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Posted By Topic: Protective Earthing of 3-phase motor

Mar 17 2020 10:04


I was inspecting the motor terminals on two separate machines that have two 110kW 3ph motors each. When I opened up the motor terminal box the 3x phases were connected to the studs but there was no earth wire. On further inspection the (separate) Protective Earth cable was run to one motor and connected externally to the motor frame and then looped via an earth cable to the second motor. The second machine was the same except the earth cable was connected to a metal plinth that the motors are bolted too.
I have never seen motors earth this way. It’s always been an earth connection inside the motor terminal box. My guess is that 3000: makes it OK? My main concern is the earthing of the metal plinth and the earth connection to the motor(s) via the 4x bolts to the motors mounting feet.



Mar 17 2020 14:26

Yes allows earthing by separate conductor. says separate PECs can be used to earth equipment on different circuits; though limits this to only equipment supplied from same DB.

At the equipment end; requires the equipment to be earthed - the equipment in question being the motor, not the frame it may be mounted on. However permits conductive framework used for mounting equipment to form part of the PEC frame as part of the PEC. Connection of PEC to motor (or frame) need not be enclosed. And a loop from PEC connection point of 1st motor to 2nd motor would be acceptable; but I'd be unhappy if there was any chance of one motor being removed, including the incoming PEC; thus leaving the other without PEC.

So while unorthodox, probably compliant provided ECC / EFLI is OK.


Mar 18 2020 21:57

Thanks for the reply :)