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Paralleling of Metering circuits

Paralleling of Metering circuits

Paralleling of Metering circuits

(OP)
In a double switched Bay at a substation we have 2 circuit breakers, each with CT's on each phase. (Bus 1 CB and Bus 2 CB) A line is terminated between the circuit breakers, hence the need to measure this line current through the breakers.

The metering cores of the CB1 and CB2 CT's are wired to various instruments on their respective secondary circuit.

Naturally, I want to 'cut' into each metering circuit and energise a new device's CT inputs for another application.

This should be acceptable, but, not using summation CT's, I want to wire them to common inputs so that they are in parallel.

This creates the question. When one of the CB's is open, will secondary current flowing in the other metering circuit now flow in the secondary circuit of the open CB, and thus indicate that current is flowing in that primary circuit. It may help to draw this.

Thanks,
Ez

RE: Paralleling of Metering circuits

(OP)
It's not a problem. Impedence of each metering circuit seen by the other circuit is so high, that each acts as an open circuit to the other.

RE: Paralleling of Metering circuits

The only problem that might come is difficulty testing or shorting the CTs in the out of service circuit breaker.  

RE: Paralleling of Metering circuits

Hi Cefez.
Could you please attach scheme of your CT connections, it will be more easy for us.
Best Regards.
Slava

RE: Paralleling of Metering circuits

A diagram would help.  Seems to me that you need to bring the individual breaker CT leads to a common point after they go through the individual breaker meters.  From the common point, run the CT leads through the line metering, then to the neutral.
 

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