CBCT On Busbar For Outgoing Feeder

Do you have a neutral cable running ? Since it is a motor load, it is unlikely. However, if a neutral is there, it should be passing through the CBCT. In such a case there should be no tripping. (Pardon me if i have stated something obvious to you).

First of all thanx for your reply

There is no Neutral cable running from contactor to motor (only three phases)

Three busbar passing through CBCT at contactor incoming and contactor outgoing connected to Motor cable (3C)

Regards,

T.D.

Yes, as expected. No issue there.

The unbalance you observed...is it vectorially zero? If not, we should wonder where the remaining unbalance current is goimg ... that might be causing the nuisance trip. If the incoming cable before the contactor has a neutral, it would be terminated at some point, and may provide a return path for the unbalanced component.

The basic point is (i am sure you know it, pardon my reiterating) that all current-carrying conductors must pass through the CBCT, and vector sum of all their currents should be zero in a healthy system, even if there is an unbalance. Please see how this can be checked at your installation.

Is this a new installation or an existing set-up which has worked OK previously?

Imbalance is not the same as an earth fault. One may easily have imbalance without a current flowing in the earth conductor; in extremis it is called a "single phase installation".

Please clarify.

I would check to make sure your voltage is correct for that motor, on all three legs...Is there a delta leg, is it required, is it wired properly?

"...false tripping of Earth Fault During motor starting and running...", if it trips on starting how can you measure the running current? If you're blocking the trip signal and getting a 7-8Amp imbalance, you still haven't told us anything, it could be on a 1,000Amp motor or a 10Amp motor.

You also need to check that the CBCT is sized properly and isn't saturating during any starting or load transients, then check that all the wiring is the same length, sized properly, the connections are correctly polarized, and made-up tightly. This assumes that you have verified that the motor is not at fault (no pun intended).

On starting, a motor can be expected to pull 5-7 times its full load current for a brief period while it accelerates from rest.

It is a fallacy to expect one protection device to provide both overload and cable protection. That is why, in BS7671 and other places, there are two protective devices, often a set of fuses to protect the wiring and often an overload device to protect the motor. The topic of coordination is well covered in the standard.

Please describe how the installation in question differs from that recommended by BS7671 and why?