Answer / amit kumar

Becoz we cant store the ac supply and at the time of fault
we need fast tripping of the coil, which can be done by dc
only.

Answer / rvg

Control circuits are sensing the healthy status of the
system, and there are many areas, where the system is to be
monitored even if there is no power supply at all.(eg: in
sub stations. during lightning stroke its very necessary
that, the system to be isolated.)
In this case, DC is the only choice that, we can provide
with battery supply. So most critical control systems
designs with DC only( pref. 24 to 48 volts DC)

Answer / girish kumar samal, hindalco,h

main thing is tripping ckt is alwayws dc as if ac fail & it
is urgent to trip an equipment we cannt do.

Answer / shafi kukkad

for controling we need continue supply in any condition

so,in Electrical Storage only by Using Battery which input
and output is DC. so whn our normal system fail this DC
comes from direct from Battery bank (which chareged by DC
UPS.) no any other arragment required.

if we using AC Controling we used AC UPS syestem, which
done first AC to DC and charged Battery. and whn Fail
normal control supply (which is AC), thn first Battery
output DC convert into AC and give control supply.which is
not reliable and complex arragement.

Answer / zulfiquar wani

DC supply we are using for the control circuit becoz it is
a continous supply for control circuitary for signaling and
even for tripping even if the AC fails.... becoz DC is
always available and also we are using DC backup for
emergency tripping of lighting system

Answer / vishal

In control ckt ,tripping ckt plays very imp role. Let us consider circuit breaker when fault occurs our relay gives trip command which lasts for fraction of second since AC has zero crossing probably DC is used & which is reliable...!

Answer / pankaj

DC supply used for control ckt because the system which we are protected by fault(i.e. o.c.,earth fault etc.)is working on A.C. at 50Hz frequency,if their is any fault & if our control system is also same on A.C. then we cant protect the H.T.line.
Hence D.C.supply which are not dependent on A.C.
(i.e.Storage Battery)is used for control ckt.

Answer / arjun kumar shukla

DC supply can be store on battery but AC supply can't store in simply DC used. Main supply, control supply would not be same.

Answer / sethar12

Can we supply AC from UPS to control circuits?

Answer / prakash suhane

Control ckts are design on DC because

1) unlike to AC, DC is constant wrt time hence no
fluctuation provide better stability in control ckt device.

2) As there is no frequency hence no leagkage current again
safety of control device.

3) DC as compare to AC is quickest means of isolation.

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