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Testing 3 phase


 
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Maxxy

from United Kingdom

Joined: 10 Sep 2006
Posts: 390
Location: Essex,
United Kingdom

PostPosted: Wed Sep 13, 2006 12:20 pm Reply with quote

Just a quick question, when installing a 3 phase balanced motor (no neutral) Is there anyway to do an Earth Fault Loop Impedance test? Or is the only way by calculation after measuring the External loop, and R1R2.
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Lectrician

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PostPosted: Wed Sep 13, 2006 4:32 pm Reply with quote

If you have a two prong EFL tester, one to live, on to earth.

If it is a 3 prong tester, put the neutral on the earth, and the earth on the earth, and the live to the live.

BE AWARE that if the motor is not actually earthed (this is why you are testing), that it will rise to mains voltage when you stick the live on, as the neutral has 'no where' to go.
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Maxxy

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Joined: 10 Sep 2006
Posts: 390
Location: Essex,
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PostPosted: Wed Sep 13, 2006 5:34 pm Reply with quote

Thanks for the quick response. The motor is fully earthed, both by the outer sheath, and a cable inside the armoured cable feeding it.

Its a 3 prong Kewtech tester that ive got. With the method you described, it would be too dangerous, so ill have to stick to calculated readings.

Thanks again
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RF Lighting

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PostPosted: Wed Sep 13, 2006 7:03 pm Reply with quote

[quote="Maxxy"]The motor is fully earthed,


Maxxy wrote:
With the method you described, it would be too dangerous


Why? If the motor is fully earthed then there will be no problem. If the motor is not earthed then it will rise to mains potential, but your tester will show this.

If you like, for piece of mind you could carry out an ELV continuity test between the earth at the motor and its source before proceding with the EFLI test.
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Maxxy

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Joined: 10 Sep 2006
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Location: Essex,
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PostPosted: Wed Sep 13, 2006 7:33 pm Reply with quote

Wouldnt attaching the Neutral and Earth probes to the Earth cable send 230v down the Earth for the duration of the test?

The motor is part of an industrial fan, so a lot of metal work around it where we fix it to the roof - which i would be kneeling on when doing the test.
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RF Lighting

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PostPosted: Wed Sep 13, 2006 7:42 pm Reply with quote

Yes the tester will 'send 230V down the earth' but it will onlt be a few mA.

Providing there is an earth connected to the motor then the metal work will not come live.

Attaching your tester in this way in no more dangerous that carring out an ELFI test using three test leads.
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Maxxy

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Location: Essex,
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PostPosted: Wed Sep 13, 2006 8:04 pm Reply with quote

Brilliant, thanks alot for clearing that up for me
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