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Topic Title: Current measured on c.p.c.
Topic Summary: Can anyone help explain this please?
Created On: 19 November 2012 06:30 PM
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 19 November 2012 06:30 PM
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weirdbeard

Posts: 1615
Joined: 26 September 2011

Hi All, today I had my clamp meter out and was taking some readings on the cores of an swa which was glanded into a short run of earthed steel trunking, they were pulling about 100A, then out of interest I put the clamp meter onto the earth cable which runs within the trunking to the glands banjo and it was reading 1.2A, yet there was no reading if I clamped the installations main earth conductor or the bonding conductor.



Does any one have any ideas of how this 1.2A reading is caused?

Thanks.
 19 November 2012 06:47 PM
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michaelbrett

Posts: 938
Joined: 28 December 2005

Could be leakage current from the load

Regards

Mike
 19 November 2012 06:58 PM
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perspicacious

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I'd check Ze first

Regards

BOD
 19 November 2012 07:16 PM
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weirdbeard

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Originally posted by: michaelbrett

Could be leakage current from the load




Hi mike thanks for the reply, that was my first thought, but the current only appears on this earth conductor within the trunking, it does not simultaneously appear on the main earth, which I would have thought it would if it was earth leakage.
 19 November 2012 07:26 PM
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weirdbeard

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Originally posted by: perspicacious

I'd check Ze first





Hi BOD, thanks for the reply, Ze's fine at 0.09ohms.

The main earth is 50mm, bolted to a switch fuse, which is bolted to the trunking which is also bolted to a DB that has a 35mm earth.

As it was under controlled conditions, I was able to disconnect the MPB and other parralel earths, so leaving only the main earth conductor as the only possible earth, yet I still got a reading on the conductor within the trunking, but no reading on the MEC.
 19 November 2012 07:41 PM
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daveparry1

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Induced current from the other conductors whithin the trunking i'd say,

Dave.
 19 November 2012 07:47 PM
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weirdbeard

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Hi Dave thanks for replying, that was my stab in the dark thought too in the end, but not sure of the physics of how to prove that that is what is happening.
 19 November 2012 07:52 PM
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daveparry1

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I suppose if there was 100 amps flowing in the other conductors an induced current of 1.25 amps wouldn't be unreasonable as they were in close proximity?
 19 November 2012 07:56 PM
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OMS

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What's the means of earthing ?

Regards

OMS

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Failure is always an option
 19 November 2012 08:04 PM
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daveparry1

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I don't think it's an earth issue OMS as the op says there's no measurable current on the cpc,

Dave.
 19 November 2012 08:07 PM
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weirdbeard

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Originally posted by: daveparry1

I suppose if there was 100 amps flowing in the other conductors an induced current of 1.25 amps wouldn't be unreasonable as they were in close proximity?


Hi dave, thats the thing, I don't why its happening so can't say if it's reasonable or not, but would like to find out the cause! The separate cpc within the trunking connected to the banjo runs for about 1 meter max alongside or thereabouts to the loaded conductors
 19 November 2012 08:08 PM
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weirdbeard

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Originally posted by: OMS

What's the means of earthing ?



It's PME
 19 November 2012 09:20 PM
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John Peckham

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Neutral to earth fault and the installation not earthed? I would have said leakage and/or network circulating currents but you say there is no current in the earthing conductor or bonds?

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John Peckham

http://www.astutetechnicalservices.co.uk/
 19 November 2012 09:41 PM
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weirdbeard

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Hi John thanks for giving this your consideration, I did wonder about a N-E problem as the SWA is 3 phase+N, but the N current was much less than this current on the cpc. The installation is defo earthed! I don't think there would be any network currents as the installation has it's own transformer.
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