Answer / elangovan t

Lightning arrester is to protect the system from any high
voltages such as lightning.
If the system voltage is 11kv, then the phase voltage will
be 11/sqrt(3)=6.35kv. It means the RMS voltage of the phase.
The maximum voltage or the peak voltage across the arrester
and earth will be 6.35*sqrt(2)=8.98kv.i.e 9kv.
Therefore, for 11kv line it is rated for 9kv.

Answer / h r jadhav

when we use ac line having system line voltage 11kv,in
this case phase voltage [phase to neutral which is earthed}
is 6.351 kv. lightening arrester is connected between phase
and neutral. lightening arester are nothing but the voltage
dependant resistors which act as insulator up to not only
the phase volatage ie 6.351 kv rms but ac peak voltage
reaches to 6.351 rms x root of 2 ie 1.414=8.98 kv. for
normal operation of system. thus the L A s are rated for
normal voltages 9 kv and discharge current generaly 10 kamp.
when ever voltage surges occures due to lightenig or either
by system abnormabilty,L A becomes for a instance as a
conductor provides path to surge voltages and prevents
failure of insulation leveles of equipment of system

Answer / nageshwar

FOR A 11KV LINE LINE TO GROUND VOLTAGE IS 11KV/SQRT(3). IT
GIVES OF THE ORDER OF 6KV ONLY. AS ARRESTER IS USED FOR
LINE TO GROUND TO DISCHARGE THE FAULT CURRENT TO GROUND ITS
RATING SHOULD BE MORE THAN 6KV. GENERALLY WE USE 9KV.

Answer / shirish

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.

Answer / gunasekaran

for solid grounding maximum voltage build up at earth faul
condition it self is not more than 80% of line voltage so
11*.8=8.8KV so we r using 9KV

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