30mm as top and bottom margins.

Most computer displays also have the same aspect ratio as TV sets. 4:3. 640 x 480 = 1024 x 768 = 1280 x 960. If you prepare presentation slides, the layout should be inside 280 x 210 mm. This will help in showing all parts of transparencies on all slides and easier printability.

One of the oldest written records regarding the two-square-root aspect ratio for paper sizes is a letter that the physics professor Georg Christoph Lichtenberg from University of Gottingen, Germany wrote to Johann Beckmann.

US and few other industrialized nations today uses non-ISO paper sizes like Letter (216 mm x 279 mm), Legal (216 mm x 356 mm), Executive (190 mm x 254 mm). Refer to the following table showing closer dimensions for ISO papers in inches.

Table 7

A Series Formats		B Series Formats		C Series Formats
4A0	66.250 x 93.625	-	-	-	-
2A0	46.750 x 66.250	-	-	-	-
A0	33 x 46.750	B0	39.375 x 55.750	C0	36 x 51
A1	23.375 x 33	B1	27.750 x 39.375	C1	25.5 x 36
A2	16.5 x 23.375	B2	19.75 x 27.75	C2	18 x 25.5
A3	11.75 x 16.5	B3	13.875 x 19.75	C3	12.75 x 18
A4	8.25 x 11.75	B4	9.875 x 13.875	C4	9 x 12.75
A5	5.875 x 8.25	B5	7 x 9.875	C5	6.375 x 9
A6	4.125 x 5.875	B6	4.875 x 7	C6	4.5 x 6.375
A7	2.875 x 4.125	B7	3.5 x 4.875	C7	3.1875 x 4.5
A8	2 x 2.875	B8	2.5 x 3.5	C8	2.25 x 3.1875
A9	1.5 x 2	B9	1.75 x 2.5	C9	1.625 x 2.25
A10	1 x 1.5	B10	1.25 x 1.75	C10	1.125 x 1.625

The dominance of the "Letter" format instead of ISO A4 as the common paper format in North America causes a lot of problems in daily international document exchange with the USA and Canada. ISO A4 is 6mm less wide but 18 mm higher than the "letter" format. Word processing documents with an A4 layout can often not be printed or modified without loss of information, loss of margins and all the styles applied, including page numbering. A4 documents must be printed with 94% magnification factor to fit into letter paper. Similar, letter documents have to be printed with 97% magnification to into 3% less wide A4 paper.