Electrical Engineering-Technical Q&A (Part.1)

Electrical Q&A Part-1

1)     Why ELCB cannot work if Neutral input of ELCB does not connect to ground?

§  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 same 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.

2)     What is the difference between MCB & MCCB, Where it can be used?

§  MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit.

§  Normally it is used where normal current is less than 100A.

§  MCCB moulded 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.

3)     Why in a three pin plug the earth pin is thicker and longer than the other pins?

§  It depends upon R=ρL/A where area (A) is inversely proportional to resistance (R), so if  area (A) increases, R decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the The First to make the connection and last to disconnect should be earth Pin. This assures Safety for the person who uses the electrical instrument.

4)     Why Delta Star Transformers are used for Lighting Loads?

§  For lighting loads, neutral conductor is must and hence the secondary must be star winding and this 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.

5)      What are the advantages of star-delta starter with induction motor?

§  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  Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced.

6)     What is meant by regenerative braking?

§  When the supply is cut off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load (resistor) across the armature winding and the motor should have maintained continuous field supply so that back e.m.f voltage is made to apply across the resistor and due to load the motor stops quickly. This type of breaking is called as “Regenerative Breaking”.

7)     When voltage increases then current also increases then why we need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?

§  No. We cannot 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 meant for sensing any internal short circuit, over load condition, earth fault thereby reducing the system failure & risk of fire. So, for a better protection of the system. It should have both over voltage & over current relay.

8)     If one lamp connects between two phases it will glow or not?

§  If the voltage between the two phases is equal to the lamp voltage then the lamp will glow.

§  When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.

9)     What are HRC fuses and where it is used?

§  HRC stand for “high rupturing capacity” fuse and it is used in distribution system for electrical transformers

10)  Mention the methods for starting an induction motor?

§  The different methods of starting an induction motor

§  DOL: direct online starter

§  Star delta starter

§  Auto transformer starter

§  Resistance starter

§  Series reactor starter

11)  What is the difference between earth resistance and earth electrode resistance?

§  Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition:

§  Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away.

§  The resistance of the electrode has the following components:
(A) the resistance of the metal and that of the connection to it.
(B) The contact resistance of the surrounding earth to the electrode.

12)  Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?

§  4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation.

§  For example: A pressure instrument gives output 4mA to indicate 0 psi  up to 20 mA to indicate 100 psi or full scale. Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.

13)  Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow bright and why?

§  Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the Bulb (P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so 40W bulb will glow brighter.

14)  What happen if we give 220 volts dc supply to bulb or tube light?

§  Bulbs or devices for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element

15)  What is meant by knee point voltage?

§  Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.

16)  What is reverse power relay?

§  Reverse Power flow relay are used in generating stations’ 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.

17)  What will happen if DC supply is given on the primary of a transformer?

§  Mainly transformer has high inductance and low resistance. In case of DC supply there is no inductance, only resistance will act in the electrical circuit. So high electrical current will flow through primary side of the transformer. So for this reason coil and insulation will burn out

§  When AC current flow to primary winding it induced alternating flux which also link to secondary winding so secondary current flow in secondary winding according to primary current.Secondary current also induced emf (Back emf) in secondary winding which oppose induced emf of primary winding and thus control primary current also.

§  If DC current apply to Primary winding than alternating flux is not produced so no secondary emf induced in secondary winding  so primary current may goes high and burn transformer winding.

18)  Different between megger and contact resistance meter?

§  Megger used to measure cable resistance, conductor continuity, phase identification where as contact resistance meter used to measure low resistance like relays, contactors.

19)  When we connect the capacitor bank in series?

§  We connect capacitor bank in series to improve the voltage profile at the load end in transmission line there is considerable voltage drop along the transmission line due to impedance of the line. so in order to bring the voltage at the load terminals within its limits i.e. (+ or – %6 )of the rated terminal voltage the capacitor bank is used in series

20)  What is Diversity factor in electrical installations?

§  Diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Diversity factor is usually more than one.

21)  Why humming sound occurred in HT transmission line?

§  This sound is coming due to ionization (breakdown of air into charged particles) of air around transmission conductor. This effect is called as Corona effect, and it is considered as power loss.

22)  Why frequency is 50 Hz only & why should we maintain the frequency constant?

§  We can have the frequency at any frequency we like, but then we must also make our own motors, transformers or any other equipment we 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.

23)  If we give 2334 A, 540V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage=11 KV?

§  As we know the Voltage & current relation for transformer-V1/V2 = I2/I1
We Know, VI= 540 V; V2=11KV or 11000 V; I1= 2334 Amps.
By putting these value on Relation-
540/11000= I2/2334
So,I2 = 114.5 Amps

24)  What are the points to be considered for MCB (miniature circuit breaker selection)?

§  I(L)x1.25=I(MAX) maximum current. Mcb specification is done on maximum current flow in circuit.

25)  How can we start-up the 40w tube light with 230v AC/DC without using any choke/Coil?

§  It is possible by means of Electronic choke. Otherwise it’s not possible to ionize the particles in tube. Light, with normal voltage.

26)  What is “pu” in electrical engineering?

§  Pu stands for per unit and this will be used in power system single line diagram there it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis. Those values are called as pu values. (p.u=actual value/base value).

27)  Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

§  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 happens the fuse connected in the particular control circuit alone will blow off.

§  If Fuse is provided in Neutral and if it is blowout and at the same time Supply is on than due to open or break Neutral Voltage is increase and equipment may be damage.

28)  If 200w, 100 w and 60 w lamps connected in series with 230V AC , which lamp glow brighter? Each lamp voltage rating is 230V.

§  Each bulb when independently working will have currents (W/V= I)

§  For 200 Watt Bulb current (I200) =200/230=0.8696 A

§  For 100 Watt Bulb current (I100) =100/230=0.4348 A

§  For 60 Watt Bulb current (I60) =60/230=0.2609 A

§  Resistance of each bulb filament is (V/I = R)

§  For 200 Watt Bulb R200= 230/0.8696= 264.5 ohms

§  For 100 Watt Bulb R100= 230/0.4348 = 528.98 ohms and

§  For 60 Watt Bulb R60= 230/0.2609=881.6 ohms respectively

§  Now, when in series, current flowing in all bulbs will be same. The energy released will be I²R

§  Thus, light output will be highest where resistance is highest. Thus, 60 watt bulb will be brightest.

§  The 60W lamp as it has highest resistance & minimum current requirement.

§  Highest voltage drop across it X I [which is common for all lamps] =s highest power.

§  Note to remember:

§  Lowest power-lamp has highest element resistance.

§  And highest resistance will drop highest voltage drop across it in a Series circuit

§  And highest resistance in a parallel circuit will pass minimum current through it. So minimum power dissipated across it as min current X equal Voltage across =s min power dissipation

29)  How to check Capacitor with use of Multi meter.

§  Most troubles with Capacitors either open or short.

§  An ohmmeter (multi meter) is good enough. A shorted Capacitor will clearly show very low resistance. A open Capacitor will not show any movement on ohmmeter.

§  A good capacitor will show low resistance initially, and resistance gradually increases. This shows that Capacitor is not bad. By shorting the two ends of Capacitor (charged by ohmmeter) momentarily can give a weak spark. To know the value and other parameters, you need better instruments

30)  What is the difference between Electronic regulator and ordinary rheostat regulator for fans?

§  The difference between the electronic and ordinary regulator is that in electronic regulator power losses are less because as we decrease the speed the electronic regulator give the power needed for that particular speed .But in case of ordinary rheostat type regulator the power wastage is same for every speed and no power is saved. In electronic regulator triac is employed for speed control. by varying the firing angle speed is controlled but in rheostat control resistance is decreased by steps to achieve speed control.

31)  What will happen when power factor is leading in distribution of power?

§  If there is high power factor, i.e if the power factor is close to one:

§  Losses in form of heat will be reduced,

§  Cable becomes less bulky and easy to carry, and very cheap to afford.

§  It also reduces over heating of transformers.

32)  What the main difference between UPS & inverter?

§  Uninterrupted power supply is mainly use for short time. Means according to ups VA it gives backup. Ups is also two types: on line and offline. Online ups having high volt and amp for long time backup with high dc voltage. But ups start with 12v dc with 7 amps. but inverter is start with 12v,24,dc to 36v dc and 120amp to 180amp battery with long time backup

33)  Which type of A.C motor is used in the fan?

§  It is Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

34)  What is the difference between synchronous generator and asynchronous generator?

§  In simple, synchronous generator supplies’ both active and reactive power but asynchronous generator (induction generator) supply’s only active power and observe reactive power for magnetizing. This type of generators is used in windmills.

35)  What is the Polarization index value?

§  Its ratio between insulation resistance (IR)i.e meager value for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5 .

36)  What is Automatic Voltage regulator (AVR)?

§  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.

37)  Difference between a four point starter and three point starters?

§  The shunt connection in four point starter is provided separately from the line where as in three point starter it is connected with line which is the drawback in three point starter

38)  What happens if we connect a capacitor to a generator load?

§  Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, otherwise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf.

§   Don’t connect a capacitor across an alternator while it is picking up or without any other load

39)  Why the capacitors work on ac only?

§  Generally capacitor gives infinite resistance to dc components (i.e., block the dc components). It allows the ac components to pass through.

40)  Why the up to dia 70mm² live conductor, the earth cable must be same size but above dia 70mm² live conductor the earth conductor need to be only dia 70mm²?

§  The current carrying capacity of a cable refers to it carrying a continuous load.

§  An earth cable normally carries no load, and under fault conditions will carry a significant instantaneous current but only for a short time most Regulations define 0.1 to 5 sec before the fuse or breaker trips. Its size therefore is defined by different calculating parameters.

§  The magnitude of earth fault current depends on:

§  (a) the external earth loop impedance of the installation (i.e. beyond the supply terminals)

§  (b) the impedance of the active conductor in fault

§  (c) the impedance of the earth cable.

§  i.e. Fault current = voltage / a + b + c

§  Now when the active conductor (b) is small, its impedance is much more than (a), so the earth (c) cable is sized to match. As the active conductor gets bigger, its impedance drops significantly below that of the external earth loop impedance (a); when It is quite large its impedance can be ignored. At this point there is no merit in increasing the earth cable size

§  i.e. Fault current = voltage / a + c

§  (c) is also very small so the fault current peaks out.

§  The neutral conductor is a separate issue. It is defined as an active conductor and therefore must be sized for continuous full load. In a 3-phase system,

§  If balanced, no neutral current flows. It used to be common practice to install reduced neutral supplies, and cables are available with say half-size neutrals (remember a neutral is always necessary to provide single phase voltages). However the increasing use of non-linear loads which produce harmonics has made this practice dangerous, so for example the current in some standard require full size neutrals. Indeed, in big UPS installations I install double neutrals and earths for this reason.