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Eath fault in low voltage motor

Eath fault in low voltage motor

Eath fault in low voltage motor

Hi Filks
Induction electric motiry 0.4kv 3ph 5.5kw keep tripping by earth fault (F64 setting 3A 1s)
Checking the motor insulation 12 Gohm
Cable insulation resistance 75 Mohm
We conducted Hipot test for cable using 1.5kv leakage was 20 michroAmp
Earth faul relay has tested it is good
What would be the causes of earth fault?

RE: Eath fault in low voltage motor

If the cable is 3 conductor unshielded you can check for fault current by placing the clamp on ammeter over all 3 phases simultaneously. If the cable is 4 conductor or shielded you'll only be able to test in enclosures where you can clamp over the 3 phases without the ground wire. Any fault current causes an imbalance and the difference will display on the ammeter. Perform this test at multiple locations to determine where the fault has occured. If everything is good as you describe I suspect you'll find someone has wired the cable incorrectly in your distribution panel.

RE: Eath fault in low voltage motor

Long conductors to the motor?
Make and model of the Earth faul relay?
Soft Start?
PF capacitors?

Ohm's law
Not just a good idea;
It's the LAW!

RE: Eath fault in low voltage motor

Did you hipot the motor? AC hipots are better at fault puncturing than DC hipots.

I suspect cracked motor terminal(s) (probably one of them cheap SMC terminal blocks which all 'reputed' OEM's use now a days because some bean counter got 'involved'.) or damaged motor leads.


RE: Eath fault in low voltage motor

Is the motor tripping during starting or during running?
Is it first time the motor is started or has it been running before without the tripping issue?
Is it a pump or fan or compressor drive?
Is the protection connected in residual circuit of three phases or is there a core balance CT?
What is the relay type - is it RCD (ELR) or standard earth fault protection? RCD/ELR have non-operating threshold (IEC 60947-3) that can be as low as 50% of the set pickup. This means the relay that is set at 3A could be operating at any current above 1.5A.
It is normally not a practice to provide stand alone earth fault protection for motors < 30kW rating. Fuses being small in rating can provide protection for earth faults too at this level.

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