CTs and 3-Phase Cables

[nSey]

I was in the field last month and was told that CTs are used for protection purposes. Being a non-electrical engineer, I assume that the CTs measure the current through the CT and trip a protection relay.
This would make sense if there was a single cable through a CT. However, it looked like there wer multiple cables through the CT Box - how would the protection system work when there are multiple cables through a single CT?

The field technician couldn't answer my questions; hope to get a better explanation from some electrical minded engineers

 

[DiscoP]

It could be that there are two cores for 2 separate protection schemes, to improve reliability.

It could be two cables connected in parallel to reduce the lead resistance, which in turn reduces the voltage requirements for the CT.

 

[nSey]

I meant multiple cables on the primary side.
I.e. in a protection scenario, would there be a dedicated CT for each cable (phase) or would there be a single CT for multiple cables being protected?

 

[DiscoP]

I guess it could be that. A good system would have separate protection so that if one cable fails, the protection just disconnects the faulted cable, and the other one can keep supplying power. But anything is possible !

Maybe it is that one cable is not rated for the load, so they have some in parallel to share it ?

Are the cables the same size ?

If one is much smaller it could be an earth/ground wire. If the cable has a sheath around it, the earth wire needs to come back through the CT for the protection to be able to measure faults between the phase and the sheath.

There are many other possibilities - any wiring diagrams, schematics, drawings or even photos could allow someone to better help you.

 

[DiscoP]

Or it might be a core balance CT

 

[waross]

Probably for ground fault detection. As long as things are normal the net current will be zero. In the event of a ground fault, there will be an current unbalance through the CT window.
BTW That is how GFCI devices function.

Bill
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"Why not the best?"
Jimmy Carter