Window CT on shielded cable 2

[nsbelectrical]

I understand that the primary (conductor) passing through the window CT creates a magnetic field that induces voltage/current in the secondary of the CT. This makes me think a window CT will work on a shielded cable because the shield is designed to create a unified electric field. My question is: Does the shield affect the magnetic field thus stopping the window CT from working properly?

Be Safe.

 

[dpc]

CT will work with the shielded cable.

BUT.....

Any current on the shield will be phasor summed with the current in the conductor by the CT. In the case of fault current, this is often directly out of phase with the conductor current.

So if the shield passes through the CT as part of the cable, it is necessary to bring a made-up shield lead back through the core in the opposite direction prior to grounding it so that the shield current is effectively canceled out.

 

[gepman]

dpc, good point (on the BUT). I never gave the current on the shield during a fault any thought on how it would affect the CT. Do you think that the fault capacity of the copper tape shield would be enough to affect the tripping of the protective relay? I am thinking of 15kV MV-105 cable.

 

[slavag]

Hi.
I very like this point, and before some time prepare pict.
In additional to DPC words.
Please see attached.

 

[slavag]

Hi Gepman.
Yes, enough.
Regards.
Slava

 

[nsbelectrical]

Thanks to all.

Be Safe.

 

[rcw retired EE]

Gepman "Do you think that the fault capacity of the copper tape shield would be enough to affect the tripping of the protective relay?" - Yes.

A typical application is a 200:5 or 50:5 CT connected to a 50/51G relay with a low pickup to provide fast clearing of downstream ground faults. If both ends of the cable shields are grounded, an external fault or normal ground currents can cause enough current in the three shields to trip the relay.

If only one end is grounded the shield will carry all fault current for a cable fault. That means the relay will not sense the fault current until the shield wires burn up.

This is a common installation problem. I know of two large industrial plants where all the 13.8 kV breakers incorrectly tripped during a transmission line fault at the adjacent utility substation. (Separate plants, different times). Enough utility fault current returned on the cable shields to pickup all of the feeder breaker ground fault relays. In both cases the problem was improper termination of shield wires back through the zero sequence CT’s.

 

[arcflash99]

We also had a false trip on a 12000 HP motor due to GF currents passing on the shields through a GFCT. Event: We had a VAC interupter partially fail on a feeder breaker at our main subsataion. The main sub also supplies power to the starter for the large motor. The result was there was enough noise from the arcing through the interupter to cause spikes on power system. The ground bond from the main sub to the distribution sub the breaker was feeding was not intact. The GF currents were trying to find a path back to the main sub. The Motor was located close enough to the distribution sub so that the ground path through the shields was enough to pick up the 50G element in the relay.