LV Motor feeder & Earthfault protection

[RRaghunath]

Is it correct to employ Earth Leakage Relay (ELR) for earthfault protection of an LV motor.
The 415V system is solidly earthed and the motor is rated 128kW.
The ELRs are governed by IEC60947-2 which stipulates 'Non operate current' for the ELR to be not less than 50%, which means, the ELR can operate at any current above 50% of the set threshold.
I am curious to know the practice followed in other utilities.
Appreciate sharing your experience / views.
Thanks in anticipation.

 

[ScottyUK]

In my opinion yes it is appropriate for larger motors. Without dedicated earth fault protection on the motor feeder you will one day find yourself in the undersirable situation where a motor circuit earth fault occurs and the motor overcurrent protection operates more slowly than the MCC incomer earth fault protection resulting in loss of the full MCC. Normally core balance E/F relays are found on LV motors larger than about 37kW / 50HP or so; below this level the overcurrent protection clears ahead of the incomer earth protection so the additional complexity may not be justified.

If you wish you can prove the above to yourself by superimposing imposing the motor O/C time-current curves on the incomer E/F curves. Repeat the exercise with E/F protection added to the motor circuit.