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Topic Title: RCD Earth loop irratic
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Created On: 13 November 2012 06:10 PM
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 13 November 2012 06:10 PM
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beaver74

Posts: 361
Joined: 07 February 2007

Hi guys

does any one know if a RCD can effect the D_lock function of
Megar MFT ,as I have today had a Tns supply feed via a piro when testing I found Zs os socket 17.6ohms when to consumer unit to test incoming ZE hi setting 1.4 ohms, test the same with D_lock funtion 16.7ohms after a bit of chasing around and evn a second tester ZE is now ok but for the life of me I can see I have done anything is it posible the RCD is saturated and due to operating it the saturation has cleared..
cheers
 13 November 2012 07:18 PM
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kj scott

Posts: 2144
Joined: 02 April 2006

beaver 74, the Megger instruments don't use D lock, that is Robin. Megger uses a low current test. Do you have any other information?

-------------------------
http://www.niceic.biz
 13 November 2012 08:10 PM
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weirdbeard

Posts: 1535
Joined: 26 September 2011

Hi beaver, I don't really know the technical ins and outs of how testers work, but as far as i know, 'saturation' of an rcd would normally be associated with affecting the operating times of an RCD rather than the EFLI reading. I may be wrong and happy to be corrected, but saturation of an rcd can occur when cables and or equipment have a capacitive effect that stores some electric, and when the device is operated for test purposes this effect can cause the RCD tester to 'see' the additional electric for a duration after the RCD has actually opened which can affect the time measured by the tester, as I believe they continue timing untill the circuit is 'dead' (This is, I believe, why it is recommended to preferably test RCD's without anything else connected)

As you have taken a Ze reading, did you do an R1+R2 measurement on the concerned circuit, as these together would give you the Zs to compare with your spurious results.
 13 November 2012 08:23 PM
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daveparry1

Posts: 6208
Joined: 04 July 2007

Poor connections during testing I reckon Beaver!

Dave.
 13 November 2012 09:51 PM
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beaver74

Posts: 361
Joined: 07 February 2007

cheers for the coments`
I have tested the ZE with the high current earth loop test and got a reading below the max allowable.when testing using the low current test it gave a high reading.after operation and a bit of buggering about the reading with the low current test cleared up I am going back tomorrow to recheck and see if the ZE using low current test has increased.
I did check using a seperate tester and got the same readings it has just stumped me why the readings have improved (but I hope the are the same tomorrow)
the ZS ant the circuit improves at the outlets with the ZE
R1+R2 reading were stable at .67ohm and all >200 M
 13 November 2012 10:22 PM
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AJJewsbury

Posts: 11462
Joined: 13 August 2003

It's not unusual for a no-trip loop test to read different (usually higher) than a high-current test - but usually by a fraction of an Ohm - not 17 Ohms!

I wonder if you have a loose/tarnished connection somewhere, which posed a high resistance to the low current test, but the high current test burned through the oxide, so improving the connection - at least temporarily.

- Andy.
 14 November 2012 12:31 PM
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beaver74

Posts: 361
Joined: 07 February 2007

been back to day and all readings are steady.
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