Mr

Having replaced the ELR [assumption: "earth leakage relay"] and CBCT [undefined] it is now possible to determine whether the fault lay in the ELR /CBCT, the motor feeder cable or the equipment downstream of it:

• If the fault persists, then the latter is the case, so then it is a case of isolating the motor from the cable and trying again, and then reconnecting the motor and trying again.
• If the above is inconclusive then carry out a "Megger" test on the above to yield more information.
• If the fault has gone away, then it was in the displaced ELR/CBCT, which now needs to be presented for recycling under the pan-European WEEE Directive before the Brexit Date of 31st October 2019.

If in doubt, consult a qualified local Electrician.

Simples. ζ

CBCT installation is a challenge when you have three nos 3-core cables to pass through its window. Assuming that the cable armour is not passed through the CBCT window, I guess the problem is with the centering of these cables in the CBCT window.

It is recommended to select a CBCT that has window as large as twice the dia of all the three nos three core cables put together.

The ELR maloperates during start as that is the time the error current out of the CBCT is highest due to residual flux in the CBCT window.

You may have to either re-engineer the CBCT/ELR scheme or resort to bypassing/disabling the protection for the time the motor takes to come to full speed.

Increasing the ELR pickup setting can be considered, but I doubt it will help.

ELRs are made to comply with IEC 60947 and can have non-operate threshold as low as 50% of the pickup setting.

Increasing time delay is counter-productive as it will allow motor to burn when there happens to be a real fault.

Check to ensure that the motor cables are passing through the ELR CT are in the order RST RST RST not with all the R phases together, all the S phases together and all the T phases together. by not keeping the cables in trefoil will produce a magnetic imbalance in the CT.

It can also be a motor winding fault causing an unbalanced motor current at start leading to the ELR CT saturating and giving a false trip. Check the winding resistance with a low ohms bridge tester.