Question { Hakel, 50575 }

RATING OF LIGHTENING ARRESTER FOR 11 KV LINE IS 9 KV BUT WE
KNOW THAT LIGHTENING ARRESTER IS USED FOR OVER VOLTAGE
PROTECTION SO WHY NOT THE RATING OF LIGHTENING ARRESTER BE
MORE THAN 11KV ???????????????//

Answer

11kv is line voltage,but we put the lighting arrester in
phase so phase voltage of 11kv line is 6.351kv,now we can
not put rating of lighting arrester to 6.351kv because it is
normal phase voltage,so we put it 1.414(square root of
2)times more than phase voltage 6.6kv means 9kv.

Question { KSK Energy, 69003 }

what is the diff. b/w restricted earth fault relay &
standby earthfault relay and where they are used?

Answer

The REF and SBEF are two separate protection functions. The
REF will protect the transformer winding and the cables to
the breaker. SBEF is standby earth fault protection. It is
backup protection in case of uncleared faults out on the
network, or earth faults outside the REF zone. It may also
protect the neutral earthing resistor if there is one, and
may act as a cheap busbar protection system by tripping the
LV bus section first and the transformer LV C/B second. Do
you really want to compromise important backup protection
for the cost of a neutral CT? You have the option to get
another CT fitted in the bushing, or if there is a neutral
cable terminated on the bushing you could install a slipover
CT on the cable, or in the NER cable box. Lots of
possibilities, and not very expensive.

Question { NCC, 111340 }

what is the function of anti-pumping in circuit breaker?

Answer

The function of closing coil is to close breaker,it is
designed to take supply for few seconds,if we give
continuous supply to it burns,so to prevent burning of
closing coil,after closing of breaker closing coil must be
bypass,and this is done by made arrangement for bypassing
closing coil after breaker closure,
so if above condition is achieved and then breaker is close
,we can not operate closing coil and it is prevented from
burning ,this system is called anti pumping
thanks

Question { 8558 }

How can we calculate the cable size???
plz give the formula !!!!!

Answer

r=v/i and r=&l/a where &=specific resistance of conductor,
now v/i=&l/a then i=va/&l
i=current.v=voltage ,a=area of conductor. &=specific
resistance of condutor ,l=length of condutor

Question { 7642 }

why we use to transmit the power in three phases intead of
single phase ?

Answer

1.generation efficency increased
2.less copper conductor is used for 3 phase power generation
as compared to 1 phase power generation.
3.more efficiency of eqvipment as compared to 1 phase