Over Current And Earth Fault Relay

What does your local code specify? You should know your codes better than us, we don't even know what country your from.

The usage of neutral CT for o/c and ef relay depends on type , make of relay configuration and wiring to the relay.

For REF applications the neutral CT is required for general 2 element OC and one element EF, neutral CT not required. For sensitive earth fault protection separate Neutral CT is required. It all depends on type of protection and application.

One post, one GA ! We need more people like you here

i have a doubt. i am into component design, and in a RCCB, we pass all four wires through a core-balance CT. So the vector sum of the three phase currents AND the neutral current if any, acts on the CT. Any residual current has to be an earth leakage, and the RCCB trips. Now here in this case, if there is an unbalance in the three phase loading, some current will flow through the neutral. Will the EF not trip in such a case (No earth fault, but just an unbalanced load) ? Granted that we are talking about the main ACB of a transformer, but still....

Hi, my take on this..... Over-current protection and earth fault/leakage protections are based on different principles. Earth fault protection - vector summation of currents in all live conductors ie. 3 phase + 1 neutral, by using individual CTs, or in the case of earth leakage relays or breakers, usually using one zero sequence transformer (ZCT). Over-current protection needs individual CTs in whatever phase/neutral you intent to protect. Hope this explanation helps. Regards.