Question { 36818 }

What is IDMT? What is the principle

Answer

its a type of overcurrent - earth fault relay --
IDMT -- inverse definite min. time , having diff types--
normal inverse / long inverse / short inverse type

the relay are of 3 types
1) electro mechanical (whose opeartion way is described by
others)
2) static and latest are 3) numericals --no moving part.

the simple way to explain its principle is that
THE RELAY OPERATIONAL TIME IS INVERSELY PROPORTIONAL TO
THE FAULT CURRENT. IN NORMAL INV IDMT FOR A 2.0 TIMES
CURRENT WITH TMS (TIME MULTIPLIER SETTING) of 1.0 THE RELAY
OPERATInG TIME IS 10 SEC,.. WHEREAS FOR THIS SAME SETTINGS
IF CURRENT IS DOUBLED THE TIME REQUIRED IS ALMOST 5 SEC..

but after 20 amp(in 1 A relay) the time reqd is same.

Question { Areva, 8866 }

we can stepdown the voltage from 230/12v ,and we use the
voltage for various electronics components why can't we
step up the voltage from 12/230v if we can shall we use the
voltage for domestic single phase supply

Answer

if you are going for a step up voltage then the voltage
source at lower level (in this case 12v ) has to be very
strong i.e the 12v side should be able to feed the high
current rating of the equipment connected to 230v such as
AC, fridge, water heater , mixer, microwave etc and imagine
if few of the equipments are running at the same time then
amount of power required which the 12v source has to
provide ...its difficult...
and also in such case the insulation level, the conductor
size etc has to be worked out for ECONOMIC point of view ...
its difficult..

Question { 7917 }

What is the purpose of taking Capacitance and Tan Delta
Test in Transformer bushing?

Answer

tan delta (loss angle ) and cap is taken of the enquipment
such as CTs and TRF bushings wherein paper and other
material is utilised in its insulation.
tan delta and cap values indicates to you the condition of
the bushing insulation.
if a plot is plotted aginst the no of years of the bushing
in service and the value of tan delta taken every year, you
shall get the trend of insulation detoriation.
the higher side of tan delta value is 0.007.

Question { 8758 }

What is correction factor in Tan delta test?

Answer

on site the ambient / atmp temperature is different.
whereas the facotry test conducted may be at different
temp .
As such to compare the value we apply a correction factor
such that the values are converted to a std test
temperature and we can have a comparsion of the obtained
results.

Question { 7779 }

what is the contact resistance of sf6 circuit breaker??

Answer

a breaker has a fixed and moving contacts.
after closing of the breaker the resistance between
this contact is called breaker contact resistance and its
value is in few micro/milli ohms.

Question { 3959 }

what is phase to phase to phase to ground fault

Answer

its a three phase to ground fault. ie R-Y-B with ground
fault.
it is a very rare fault. it may happen if the earth wire
from the tower falls on all three phases at a time. or any
phase conductor broke and fall on other 2 phase conductor
alogwith ground.