Protection Engineering Basics and Schemes Protection Engineering | Page 278

9.6 Short circuit Protection It is provided by Overcurrent relays and earth fault relays, already discuss in the Protection chapter of this book

Earth fault protection can be provided by core balance current transformer and earth fault relay.

Core balance current transformer is mounted on cables and it is calculated the vector sum of phase currents

If system remains balance, then I a + I b + I c = 0 = I n = I e … … ….( A)

If system unbalance due to earth fault let suppose we have below line to ground fault

I a = I o + I 1 + I 2 I b = I o + a 2 I 1 + aI 2 pg. 278