Basic Electrical Laws

Joule's Law

The heat produced by a current I flowing through a resistance R for a time t, is proportional to l²Rt  or,  Heat = (I² R t) / J   where J = 4.2 Joules / cal

Ampere's Rule

This rule gives the direction in which a magnetic needle deflect when placed near a current carrying conductor. Imagine a person swimming in the direction of the current, facing the magnetic needle. Then the direction of deflection of the north pole of the needle will be towards his left side.

Coulomb's Law

The force between two electric charges is directly proportional to the product of the magnitude of the charges and inversely proportional to the square of the distance between them.

Ampere's right hand rule

This gives the direction of magnetic field around a current carrying conductor. Imagine that the wire is gripped in the right hand with thumb pointing along the wire in the direction of current. Then the direction of the fingers will give the direction of the magnetic field.

Maxwell's cork-screw rule

This also gives the direction of the magnetic field around a current carrying conductor. Imagine that a right-handed corkscrew is driven in the direction of the current flow. Then the direction of magnetic field due to the current will be the direction of rotation of the head of the screw.

Laws of Parallel Currents

1.    Two parallel conductors attract each other if the currents through them flow in the same direction and repel each other if the currents through them flow in the opposite direction.

2.    The force between two such parallel conductors is proportional to the product of the current strengths and to the length of the conductors considered and varies inversely as the distance between them

Faraday's Laws of Electromagnetic Induction

1.    Whenever the magnetic flux linked with a circuit changes, an EMF is always induced in it.

2.    The magnitude of the induced EMF is proportional to the rate of change of flux-linkage

Lenz's Law

The electromagnetically induced current always flows in such a direction as to oppose the very cause which produces it.

Fleming's Left-hand Rule (for motor action)

This gives the direction of force on a current carrying conductor placed in a magnetic field. Hold the left hand with forefinger, middle finger and thumb at right angles to one another. If the forefinger represents the direction of the field and the middle finger that of the current, then the thumb gives the direction of motion of the conductor.

Fleming's Right-hand Rule (for generator action)

This gives the direction of induced current in a conductor, which is moved in a magnetic field. Hold the right hand with forefinger, middle finger and thumb at right angles to one another. If the forefinger represents the direction of the field and the thumb that of the conductor motion, then the middle finger gives the direction of induced current.

Kirchhoff's Laws

1.    In an electrical network, the sum of the currents entering a junction is equal to the sum of the currents leaving the junction.

2.    The algebraic sum of the potential differences around a closed circuit is zero.

Maxwell's Law

1.    Any two circuits carrying current tend so to dispose themselves as to include the largest possible number of lines of force common to the two.

2.    Every electromagnetic system tends to change its configuration so that the exciting circuit embraces the largest number of lines of force in a positive direction

Hall Effect

It states that if a magnetic field is applied perpendicular to a metal plate, which carries a current, then a transverse voltage is set up in the plate perpendicular to both the current and the magnetic field. This transverse voltage is known as Hall Voltage.

Seebeck Effect

When two dissimilar metal wires are joined at the ends to form two junctions (thermocouple) and when these junctions are kept at different temperatures, an EMF is produced in the circuit. This is called Seebeck Effect.

Peltier Effect

When an electric current is passed through a thermocouple, then heating is produced at one junction and cooling at the other junction.

Colour coding of resistors

Colour	Number	Colour	Number	Tolerance
Black	0	Green	5	Gold: ± 5% Silver: ± 10% No colour: ± 20%
Brown	1	Blue	6
Red	2	Violet	7
Orange	3	Grey	8
Yellow	4	White	9

Assessment of the value of resistance

First two bands from the end indicate the first two significant figures of resistance in ohm.

The 3rd band indicates the decimal multiplier.

The last band stands for the tolerance in percentage.

Example

First band - Yellow                 = 4

Second band - Violet             = 7

Third band - Brown                = 1       i.e. 10¹

Fourth band - Gold                = ±5% tolerance

Therefore the value of resistance   = 47 x 10¹ ± 5% = 470 ohm ± 5% tolerance

Inspection of Industrial Establishments

Readers who can’t follow Malayalam language may please excuse me for the video clip below, featuring an electrical inspection of a proposed biscuit factory. This hilarious film that also exposed the pathetic industrial atmosphere prevailing in Kerala was a big box office hit and the message is relevant even today. This clip is from the film, ‘Mithunam’, released in 1993, featuring actor Mohanlal in the lead role of an entrepreneur whose plans are thwarted by bureaucracy bent on extorting bribes and other pecuniary benefits. The portion shown in the clip narrates the inspection of the factory by an electrical engineer of the distribution company, prior to energization. The technical points raised by the inspector and the reply are given below. The supply is low-tension, as gathered from an earlier scene in which the engineer asks the entrepreneur to subscribe to an insurance policy of Rs. 1 Lakh (in 1993!) to increase the rated capacity of the local transformer.

(Inspector enters the factory premises)

Inspector: “Where is the diagram of the electrical circuit?”

Entrepreneur: (hesitates) “diagram….diagram….diagram is there, sir” (pointing to some other place)

Inspector: “Which are the cables used? Aren’t they armoured?”

Entrepreneur: (silent)

Inspector: (looking at the main isolator) “Where is the ISI mark? It is mandatory”

Entrepreneur: “Sir, sir, best switches from Delhi are used sir..”

Inspector: (looking at a machine) “What? The same connection for lighting and machine? Don’t you know that it should not be done?”

Entrepreneur: (folding his hands) “Sir…sir…”

Inspector: (irritated) “No, I am not satisfied. All this wiring should be changed”

(The inspector then starts moving out of the factory, at which time, the entrepreneur’s elder brother, who had a personal vendetta against the inspector intervenes and slaps him on the face asking to sign without raising unnecessary objections. He runs for his life and the scene ends here)

I first saw this movie immediately after graduation. At that time itself, I wondered whether the objections raised by the inspector are indeed valid and legal, even though the film portrays him as a venal official readily accepting bribes. Four points are involved in this issue.

a)    Armoured cables shall be used in industrial installations

b)    Electrical circuit diagram shall be readily available in an installation

c)    Lighting and power circuits shall be separated

d)    Switches conforming to BIS standards shall be used

Practicing electrical engineers know that all these points are strictly adhered to, in any industrial installation. But many people are confused as to where these conditions appear in statutes. So I made a little research on the standards and other rules to find out the primary source, from where these regulations have emerged as standard practice.

Two documents that act as guide for electrical installations in India are,

a)    IS 732: 1989, Code of Practice for Electrical Wiring Installations

b)    SP 30: 2011, National Electrical Code (NEC) 2011

It may be noted that these documents will be amended periodically and obviously, the latest amendment shall be used wherever applicable.

Now, we will see one by one as to where in these documents our four issues appear.

1. Use of armoured cables

Clause E-1.1.4.5 of IS 732:1989 states that “The circuits shall be checked whether lighting wiring in factory area is taken enclosed in conduit and conduit properly earthed, or alternatively, armoured cable wiring is used”

Moreover, Clause 6.3.1 of NEC 2011 specifies that “For power distribution from a substation or main switchboard to a number of separate buildings, use shall preferably be made of armoured cables of mineral-insulated and metal-sheathed or PVC/XLPE insulated cables“.

2. Electrical circuit diagram

Clause 7.4.3 of IS 732:1989 states that

 Diagrams, charts or tables shall be provided indicating in particular:

 a)    the type and composition of circuits (points of utilization served, number and size of conductors, type of wiring ); and

b)    the information necessary for the identification of the devices performing the functions of protection, isolation and switching, and their locations.

For simple installations the foregoing information may be given in a schedule.

3. Separate switches for lighting and power circuits

Clause 3.1.5.2 of IS 732:1989 stipulates that “Separate circuits shall be provided for parts of the installation which need to be separately controlled, in such a way that these circuits are not affected by failure of other circuits”

4. Switches with ISI mark

Clause 7.1 of IS 732:1989 states that “Compliance with Standards - All equipment fittings and accessories, materials, etc, selected for the wiring installation shall conform to the relevant Indian Standard”

Also, this is mandated by NEC as well as seen by clause 2.1, “Every item of electrical equipment used in the installation shall conform to the relevant Indian Standards, wherever available”

See? Everything the engineer demanded in the above scene was valid and mandatory!

Insulating Mats

While everyone was absorbed in the landslide technological developments taking place in communication and protection fields of Electrical Engineering, a silent revolution has taken place in rubber mats laid in front of switchboards as a means of providing safety for operators from electric shock. Gone are the days when mats looked like foam beds! The new mats are so thin, that you would wonder whether they would fly off if you switch on a man-cooler. Yet, these ultra thin mats are so effective that the industry has switched over to them. What follows below is a comparison between the old and new mats and a synopsis of their characteristics.

New Mats conforming to IS 15652:2006	Old Mats conforming to IS 5424:1969
Made of synthetic elastomer	Made of vulcanized rubber
Notified for use in India for all installations after Nov 1, 2007 through Gazette notification SO-2086	Superseded from Nov 1, 2007
High electrical insulation  resistance 10,00,000 mega ohm when measured with 500 V meggar	Comparatively far lower insulation resistance
100%  shock  proof  under  leakage  current 10 mA	Unsafe     in     case     of     leakage     of     current. Tested   from   small   electrodes   as   per   IS: 2584-1963 the value of the leakage current is 160 mA per sq. mtr.
Fire Retardant	High fire prone & fire encouraging
No effect of (a)  Transformer oil (b) Acid (c) Alkali (d)  Diesel	Affected by all these
The withstand voltage is 36kV  for one minute for 3mm mat	The withstand voltage is 15 kV for one minute
High dielectric strength 65KV for 3mm mat	Only 40 KV
Moisture proof	Absorbs moisture
Pastable type permanent fixed smooth trolley movement	Cannot be fixed permanently
Washable - Easy to clean	Not possible to clean
High tensile & elongation properties to withstand good mechanical properties.	No such properties as mat has to be removed for movement of trolleys

The voltage class and thickness of mats are as follows

New Mats conforming to IS 15652:2006		Old Mats conforming to IS 5424:1969
Thickness (mm)	Voltage class (kV)	Thickness (mm)	Voltage class (kV)
2.0	3.3	not less than 6.5 mm	not exceeding 3.3 kV
2.5	11
3.0	33
3.5	66

Spatial Clearances for Transformer Installation

Transformers are either installed indoors or outdoors. If two transformers are installed side-by-side, they shall be separated by fire-separation walls. Fire separation walls are deemed to be adequate even if oil capacity of individual transformers does not exceed 2000 litres, and total capacity of all transformers installed side by side exceeds 2000 litres^{Note 1}. Ventilation is the most important thing to be ensured in indoor installations. The area required for ventilation is also specified as 2 m² for outlet and 1 m² for inlet per 1000 kVA of transformer capacity^{Note 2}. This area is required for natural ventilation. If that much space is not physically available, fans may be used.

Vertical Clearance

Minimum clearance between the highest point of the conservator tank to the ceiling of the transformer room should be sufficient to remove the mounting on the transformer like the conservator. However, this clearance should not be less than 0.5 m ^{Note 3}.

Horizontal Clearance

Transformers shall be kept well away from walls. The minimum recommended spacing between the walls and transformer is shown in Fig 1 below.

Fig 1: Minimum horizontal clearances for transformers

 The wall indicated in the figure can also be fire separation wall. As such, there is no separate requirement for clearance from fire separation wall.

When fire safety is also taken into the picture, some more spatial clearances are to be observed. If transformer oil capacity is more than 2000 litres, the building housing the oil-filled transformer shall be separated by a distance of not less than 6 m from all other buildings ^{Note 4}. If however, a building is existing within 6 m, there shall not be any door or window opening in the substation or the adjacent building. Again, if such communication also exists, the substation shall be segregated by separating walls of 355 mm thick brick wall or 230 mm thick RCC, carried up to roof level. Doors, if any, shall be single fireproof with 2 h rating.

In the Code of Practice for Fire Safety: Electrical Installations (IS 1646), the minimum clearance specified is not in agreement with Code of Practice for Installation of Transformers (IS 10028 Part 2) when compared with Fig 1 reproduced from the latter standard. This specifies a minimum clearance of 750 mm (0.75 m) between the transformer or other apparatus and enclosing or separating walls ^{Note 5}. As is usual in the case of conflicts among provisions in standards, we may take the more stringent requirement of the two, which is, 1.25 m for transformers with enclosing walls on all four sides.

No mention of spatial clearances for transformers was found in the Indian Electricity Rules, 1956 and the Central Electricity Authority (Measures Relating to Safety and Electric Supply) Regulations, 2010.

American regulations seem to have included clear instructions on clearances as shown in this excellent article (Safety Clearance Recommendations for Transformer) on the Electrical Engineering Portal.

References

1. Code of Practice for Selection, Installation and Maintenance of Transformers, Part 2 Installation, IS 10028 (Part 2) – 1981, Reaffirmed 2001.
2. Code of Practice for Fire Safety of Buildings: Electrical Installations (Second Revision), IS 1646 – 1997, Reaffirmed 2002
3. Indian Electricity Rules, 1956
4. Central Electricity Authority (Measures Relating to Safety and Electric Supply) Regulations, 2010

Notes:

1. Clause 3.6.2 of IS 10028 Part 2 – 1981
2. Clause 7.3.1.1, ibid
3. Clause 7.3.1.5, ibid
4. Clause 7.1 of IS 1646 – 1997
5. Clause 7.4, ibid

Unused Taps of CT and Differential Relay

This is a case study of the unusual false tripping of a 110/11 kV, 15 MVA transformer in a large chemical plant. As a part of renovation, the old air-blast circuit breaker and current transformer were replaced as a retrofit. Brand new SF6 breaker and CT (both Crompton Greaves make) were installed on the 110 kV side. After commissioning, the load on the transformer was very low, normally about 1 MW as the transformer was feeding power to a section of the plant which was mothballed for a long time. When it was paralleled with two other similar transformers and loaded, it tripped on differential protection, causing production loss. Tests of insulation resistance showed no problem and after two spurious trips, it was decided to conduct a thorough study of the issue.

The details of the current transformer (CT) was 150/75/1 A, with three cores and three windings – The Class 0.5 winding for metering, Class 5P20 for protection and Class PS for differential relay. It was a multi-tap CT, with the terminal connections as shown below.

Nobody knew how the shorting link between 75A and 150A tap came about. It was assumed that since the CT came from the manufacturer with the terminals short-circuited, the commissioning staff might have let the shorting in place in order not to keep any portion of the CT winding in open circuit. It was suspected that this shorting link might be the cause of false tripping. A search on the Internet showed some examples where similar tripping of differential relay had occurred due to identical causes. But, nobody came out with a theoretical explanation or a practical illustration of the phenomenon. Those people simply declared that when they removed the shorting link, the differential relay did not trip thereafter.

So we decided to demonstrate what would happen with the shorting link in place. We conducted a primary current injection test on the CT terminals and measured the secondary current to the differential relay and in the shorting link. The readings are tabulated below.

Case A: With Shorting Link

Primary Injected Current (A)	Current to Differential Relay (A)	Current in Shorting Link (A)
30	0	0.37
60	0.02	0.65
75	0.2	0.78

 Case B: Without Shorting Link

Primary Injected Current (A)	Current to Differential Relay (A)
30	0.35
60	0.77
75	0.98

It is easily seen that practically no current flows to the differential relay when shorting link is present. The relay is set to trip at 30% of the normal current (1A), that is, 0.3 A. The second CT connected to the relay is on the transformer secondary (1000/1A) where no shorting link was present. So, when the secondary current reaches 300A, corresponding to a load of approximately 5 MVA, the differential relay, which is seeing only the secondary current will pick up and cause the transformer to trip. Only after the shorting link was removed did the CT supply the required current to the relay. The transformer was then successfully loaded.

Result: Don’t short-circuit the unused taps of CT winding if a differential relay is connected across one of the taps.

Electrical Engineering Portal

This is an excellent site for very good technical articles ranging from common issues to guides.

http://electrical-engineering-portal.com