Proper Location of an Earthfault CT

My diagram is not clear here, anyway, I posted like this. Does it matter if the earthfault CT(1 pc.) is located at "A", "B" or between the Neutral and earth cable/bus?. My package Substation is delt-wye, 13.8/380kV, TNS earthing system. My point of interest is the earth fault detection, in my own opinion, the earthfault CT should be located between the neutral and earth cable? am I correct sir? thank you for your comment.

If the CT is to be used in conjunction with the others to measure the residual current, it will detect the current in the neutral conductor at B such that IL1 + IL2 + IL3 + Ineutral = Iresidual. Normally, of course, the vector sum of the four CTs should be zero, but imbalance resulting from a fault to earth will result in a residual current at the relay, equivalent to the current in the earth fault.

Note that, if there is any imbalance between phases resulting in a voltage on the neutral conductor, a fault from the neutral conductor to earth will show as a residual current.

Single phase residual current devices work in the same way, but both line and neutral conductors pass through the same CT, so that separate overcurrent protection is necessary.

The difficulty about direct detection of the neutral-earth current is that it can be masked if there are parallel neutral-earth faults or it there is other PME.

Geoffrey36;

Thanks for sharing. I agree with your point regarding the imbalances of current resulting in risidual current in the neutral due to earthfault, etc. the thing however is if the CT is located in the Earth cable, it certainly will sense the most eartfault current and will trigger activation of the relay and breaker than when it is located in the Neutral cable. Thanks.

If the fourth CT is in the neutral cable and connected so that a Residual Current Relay combines the vector sum of the three phases and neutral, it will detect any fault to earth (including a neutral-earth fault if there is any voltage at the neutral).

If you rely on a CT in the earthing connection, neutral-earth faults on the external system may shunt the CT and avoid detection.

From your sketch of the TNS system we understand that this is a three phase five wire (3 phases + N+ PE) uniground system where unlike multiground system ground fault current will flow through only PE and load unbalance current will flow through the isolated neutral conductor N.

Hence the CT should be located in the grounding circuit after the bifurcation as proposed by you to sense only the ground fault current

Thank you for your reply

In A & B the CT would also see the system unbalance current and thus the relay setting shall have to take account of the same. The relay sensitivity would be affected. The CT location between the neutral & the ground connection is the best, for, it would only sense genuine earth fault currents and no unbalance currents.

Electricalexpert65;

Thanks for sharing. I think you have suggested a proper location of CT when it comes to earth fault detection. thanks

correct answer

The CT between neutral and earth would be analogous to a voltage sensitive circuit breaker. They were tried for a period in domestic and other installations and then discontinued as unsafe circumstances could arise, especially if the earth connection failed or was not good. The measurement of an out-of-balance current seems a preferred optiion.