EARTHING

          

        

WHAT IS EARTHING?

The main reason for doing earthing in electrical network is for the safety. When all metallic parts in electrical equipments are grounded then if the insulation inside the equipments fails there are no dangerous voltages present in the equipment case. If the live wire touches the grounded case then the circuit is effectively shorted and fuse will immediately blow. When the fuse is blown then the dangerous voltages are away.

PURPOSE OF EARTHING

SAFETY FOR HUMAN/EQUIPMENTS/BUILDINGS:-

·        To save human life from danger of electrical shock or death by blowing a fuse i.e.

·        To protect buildings, machinery & appliances under fault conditions

·        To  provide an alternative path for the fault current to flow so that it will not endanger the user

·        To ensure that all exposed conductive parts do not reach a dangerous potential.

·        To provide stable platform for operation of sensitive electronic equipments   i.e.

·        To provide safe path to dissipate lightning and short circuit currents.

·        To maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment.

VOLTAGE STABILIZATION:-

There are many sources of electricity. Every transformer can be considered a separate source. If there were not a common reference point for all these voltage sources it would be extremely difficult to calculate their relationships to each other. The earth is the most omnipresent conductive surface, and so it was adopted in the very beginnings of electrical distribution systems as a nearly universal standard for all electric systems.

OVER VOLTAGE PROTECTION:-

Lightning, line surges or unintentional contact with higher voltage lines can cause dangerously high voltages to the electrical distribution system. Earthing provides an alternative path around the electrical system to minimize damages in the System.

DIFFERENT METHOD OF EARTHING

PLATE TYPE EARTHING:-

·        Generally for plate type earthing normal Practice is to use

·        Cast iron plate of size 600 mm x600 mm x12 mm. OR

·        Galvanized iron plate of size 600 mm x600 mm x6 mm. OR

·        Copper plate of size 600 mm * 600 mm * 3.15 mm

·        Plate  burred at the depth of 8 feet in the vertical position and GI strip of size 50 mmx6 mm bolted with the plate is brought up to the ground level.

·        These types of earth pit are generally filled with alternate layer of charcoal & salt up to 4 feet from the bottom of the pit.

PIPE TYPE EARTHING:-

·         For Pipe type earthing normal practice is to use

·         GI pipe [C-class] of 75 mm diameter, 10 feet long welded with 75 mm diameter GI flange having 6 numbers of holes for the connection of earth wires and inserted in ground by auger method.

·         These types of earth pit are generally filled with alternate layer of charcoal & salt or earth reactivation compound.

DESCRIBE THE METHOD OF CONSTRUCTION OF EARTH PIT

·         Excavation on earth for a normal earth Pit size is 1.5M X 1.5M X 3.0 M.

·         Use 500 mm X 500 mm X 10 mm GI Plate or Bigger Size for more Contact of Earth and reduce Earth Resistance.

·          Make a mixture of Wood Coal Powder Salt & Sand all in equal part

·          Wood Coal Powder use as good conductor of electricity, anti corrosive, rust proves for GI Plate for long life.

·         The purpose of coal and salt is to keep wet the soil permanently.

·         The salt percolates and coal absorbs water keeping the soil wet.

·         Care should always be taken by watering the earth pits in summer so that the pit soil will be wet.

·         Coal is made of carbon which is good conductor minimizing the earth resistant.

·         Salt use as electrolyte to form conductivity between GI Plate Coal and Earth with humidity.

·         Sand has used to form porosity to cycle water & humidity around the mixture.

·         Put GI Plate (EARTH PLATE) of size 500 mm X 500 mm X 10 mm in the mid of mixture.

·         Use Double GI Strip size 30 mm X 10 mm to connect GI Plate to System Earthling.

·          It will be better to use GI Pipe of size 2.5″ diameter with a Flange on the top of GI Pipe to cover GI Strip from EARTH PLATE to Top Flange.

·         Cover Top of GI pipe with a T joint to avoid jamming of pipe with dust & mud and also use water time to time through this pipe to bottom of earth plate.

·         Maintain less than one Ohm Resistance from EARTH PIT conductor to a distance of 15 Meters around the EARTH PIT with another conductor dip on the Earth at least 500 mm deep.

·         Check Voltage between Earth Pit conductors to Neutral of Mains Supply 220V AC 50 Hz it should be less than 2.0 Volts.

THE FOLLOWING FACTORS ARE EFFECTING THE EARTH PIT

SOIL RESISTIVITY:-

·        It is the resistance of soil to the passage of electric current. The earth resistance value (ohmic value) of an earth pit depends on soil resistivity. It is the resistance of the soil to the passage of electric current.

·        It varies from soil to soil. It depends on the physical composition of the soil, moisture, dissolved salts, grain size and distribution, seasonal variation, current magnitude etc.

·        In depends on the composition of soil, Moisture content, Dissolved salts, grain size and its distribution, seasonal variation, current magnitude.

MOISTURE:-

·         Moisture has a great influence on resistivity value of soil. The resistivity of a soil can be determined by the quantity of water held by the soil and resistivity of the water itself. Conduction of electricity in soil is through water.

·         The resistance drops quickly to a more or less steady minimum value of about 15% moisture. And further increase of moisture level in soil will have little effect on soil resistivity. In many locations water table goes down in dry weather conditions. Therefore, it is essential to pour water in and around the earth pit to maintain moisture in dry weather conditions. Moisture significantly influences soil resistivity

SOIL CONDITION:-

·         Different soil conditions give different soil resistivity. Most of the soils are very poor conductors of electricity when they are completely dry. Soil resistivity is measured in ohm-meters or ohm-cm.

·         Soil plays a significant role in determining the performance of Electrode.

·         Soil with low resistivity is highly corrosive. If soil is dry then soil resistivity value will be very high.

·         If soil resistivity is high, earth resistance of electrode will also be high.

DISSOLVED SALT:-

·        Pure water is poor conductor of electricity.

·        Resistivity of soil depends on resistivity of water which in turn depends on the amount and nature of salts dissolved in it.

·        Small quantity of salts in water reduces soil resistivity by 80%. common salt is most effective in improving conductivity of soil. But it corrodes metal and hence discouraged.

  

PHYSICAL COMPOSITION:-

·        Different soil composition gives different average resistivity. Based on the type of soil, the resistivity of clay soil may be in the range of 4 – 150 ohm-meter, whereas for rocky or gravel soils, the same may be well above 1000 ohm-meter.

WEATHER CONDITION:-

 ·         Increase or decrease of moisture content determines the increase or decrease of soil resistivity.

·         Thus in dry whether resistivity will be very high and in monsoon months the resistivity will be low.

LOCATION OF EARTH PIT:-

·         The location also contributes to resistivity to a great extent. In a sloping landscape, or in a land with made up of soil, or areas which are hilly, rocky or sandy, water runs off and in dry weather conditions water table goes down very fast. In such situation Back fill Compound will not be able to attract moisture, as the soil around the pit would be dry. The earth pits located in such areas must be watered at frequent intervals, particularly during dry weather conditions.

·         Though back fill compound retains moisture under normal conditions, it gives off moisture during dry weather to the dry soil around the electrode, and in the process loses moisture over a period of time. Therefore, choose a site that is naturally not well drained.

EFFECT OF CURRENT MAGNITUDE:-

·         Soil resistivity in the vicinity of ground electrode may be affected by current flowing from the electrode into the surrounding soil.

·         The thermal characteristics and the moisture content of the soil will determine if a current of a given magnitude and duration will cause significant drying and thus increase the effect of soil resistivity

  

 EFFECT OF GRAIN SIZE AND DISTRIBUTION:-

·         Grain size, its distribution and closeness of packing are also contributory factors, since they control the manner in which the moisture is held in the soil.

·         Effect of seasonal variation on soil resistivity: Increase or decrease of moisture content in soil determines decrease or increase of soil resistivity. Thus in dry weather resistivity will be very high and during rainy season the resistivity will be low.

 OBSTRUCTIONS:-

·        The soil may look good on the surface but there may be obstructions below a few feet like virgin rock. In that event resistivity will be affected. Obstructions like concrete structure near about the pits will affect resistivity. If the earth pits are close by, the resistance value will be high.

AREA AVAILABLE:-

·        Single electrode rod or strip or plate will not achieve the desired resistance alone.

·         If a number of electrodes could be installed and interconnected the desired resistance could be achieved. The distance between the electrodes must be equal to the driven depth to avoid overlapping of area of influence. Each electrode, therefore, must be outside the resistance area of the other.

CURRENT MAGNITUDE:-

·        A current of significant magnitude and duration will cause significant drying condition in soil and thus increase the soil resistivity.

MEASUREMENT OF EARTH RESISTANCE BY USING EARTH TESTER OR EARTH MEGGER:-

 ·         For measuring soil resistivity Earth Tester is used. It is also called the “MEGGER”.

·         It has a voltage source, a meter to measure Resistance in ohms, switches to change instrument range, Wires to connect terminal to Earth Electrode and Spikes.

·         It is measured by using Four Terminal Earth Tester Instrument. The terminals are connected by wires as in illustration.

·         P=Potential Spike and C=Current Spike. The distance between the spikes may be 1M, 2M, 5M, 10M, 35M, and 50M.

·         All spikes are equidistant and in straight line to maintain electrical continuity.  Take measurement in different directions.

·         Soil resistivity =2πLR.

·         R= Value of Earth resistance in ohm.

·         Distance between the spikes in cm.

·         π  =  3.14

·         P = Earth resistivity ohm-cm.

·         Earth resistance value is directly proportional to Soil resistivity value

THREE POINT METHOD OF MEASURING EARTH RESISTANCE:-

·         In this method earth tester terminal C1 & P1 are shorted to each other and connected to the earth electrode (pipe) under test.

·         Terminals P2 & C2 are connected to the two separate spikes driven in earth.  These two spikes are kept in same line at the distance of 25 meters and 50 meters due to which there will not be mutual interference in the field of individual spikes.

·         If we rotate generator handle with specific speed we get directly earth resistance on scale.

·         Spike length in the earth should not be more than 1/20th distance between two spikes.

·         Resistance must be verified by increasing or decreasing the distance between the tester electrode and the spikes by 5 meter. Normally, the length of wires should be 10 and 15 Meter or in proportion of 62% of ‘D’.

·         Suppose, the distance of Current Spike from Earth Electrode D = 60 ft, Then, distance of Potential Spike would be 62 % of D = 0.62D i.e.  0.62 x 60 ft = 37 ft.

FOUR POINT METHOD OF MEASURING EARTH RESISTANCE:-

·        In this method 4 spikes are driven in earth in same line at the equal distance.  Outer two spikes are connected to C1 & C2 terminals of earth tester.  Similarly inner two spikes are connected to P1 & P2 terminals.  Now if we rotate generator handle with specific speed, we get earth resistance value of that place.

·        In this method error due to polarization effect is eliminated and earth tester can be operated directly on A.C.

PIPE EARTHING VS PLATE EARTHING:-

 ·         Suppose Copper Plate having of size 1.2m x 1.2m x 3.15mm thick. soil resistivity of 100 ohm-m,

·         The resistance of Plate electrode to earth (R)=( r/A)X under root(π/A) = (100/2.88)X(3.14/2.88)=36.27 ohm

·         Now, consider a GI Pipe Electrode of 50 mm Diameter and 3 m Long. soil resistivity of 100 Ohm-m,

·         The resistance of Pipe electrode to earth (R) = (100r/2πL) X loge (4L/d) = (100X100/2X3.14X300) X loge (4X300/5) =29.09 Ohm.

·         From the above calculation the GI Pipe electrode offers a much lesser resistance than even a copper plate electrode.

·         As per IS 3043 Pipe, rod or strip has a much lower resistance than a plate of equal surface area.

GI EARTHING VS COPPER EARTHING:-

·         As per IS 3043, the resistance of Plate electrode to earth (R) = (r/A) X under root(P/A).

·         Where r = Resistivity of Soil Ohm-meter.

·         A=Area of Earthing Plate m3.

·         The resistance of Pipe electrode to earth (R) = (100r/2πL) X loge (4L/d).

·         Where L= Length of Pipe/Rod in cm

·         d=Diameter of Pipe/Rod in cm.

·         The resistivity of the soil and the physical dimensions of the electrode play important role of resistance of Rod with earth.

·         The material resistivity is not considered important role in earth resistivity.

·         Any material of given dimensions would offer the same resistance to earth. Except the sizing and number of the earthing conductor or the protective conductor.

LENGTH OF PIPE ELECTRODE AND EARTH PIT:-

·         The resistance to earth of a pipe or plate electrode reduces rapidly within the first few feet from ground (mostly 2 to 3 meter) but after that soil resistivity is mostly uniform.

·        After about 4 meter depth, there is no appreciable change in resistance to earth of the electrode. Except a number of rods in parallel are to be preferred to a single long rod.

AMOUNT OF SALT AND CHARCOAL (MORE THAN 8KG):-

·         To reduce soil resistivity, it is necessary to dissolve in the moisture particle in the Soil.

·         Some substance like Salt/Charcoal is highly conductive in water solution but the additive substance would reduce the resistivity of the soil, only when it is dissolved in the moisture in the soil after that additional quantity does not serve the Purpose.

·         5% moisture in Salt reduces earth resistivity rapidly and further increase in salt content will give a very little decrease in soil resistivity.

·         The salt content is expressed in percent by weight of the moisture content in the soil. Considering 1M3 of Soil, the moisture content at 10 percent will be about 144 kg. (10 percent of 1440 kg). The salt content shall be 5% of this (i.e.) 5% of 144kg, that is, about 7.2kg.

AMOUNT OF WATER POURING:-

·          Moisture content is one of the controlling factors of earth resistivity.

·         Above 20 % of moisture content, the resistivity is very little affected. But below 20% the resistivity increases rapidly with the decrease in moisture content.

·         If the moisture content is already above 20% there is no point in adding quantity of water into the earth pit, except perhaps wasting an important and scarce national resource like water.

LENGTH VS DIAMETER OF EARTH ELECTRODE:-

·         Apart from considerations of mechanical strength, there is little advantage to be gained from increasing the earth electrode diameter with the object in mind of increasing surface area in contact with the soil.

·         The usual practice is to select a diameter of earth electrode, which will have enough strength to enable it to be driven into the particular soil conditions without bending or splitting. Large diameter electrode may be more difficult to drive than smaller diameter electrode.

·         The depth to which an earth electrode is driven has much more influence on its electrical resistance characteristics than has its diameter.

MAXIMUM ALLOWABLE EARTH RESISTANCE:-

·         Major power station= 0.5 Ohm.

·         Major Sub-stations= 1.0 Ohm

·         Minor Sub-station = 2 Ohm

·         Neutral Bushing. =2 Ohm

·         Service connection = 4 Ohm

·         Medium Voltage Network =2 Ohm

·         L.T.Lightening Arrestor= 4 Ohm

·         L.T.Pole= 5 Ohm

·         H.T.Pole =10 Ohm

·         Tower =20-30 Ohm

METHOD OF MINIMIZING EARTH RESISTANCE:-

·         Remove Oxidation on joints and joints should be tightened.

·         Poured sufficient water in earth electrode.

·         Used bigger size of Earth Electrode.

·         Electrodes should be connected in parallel.

·         Earth pit of more depth & width- breadth should be made.

CABLE GLANDS

Q: What is Cable Gland?

A:A device designed to permit the entry of cable in to electrical equipment which provides sealing, retention and ear thing, bonding, grounding, insulation, strain relief or combination of all these. Gland should maintain overall integrity of enclosure in to which it is to be fitted.

    Q: How to select the Cable Gland?

A: Gland should be selected on following Points

1.     Type of Cable

2.     Gland Size

3.     Entry Type/Thread Specification of application

4.     Ingress Protection required.

5.     Material

   

 Q: What are the type of Cable and how the Gland to be used?

A: Unarmored: Unarmored Cable will require outer seal within Gland to not only Provide ingress protection but also degree of retention.

• Armored: Gland that required clamping mechanism to terminate the armored both mechanically and electrically.
• The Gland will usually be required to provide ingress protection by sealing outer sheath and retention by clamping amour.

Q: what are the types of Gland?

1.     Brass Indoor Type Gland

2.     Brass Outdoor Type Gland

3.     Brass Straitening Unarmored Cable Gland

4.     Brass Weather Proof Gland

5.     PG Threaded Gland:

6.     Industrial Type Gland

 Q: Describe the types of Gland?

A1)    Brass Indoor Type Gland

• This Gland is quite handy in use with various types of cable whether plastic, rubberized, metal or any other.
• Application: Dry indoor, for use with all type of SWA cables, plastic or rubber sheathed cable.
• Brass indoor gland suitable for single wire armored, plastic or rubber sheathed cable. Recommended to use with shroud for additional ingress protection.
• Cable Type: Steel Wire Amour.
• Amour Clamping: Two Part Amour Lock.

A2)      Brass Outdoor Type Gland

 This come in stunning high quality material for use in outdoor or indoor application with various types of cables sheathed or unsheathed.

• Brass indoor and outdoor gland popularly used with single wire armored.
• Plastic or rubber sheathed cable. Terminates and secure cable armoring and outer seal grips sheath of cable thus ensuring mechanical strength and earth continuity.
• CW brass glands are also supplied with integral earth facilities.
• Recommended to use PVC shroud for additional ingress protection

• Application: 

• a) Outdoor or indoor, for use with all type of SWA cables, plastic or rubber sheathed cable.
• b) Most suitable for SWA, plastic of rubber (Elastomeric) sheathed cables.
• c) Used in dry indoor conditions.
• d) No loose parts and easy to install.
• e)Save times & money.

• Gland size: 20 mm to 75 mm (S & L)
• Accessories :Earth Tag, PVC Shroud, Neo prime Rubber & LSF Rubber, PVC Washer, Brass Lock Nut.
• Cable Type: Wire Braid Armor.
• Armor Clamping: Three Parts (With Lock Nut).

A3) Brass Straitening Unarmored Cable Gland

 Nickel plated or natural brass A2 type cable glands are used with variety of unarmored or rubber sheathed cables.

• Brass indoor and outdoor cable gland suitable for all types of unarmored cables, plastic or rubber sheathed cables.

• Application:

1.     For use with unarmored elastomeric and plastic insulated cables. 

2.     Indoor & Outdoor whenever it is required to provide sealing on cable outer sheath.

• Size  : Metric – 20mm to 75mm (S/L)
• Accessories: Earth Tag, PVC Shroud, Neo prime Rubber & LSF Rubber, PVC Washer, Brass Lock Nut.
• Cable Type : Unarmored

A4)    Brass Weather Proof Gland

Unlike other types of cable glands, This type cable gland is used precisely with single armored various types of swa cables whether plastic or rubber sheathed ones. this type cable gland is known for its uninterrupted services once the gland is fixed to the desired wires and wire components.

• Suitable for SWA or rubber sheathed cables.
• Outer seal grips bedding layer of cable for use in most climatic conditions.
• Weather proof and water proof.
• Design has separate armor lock rings. Can be supplied with integral earth facility.
• Gland size: 20 mm to 75 mm (S & L)

• Application

1.     Outdoor or indoor, for use with single armored, all type of SWA cable, plastic or rubber sheathed cable.

2.     E1W Gland is Weatherproof & Waterproof Cable Gland

• Cable Type :  Steel Wire Armour
• Armour Clamping: Three Part Armour Lock
• Sealing Technique: Compression & Displacement Type
• Sealing Area(s): Inner & Outer Sheath

A5)    PG Threaded Gland:

Nickel chrome plated PG threaded cable gland is a custom made threaded gland to meet the needs from the meet industries. Apart from the round headed PG threaded cable gland, we also offer hexagonal gland or any other like spherical rectangular or any other dimensional PG threaded cable gland as per the specification of the customer.

A6)  Industrial Cable Gland

• Brass gland suitable for wire braid armored, plastic or rubber sheathed cable. Terminates and secure cable armoring and outer seal grips sheath of cable thus ensuring mechanical strength and earth continuity.
•  Recommended to use PVC shroud for additional ingress protection
• Cable Type: Wire Braid Armour
• Armour Clamping : Three Part (With Lock Nut)
• Sealing Technique: Compression Type.
• Brass gland suitable for steel tape armored, plastic or rubber sheathed cables. Terminates and secure cable armoring and outer seal grips sheath of cable thus ensuring mechanical strength and earth continuity.
• Recommended to use PVC shroud for additional ingress protection
• Cable Type : Steel Tape Armour
• Armour Clamping : Three Part (With Lock Nut)
• Sealing Technique: Compression Type.

What is difference between Single Compression and Double Compression?

• Double compression glands provide extra support to the heavy armored cables entering or exiting the panel while single compression glands are used for light armored cables.
• Normal Cable Gland is also called Single Compression Cable Gland. As the name suggests, while you tighten the gland, the grip or compression is effected only at one p [lace (i.e.) at the cable armour only. There is scope for moisture and corrosive vapour to enter the gland and thus into the cable.
• Whereas in Double-Compression Gland, the compression happens both at the cable armour as well as at the inner sheath. This is sort of two sealing. Hence, chances of moisture or vapour entry are minimized. Hence these glands are also known as Weather-proof cable glands or Flame-proof cable glands.
• The basic difference between single and double compression

Q: what are the parts of Double compression Gland?
  
  A:   Parts of Double comp

• Gland body
• Gland body Nut
• Cone
• Cone Ring
• Neoprene Rubber seal.
• Rubber Washer
• Check Nut.

Q: what are the parts of Double compression Gland?
  
 A:    Single Comp Parts

• Gland body
• Gland body Nut
• Neoprene Rubber seal.
• Rubber Washer
• Check Nut
• Flat washer
• The Basic difference between Single and Double Comp is in Single comp there no cone and cone ring.
• The mechanical support for the cable is only Neoprene rubber seal, When you tighten the cable.
• In double camp gland the mechanical support to the cable only cone and cone ring. When we are doing glanding the cable armor sits on the cone and cone ring act as a lock for armor.
• Single compression and double compression glands are used on the basis of area classification. Those who are affiliated with oil and gas sector they will easily understand about area classification.
• In zone 0 where the presence of hydrocarbon is obvious (IIC) double compression gland is used because the flame path in case of double compression gland is much more than in case of single compression gland.
• The logic behind this is that if there is any explosion inside the terminal box of the motor no flame should be able to come out through the cable gland in order to prevent fire hazards but where there is no presence of hydrocarbons i.e. no danger of fire hazards (IIA/ IIB) single compression glands are used.
• It has nothing to do with mechanical strength. Even in case of lighting fixtures used in IIC zone double compression glands are used.

ELECTRICAL ENGINEERING QUESTION AND ANSWERS

Q: What is vaccum currcuit breaker.define with cause and where be use it Device?

A: A breaker is normally used to break a circuit. While breaking the circuit, the contact terminals will be separated. At the time of separation an air gap is formed in between the terminals. Due to existing current flow the air in the gap is ionized and results in the arc. Various mediums are used to quench this arc in respective CB's. But in VCB the medium is vaccum gas. Since the air in the CB is having vaccum pressure the arc formation is interrupted. VCB's can be used up to 11kv.

Q: What is SF6 Circuit Breaker?

A: SF6 is Sulpher Hexa Flouride gas..this gas is used as arc quenching medium in a Circuit breaker means SF6 CB.

Q: What is the difference between MCB & MCCB, Where it can be used?

A: MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit. MCCB molded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A.

Q: Why ELCB can't work if N input of ELCB do not connect to ground?

A: ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that much current will have to return neutral so resultant current is zero. Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded, fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.

Q: What is use of lockout relay in HT voltage?

A: A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by the same electrical source as the control power and is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.

Q: What is reverse power relay?

A: Reverse Power flow relay are used in generating station’s protection. A generating station is supposed to feed power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

Q: What is difference between fuse and breaker?

A: Fuses are burned at the time of over current flows in the circuit but breakers are just open (not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple numbers of times.

Q: How tube light circuit is connected and how it works?

A: A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided, the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000volts. This volt will capable of to break the electrons inside the tube to make electron flow. Once the current passes through the tube the starter circuit will be out of part. Now there is no change of supply causes choke voltage normalized and act as minimize the current.

Q: Why when birds sit on transmission lines or current wires doesn't get shock?

A: It’s true that if birds touch the single one line (phase or neutral) they don't get electrical shock... if birds touch 2 lines than the circuit is closed and they get electrical shock. so if a human touch single one line (phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we standing is like line (ground bed - like neutral) and in the most of electric lines the neutral is grounded. so that means that human who touch the line closes the circuit between phase and neutral.

Q: Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, pedestal fan, bracket fan etc) which are find in the houses?

A: Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

Q: What is an exciter and how does it work?

A: There are two types of exciters, static exciter and rotary exciter. Purpose of exciter is to supply the excitation dc voltage to the fixed poles of generator.Rotory exciter is an additional small generator mounted on the shaft of main generator. If it is dc generator, it will supply dc to the rotory poles through slip ring and brushes (conventional alternator). if it is an ac exciter, out put of ac exciter is rectified by rotating diodes and supply dc to main fixed poles.AC exciter is the ac generator whose field winding are stationary and armature rotates. Initial voltage is built up by residual magnetism. It gives the starting torque to the generator.
 

Q: What happens if we connect a capacitor to a generator load?

A: Connecting a capacitor across a generator always improves powerfactor, but it will help depends up on the engine capacity of the alternator, other wise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf. Secondly, don't connect a capacitor across an alternator while it is picking up or without any other load.

Q: Explain the working principal of the circuit breaker?

A: Circuit Breaker is one which makes or breaks the circuit. It has two contacts namely fixed contact & moving contact. Under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty conditions (when current exceeds the rated value) an arc is produced between the fixed & moving contacts & thereby it forms the open circuit Arc is extinguished by the Arc Quenching media like air, oil, vaccum etc.

Q: What is CT and PT? Where it is used?

A: The term C.T means current transformer, and the term P.T means potential transformer. In circuit where measurements of high voltage and high current is involved they are used there. Particularly when a measuring device like voltmeter or ammeter is not able to measure such high value of quantity because of large value of torque due to such high value it can damage the measuring device. So CT and PT are introduced in the circuits. They work on the same principle of transformer, which is based on linkage of electromagnetic flux produced by primary with secondary. They work on the ratio to they are designed.E.g if CT’s of ratio 5000\5A and it has to measure secondary current of 8000A.then ANS=8000*5\5000=8Aand this result will be given to ammeter and after measuring 8A we can calculate the primary current. Same system of the operation followed in  PT but it is for measuring voltage.

Q: what is meant by insulation voltage in cables? Explain it?

A: It is the property of a cable by virtue of it can withstand the applied voltage without rupturing it is known as insulation level of the cable.

Q: Where should the lighting arrestor be placed in distribution lines?

A: Near distribution transformers and out going feeders of 11kv and incoming feeder of 33kv and near power transformers in sub-stations.

Q: Why Delta Star Transformers are used for Lighting Loads?

A: For lighting loads, neutral conductor is must and hence the secondary must be star winding.The lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads.

Q: How electrical power is generated by an A.C Generator? 

A:For the generation of elect power we need a prime mover which supplies mechanical power input to the alternator, can be steam turbines or hydro turbines .When poles of the rotor moves under the armature conductors which are placed on the stator ,field flux cut the armature conductor ,therefore voltage is generated and is of sinusoidal in nature...due to polarity change of rotor poles(i.e.) N-S-N-S.

Q: When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?
 

A: No, We can't sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage (As most of the loads are non-linear in nature).So, the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay means for sensing any internal short circuit, over load condition, earth fault there by reducing the system failure & risk of fire. So for the better protection of system. It should have both over voltage & over current relay.

Q: What is the power factor of an alternator at no load?

A: At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.

Q: In three pin plug 6 Amp. 220v AC rating. Why earth pin diameter is higher than other two pin? What its purpose? 

A: Because Current flow in the conductor is inversely proportional to the conductor diameter. So if any short circuits occur in the system first high currents bypassed in the Earth link terminal. ( R=Pl/a area of the conductor increases resistance value decreases) 

Q: Why should be the frequency 50 Hz 60Hz only why not others like 42, 90,58,35 or any thing, why should we maintain the frequency constant if so why it is only 50 Hz 60Hz?

A: We can have the frequency at any frequency you like, but than you must also make your own motors, high voltage transformers or any other equipment you want to use. We maintain the frequency at 50 Hz or 60 Hz because the world maintains a standard at 50 /60 Hz and the equipments are made to operate at these frequency

Q: Why we use AC system in India why not DC? 

A: The output of power stations comes from a rotary turbine, which by its nature is AC and therefore requires no power electronics to convert to DC. Secondly it is much easier to change the voltage of AC electricity for transmission and distribution. Thirdly the cost of plant associated with AC transmission (Circuit breakers, transformers etc) is much lower than the equivalent of DC transmission AC transmission provides a number of technical advantages. When a fault on the network occurs, a large fault current occurs. In an AC system this becomes much easier to interrupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

Q: In a Tap changing transformer where is the tap connected, is it connected in the primary side or secondary side?

A: Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current.

Q: Capacitor is load free component but why ampere meter shows current when capacitor bank’s breakers close?

A: As we know that Electrical is having two type of load, Active and Reactive .Capacitor is a reactive load which is not considering as a load & its factor is Isin@ .Meter is design based on Current RMS value because of it meter is showing the current RMS value.

Q: What are the types of power in electrical power?

A: There are normally three types of power are counted in electrical power. They are,

· Apparent power

· Active power

· Reactive power

Q: Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

A: Link is provided at a Neutral common point in the circuit from which various connections are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e. load) only. So if any malfunction happen the fuse connected in the particular control circuit alone will blow off.

Q: Why use the VCB at High Transmission System? Why can't we use ACB?

A: Actually the thing is vaccum has high arc quenching property compare to air because in VCB, the die electric strengths equal to 8 times of air. That y always vaccum used as in HT breaker and air used as in LT.

Q: What are the power losses in rotating electrical machines?

A: Power losses in rotating electrical machines are Copper losses, core losses, mechanical losses and stray losses.

 Q: What is the difference between delta-delta, delta-star transformer?

A: Delta-delta transformer is used at generating station or a receiving station for Change of Voltage (i.e.) generally it is used where the Voltage is high & Current is low. Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena

 Q: What is the difference between generator and alternator?

A: Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into DC current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.

Q: Why transformer ratings are in KVA? 

A: Since the power factor of transformer is dependent on load we only define VA rating and does not include power factor .In case of motors, power factor depend on construction and hence rating of motors is in Kilo Watts and include power factor.

Q: Which type of motor is used in trains, what is the rating of supply used explain Working principal?

A: DC series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500v dc.

Q: What are the advantages of star-delta starter with induction motor?

A :( 1) the main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times of current of Direct online starting. (2). Hence the starting current is reduced; the voltage drops during the starting of motor in systems are reduced.

Q: What are the transformer losses?

A: Transformer losses have two sources-copper loss and magnetic loss. Copper losses are caused by the resistance of the wire (I2R). Magnetic losses are caused by eddy currents and hysterics in the core. Copper loss is a constant after the coil has been wound and therefore a measurable loss. Hysterics loss is constant for a particular voltage and current. Eddy-current loss, however, is different for each frequency passed through the transformer.

Q: what is power factor? Whether it should be high or low? Why?

A: Power factor should be high in order to get smooth operation of the system. Low power factor means losses will be more. It is the ratio of true power to apparent power. It has to be ideally 1. if it is too low then cable over heating & equipment overloading will occur. if it is greater than 1 then load will act as capacitor and starts feeding the source and will cause tripping.(if pf is poor ex: 0.17 to meet actual power load has to draw more current(V constant),result in more losses if pf is good ex: 0.95 to meet actual power load has to draw less current(V constant),result in less losses).

Q: How many types of cooling system it transformers?

A: 1. ONAN (oil natural, air natural)

2. ONAF (oil natural, air forced)

3. OFAF (oil forced, air forced)

4. ODWF (oil direct, water forced)

5. OFAN (oil forced, air forced)

Q: What is the difference between surge arrestor and lightning arrestor?

A: Lighting Arrestor is installed outside and the effect of lightning is grounded, where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

Q: What is Automatic Voltage Regulator (AVR)?

A: AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.

Q: Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?

A: DC Series motor has high starting torque. We can not start the Induction motor and Synchronous motors on load, but can not start the DC series motor without load.