Question { 11480 }

Differance between ELCB and RCCB?

Answer

ELCB is actually voltage operated ie the leaked voltage difference between the faulty equipment's earth body and the earth electrode in the ground.If it exceeds about 40 volts it would trigger the ELCB to trip.If the cable to the earth electrode in the ground is broken or earth electrode is of very high resistance the ELCB will not trip even if there is a big voltage leak.So this is no good.
RCCB is actually seeing the difference in the current flowing in and out of the circuit.If the current that flows into a circuit and comes out is the same than no leak had taken place.If there is a leakage than the current that flows out of the circuit will be less.That means some current had flowed out to the earth or some where,than there is a fault the sensing circuit will detect the difference and trigger the RCCB to trip.RCCBs are rated by the sensivity of the leakage current normally by mAmps. If it is rated as 60 AMPS / 30 milliamps that means it can be used for a circuit of up to 60 Amps but if there is a leakage of 30 milliamps and above it will cause the sensing circuit to trigger the trip.Even if the earth electrode is open it will trip so long there is a flow of current away from the circuit maybe while electricuting somebody.

Question { NTPC, 11605 }

How the Transformer oil testing is done & Why oil dielectric
goes LOW & what is solution for that ??

Answer

The sample oil to be tested for dielectric test is collected
and put into the tester and the average of 6 tests is
taken.The minimum withold voltage is as per local regulations.
causes of low dielectric results are usually exposure of the
insulating oil to moisture through the breather or there is an
exposed opening, poor quality oil being used. high acidic
content.The insulating oil should be replaced or dehydrated.
breathers silica gels should be maintained good all the time
and check for any leaks or any exposure to atmosphere must be
rectified.

Question { 22956 }

WHY IS THE POWER FACTOR NOT MORE THAN UNITY.

Answer

power factor is said to be lagging say 0.2 to leading say o.2.Unity means it is not leading or lagging. Now pf is the angular difference between the voltage and amperes. When a voltage is applied across a resistive load the current travels along the voltage that means unity pf. when the voltage is applied across an inductor the current will flow later than the voltage this is said to be lagging pf, but when you apply across a capacitive load the current will flow faster than the voltage which means it is leading. so you will get a pf lag"g from o.o1 to 1 or lead"g o.o1 to 1(unity) So there is NO not more than 1.

Question { 6835 }

HAT IS THE EFFECT OF LOW POWER FACTOR ON THE GENERATING
STATION?HOW WILL YOU IMPROVE THE LOW POWER FACTOR?

Answer

power is sold in units of KWH.The capacity of the generator is
limited by the max ampere load on the alternator.So a load of
low PF will mean a high ampere .That means you can only
generate lower KWH FROM THE GENERATOR IF THE PF IS LOW BUT BY
IMPROVING THE PF YOU CAN GENERATE A HIGHER KWH. THAT MEANS IT
IS MORE PROFITABLE.
PF CAN BE IMPROVED BY CONNECTING CONDENSORS OR PF IMPROVING
SYNCHRONOUS MOTORS.

Question { 2797 }

I have a shop heater with a 1/3 hp capacitor start,
induction motor driving the fan. Intermittently, the fan
motor speeds up significantly for several seconds (15-60
sec) then slows to normal operating speed. The manufacturer
suggested changing the capacitor which made no sense, but I
did it to no avail.
Could this possibly be caused by the centrifugal switch
intermittently reconnecting the start winding?

Answer

Just a possibility.How big is the heater May be when the
heater is "on" the voltage drops so the fan is actually slow
but when the heater cuts out due to thermostat the voltage is
near normal and the motor is functioning at normal speed.

Question { 5502 }

What will happen if the oil level in the buchholz realy of
a transformer gets down?

Answer

When th oil level drops below the pre determined level the buchholtz relay will be trigered
A)if alarm level the alarm will be switched on
B)if trip level the transformer will be tripped off

Question { Power Grid, 12313 }

1.generator rated as 1000 mva,11kv .8 pf what will 0.8 pf
will indicate?
1.max pf it can supply 2.min pf it can supply
like this 4 options there what is answer.explain

Answer

i agree with Mr Reddy.
Generator rated at 1000MVA at 0.8 pf 11Kv
In terms of KW the prime mover had been designed for this Gen is 1000 MVA X 0.8 = 800kw, any thing more can destroy your prime mover in the long run
now the alternator part is 1000 MVA AT 11KV
The max amps it can carry is 1000000/1.732/11000= 52.5A /phase
now you can operate the generator at any pf you want as long as you do not exceed 52.5 amps per phase.

Question { 4596 }

what happens if DG N earthing fails while running and N
Earthing fails while Transformer on load ???what will be the
impact ???????/

Answer

In both cases if the Neutral connection from the DG set or
from the Transformer to the load is not interrupted than there
is no impact at all.

Question { Mepax, 6284 }

how to calculate the no. of earth pits required for an
electrical installation?

Answer

The no of earth pits is not the determining factor it is the
earth resistance.The number of earth pits is increased till
the required minimum earth resistance is met.

Question { Shakthi Power Systems, 54284 }

How to select a D.G set for calculated load of 110kW..?

Answer

D.G if ac is rated in KVA.Usually the alternator is rated at 0.8 pf. So KW/PF=KVA. 110/0.8=137.9.KVA.
Now the D.G must have some spare power to take care of starting current and a bit of future expansion so we load the D.G at 75%.
137.9/.75=183.8KVA
Now you can buy the nearest available D.G in the market.
Maybe 175KVA or 185 KVA

Question { 4223 }

what is the means of dedicate earthing?

Answer

Dedicate means devoted.That means an earthing specially done
for one particular purpose.For example an earthing system
is made just to earth the generators neutral and another for
the frame so there are two dedicated earthing.

Question { 2805 }

Why not LED burn after showing alarm,as Alarm volt is 11-
13,But LED is of3 volt,why it isnt Burn??

Answer

You are right but the voltage to the LED is reduced by
connecting a resistor in series or a few LED are connected
in series so the voltage across each LED IS about the rated
voltage.

Question { ATS, 45532 }

what is amf panel & how it is working

Answer

AMF panel is Autamatic Mains Fail panel.

It is not just an electrical panel.A suitably sized gen set is connected to it.The AMF panel has a sensing circuit which can detect a power failure.Now the moment the panel detects a power failure the genset is started automatically. Automatic change over isolators are connected, where the power supply from the mains is changed over to that of the genset.The genset will run and provide the power supply until mains power is available again.The sensing circuit in the AMF panel will detect this and after an adjustable time the automatic changeover isolators will revert back to the mains supply.After a while the genset will switch off on its own.

The AMF panel is connected with certain protections electrically for the genset. It is suppose to operate on its own.The AMF panel is usually connected to important services like security, hospitals and important businesses.

The AMF panel must be tested regularly to ensure it is in working order when the mains fail Otherwise all the money invested in the AMF panel is wasted

Question { 7476 }

How to select KW/HP rating of motor for particular
application? Is there any relationship od RPM?

Answer

KW / HP is is the unit for work done or to be done.
The kw rating of the motor is the max designed rating of
the motor.That means you must know what will be the max
load of the PARTICULAR APPLICATION before you get a motor.
RPM has a direct relationship with the load . Higher the RPM
faster the work is done so higher the KW.

Question { 4216 }

how we can save the power ?
suppose the electricity board gives 11 kw , but the
machine require only 8 kw means how we can save the exese energy

Answer

The EB gives 11 kw means that is the max load you can put on
EB but you only pay for what you use so ther is no saving
unless you find ways to reduce the 8 kw of the machine.