Question { 6029 }

If the positive terminal of batteries in 125 v dc system is
connected to earth what will happen ?

Answer

Nothing wil happen, such system are adopted in
Telecommunication power supply since its inception and may
be for avoiding over voltage due to positive nature noise
added in the circuit. Always there will be less voltage
that (-) 125 Volt when any (+) volts noise is added.

Question { BARC, 10379 }

why the earth terminal in three pin top is longer than rest
of the two terminal and of bigger size?

Answer

As we know earth is connected to the body of
equiment/machine hence if earth terminal is longer then
earth will be connected to body of equipment/machine first
thereafter phase and neutral. It is bigger size because it
must not go in phase & neutral terminal by any mistake so
as to avoid any direct contact of body of machine with
phase/neutral.

Question { BARC, 6848 }

how to find out speed of single phase motor

Answer

In single phase induction motor it is 120 * Frequency/Pole.

Question { Adani, 17791 }

what will happen if u run induction motor beyond its rated speed

Answer

It will act as induction generator and it will feed power
supply from stator.

Question { 5950 }

how to calculate H.P of a moter from speed of a moter?

Answer

Power = 2 * pi * Speed *Torque/4500 Watt
Power in HP = Power in watt/746

Question { BARC, 6718 }

What happen when ct secondary are open circuited? why same
thing is not happen in ordinary transformer?

Answer

If secondary of CT is opened and as we know that primary
always flow the current and by transformaton action
secondary must flow the current but in our case secondary
is opened hence Extra high voltage is induced in terminals
of secondary which can be fatal.
In case of ordinary transformer if secondary is open
primary flows only no load current hence secondary voltage
is only rated secondary votage which is not as danger as in
the case of CT.

Question { BARC, 13200 }

difference between over current and over load protection?

Answer

Over current and over load protection might be same in
electrcal engg. point of view.

Question { TCS, 19207 }

what are advantages of dc over ac supply?

Answer

DC supply does not have skin effect, proximity effect,power
factor, inductive and capacitive reactance and hence low
loss in transmisson and distribution.

Question { CDS, 23073 }

Difference between MCC and PDB

Answer

MCC is motor control centre hence it must have back-up fuse
or back-up MCCB/MCB and starter whereas PDB is Pwer DB
which generally feed lighting and normal power load.

Question { BARC, 26914 }

What should be the earth pit resistance value?

Answer

For important installations such as Sub station and data
centre, it should be less than 1 Ohm. It can be upto 5 ohm
for residential aplication.

Question { L&T, 15416 }

single phase can run in PVC conduit. Three phase can also
run in one PVC conduit. But two phase cannot run in one PVC
conduit. WHY? reply plz....awaiting your answer.

Answer

In PVC conduit, one can run single phase, two phase and
three phase but two phases is not allowed in metalic
conduits as two phases can not cancel out magnetic field
and metalic conduit can induce voltage which can lead to
fatal accident.

Question { BARC, 12931 }

how to calculate the current carrying capacity of a
conductor. How it is related to diameter of cable or
conductor

Answer

current carrying capacity of conductor depends on many
factors,
size, shape and material of the conductor, ambient
temperature, insulation and laying method:
As area of the conductor increases current density is
decreasing hence 2.5 times per sqmm. for coer and 1.5 times
per sq.mm may be alright for smaler size conductor but at
400 sq.mm. it may be only around one time as perimeter and
area also affect the current carrying capacity.

Question { BARC, 86974 }

how to measure earth pit resistance

Answer

Earth pit resistance is measured by Earth tester, Earth
tester have four terminals P1,P2,C1,C2 i.e. Two potential &
Two Current terminals. while earth pit testing, P2 & C2 are
shorted and C1 connected to current spike(Conductive rod)
which should be drived into soil at around 30 meter
distance from earth pit and P1 is connected to potential
spike(Conductive rod) drived at around 15 mtr away from
earth pit in the direction of current spikes.By rotating
the handle of meter(in old meters) or by pressing test
button (in new type meter) one can get earth resistance.Now
Potential spikes are drived at different locations toward
earth pit and current spike side and a curve between
distance and resistance is plotted. Earth resistance will
be platue of the graph.In this meter current is passed
between earth pit and current spike and potential is
measured between potential spike and earth pit. Ratio
between Voltage measured and current passed thrugh the
earth is the resitance of earth pit.

Question { ABB, 8477 }

Why transformer ratings in KVA not in KW?

Answer

As we know,capacity of conductor carrying conductor is
fixed in a perticular atmospheric conditions.Current in
transformer or DG set or alternators depends on load power
factor, current drawn by a perticular KW machine is more if
power factor is low whereas same KW rating machine wil draw
less current if it has high power factor.Hence
Transformer/DG/Alternator can not be designed for KW but
for KVA rating.It will feed high KW with high power factor
and low KW with low power factor without damaging winding
conductor/insulatin.