CT Location on Outgoing MV Feeder Breaker 1

[Sparky1963]

In MV metal-clad switchgear, are the CT's for a 50/51 relay on an outgoing feeder breaker usually located on the line side or load side of the feeder breaker? What is the most common practice and why?

 

[davidbeach]

CTs should be located, if at all possible, such that the circuit breaker is in the zone of protection. So, for a feeder position you would always want your CTs on the bus side of the breaker.

 

[slavag]

Hi.
David, I think it was some missunderstanding.
As fa5r, as I know, CT location in MV metal-clad SWG is always bottom section, line side, after ES.
Best Regards.
Slava

 

[davidbeach]

At least in ANSI MV metal-clad gear there is room for two CTs per bushing of the breaker, so two sets on the bus side and two sets on the line side. Usually not all are provided.

 

[slavag]

Thanks David.
Good to know,
Best Regards.
Slava

 

[davidbeach]

Meant to say "... metal-clad gear there is usually room for two CTs..." There are always exceptions to everything.

 

[rbulsara]

As to why on the bus side; Two main reasons:

1. Because it is very convenient to install CTs around the insulating bushings which are already there to accept breaker stabs. This even allows low voltage (600V)CTs to be used as the insulation to the bus is provided by the bushings.

2. Usually the load sides will be cables and it is very inconvenient to install CTs around them, plus you end up with a portion of the cable and breaker load terminals outside the protection zone. That is not desirable.

 

[marks1080]

From a protections point of view, we put the CT's on the bus side so we can overlap protection zones. If the CT's protecting the line were on the line side, and the CT's protecting the bus were on the bus side, than a fault in the breaker wouldn't be detected.