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

Answer Posted / amit

The system voltage is always specified in RMS value or we can say that we can measure only RMS value but actually the system is subjected to peak voltage. in case of 11 kv system if the LA is provided between line to neutral, which is normally the case is, the phase value will be 11kv/sqrt(3)= 6.35kv (the phase value. but the peak value for this phase voltage will be sqrt(2)*6.35kv = 8.98kv. hence the rating of LA will be 9kv

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