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Topic Title: earth fault loop impedance test for 3ph motors with no cpc
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Created On: 06 August 2014 08:33 PM
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 06 August 2014 08:33 PM
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johnf23

Posts: 1
Joined: 06 August 2014

can someone explain the procedure in obtaining Zs on a 3ph motor with no cpc?

I have seen motors being bonded to the plant steel as the means of earthling. (on drill ships and rigs)

to simplify, I already have Ze and there's no overload, 3 core SWA/braided cable fro m db to motor terminal box. what's the best/easiest way of carrying out this test.

thanks
 06 August 2014 10:28 PM
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AJJewsbury

Posts: 11547
Joined: 13 August 2003

I'm not sure I follow. Are you saying the c.p.c. is formed by something other than a conventional core of a cable (like cable braid/armour or exposed/extraneous-conductive-parts), or there really is no permanent connection to earth for the motor frame? (e.g. it's class 2 equipment)

If there isn't an earth connection, it seems a bit pointless to attempt to test it.

If the earth connection is there, if unconventional, then just test from the motor terminals in the normal way.

- Andy.
 06 August 2014 10:39 PM
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sparkingchip

Posts: 6177
Joined: 18 January 2003

This being a BS7671 discussion forum ships and drill rigs are out of our remit.

Though I'm sure someone who can advise will appear.

The test would be from a live phase to the exposed metal work of the motor, just like testing a cooker.

Andy
 07 August 2014 04:41 AM
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industryspark

Posts: 559
Joined: 23 October 2006

Just test to the metalwork of your motor as the cpc.

Do you want to be doing a zs test on a loaded motor though really? Is a dead continuity test not better and then go down the calculated zs route?
 07 August 2014 07:27 AM
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IanLloyd

Posts: 63
Joined: 17 April 2002

From 3.10.3 of GN3:

"Loop impedance tests on motors can only be carried out on the supply side of isolated motor controlgear. A continuity test between the circuit protective conductor and the motor is then necessary."
 07 August 2014 08:29 AM
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sparkingchip

Posts: 6177
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That's the correct BS7671 answer, but there isn't a CPC as we know it.

A loop test from the live phase upfront of the control gear to the motor casing should give the result you're looking for.

It's not our regulations book you are working to.

Andy
 07 August 2014 09:11 AM
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AJJewsbury

Posts: 11547
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"Loop impedance tests on motors can only be carried out on the supply side of isolated motor controlgear. A continuity test between the circuit protective conductor and the motor is then necessary."

OP said "there's no overload".
- Andy.
 07 August 2014 09:18 AM
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industryspark

Posts: 559
Joined: 23 October 2006

How about undo the 3 phases from the motor terminals and then put them in choc blocks.....then test between these and the motor metalwork/cpc.
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