CT Placement

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strebej

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Neenah, WI
Does anybody know if there is any advantage or disadvantage to placing the CT's on the line side or the load side of a Circuit Breaker? The CT"S would be used by a multifunction relay that will open the breaker in case of Instantaneous Overcurrent or Rate of Rise and for Time Overcurrent faults.
 
strebej said:
Does anybody know if there is any advantage or disadvantage to placing the CT's on the line side or the load side of a Circuit Breaker? The CT"S would be used by a multifunction relay that will open the breaker in case of Instantaneous Overcurrent or Rate of Rise and for Time Overcurrent faults.
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If on the load side you could include the breaker in the differential scheme.
 
One issue I see is safety when servicing the CTs. On the load side of the breaker the CTs will be on a de-energized bus when the breaker is open. If it is on the line side then an upstream breaker will have to be opened to de-energize the bus & CTs.

If the CTs are ahead of the breaker they will be protecting a few more feet of cable or bus than if installed on the load side. I suppose the question is how far from the breaker will the CTs be mounted on the load side ?

The CTs & relay's zone of protection is everything after the breaker they control in either case.
 
Due to spacing requirements, it is not uncommon to see (2) CTs on the line side and (1) CT on the load side.
 
OK, lots of good ideas here all based on assumptions.

strebej, what type of breaker are you discussing? Can you tell us a little more about your system?
 
Good point zog. I was thinking about a substation differential using the feeder breaker.
 
In general you want to include the circuit breaker itself inside the zone of protection.

This means that CTs should be on the line side of a feeder breaker for standard instantaneous and overcurrent protection, as well as for transformer differential protection for the breaker feeding the primary of the transformer. For bus differential of a feeder breaker on the bus the CTs should be on the load side of the breaker, but for a main breaker feeding the bus the differential CTs should be on the line side.

For main breakers on the secondary of the transformer you will typically see CTs on the line and load side - the line side CTs are for instantaneous and overcurrent protection, and the load side CTs for transformer differential of the transformer that feeds the main.
 
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