Restricted Earth Fault Relay CT

The ideal location of CTs are in the Transformer LT Terminal box at the tapping point of busduct.

Generally the transformer manufacturer gives the neutral CT before earthing of the neutral,in such case 3 CTs to be installed in the LT terminal box.

The restricted earth fault relay to be made to trip the HT side breaker

REF only operate for earth fault in protected zone

The location of CT for REF shall be at the end of zone to be protected with REF function. In your case Incoming terminals of LV main Circuit breaker including bus duct.

It is very important to note that the neutral shall be earthed after the CT for REF are installed. In other words, The neutral shall not be eartherd at transformer Neutral bushing but after REF neutral CT.

If Neutral is earthed at transformer and CT are provided at end of Bus duct, the REF would not give any protection and not operate at all.

HS,

Thanks for your response. Actually your recommendation to install the CT on the line side of main circuit breaker is according to the restricted earth fault protection relay manufacturer's recommendation.

However, the authority where I currently work in a foreign country, the recommendation is, CT should be on the load side of the breaker.

Best regard,

rudy_fernando5r

HS,

For the benefit of all CR4 users, it would be a good idea to show in a 3 line diagram (transformer, LV bus duct and LV swithcgear breaker) the location of the 3 phase CTs and the neutral CT.

Thanks and regards,

rudy_fernando5r

Hmmm, two different answers.

Ramvinod's answer will protect the transformer only.

Happy singh's the transformer and bus-duct.

I'd go for HS's method.

Thanks TonyS,

In plant I worked, we had same arrangement of REF for protection of Transformer and Bus-duct.

I think the breaker side installation of CT is more common practice which covers Trafo & busduct, the zone coverage is more.