why should we place 9KV lightning arrestors in 11KV
substation ?

Answer Posted / a.umapathy

The reason is when ligtning impulse is more than 9kV the
lightning arrestor by pass the lightning impuluse so that
the 11kV system will be affected. If lightning arrestor
rating is more than the system working voltage (say 15kV)
then 11kV system will experience higher voltage which will
cause the damage in apparatus (as this designed for 11kV).
So insulation coordination of protective device must be
less than the device to be protected.

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