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Topic Title: Very low earth loop
Topic Summary: Faulty tester?
Created On: 04 February 2013 11:47 AM
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 04 February 2013 11:47 AM
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Avatar for Approvedspark.
Approvedspark

Posts: 107
Joined: 09 March 2008

To cut a long story short, my ethos 7920 failed its last calibration for reading to low on the EFLI (something I wasn't aware of) , I was given a brand new machine through a warranty deal the calibration company has with ethos. Now the new machine is regualry giving me EFL readings of 0.00 ohms and kA's of >30. I rang the calibration company and they assured me the new machine was tested and was working perfect and even offered to send out a representative to show me how to use it!! (Very kind of them.. I did wonder where to put the leads? ) He also said if you are very close to a main sub it would give these low readings but its happening to often and even getting them on sub boards within buildings.

Does any one else own one of these and experienced similar problems? I can take Zs on individual circuits and get reliable readings most of the time but Ze & Zdb is hit & miss.

Cheers

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 04 February 2013 12:16 PM
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zeeper

Posts: 1409
Joined: 11 July 2008

If you have a local 11kv to 400v transformer is on site you would get low Zs. Because the earth loop is very short.

yours do seem very low though.

On my site the Ze is 0.01 i got this from the transformer boys who service it not from measuring it.

And I have measure less than 0.1 on submains. only way to know for sure is try a different meter.
 04 February 2013 12:19 PM
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zeeper

Posts: 1409
Joined: 11 July 2008

the other thing to check , what it the resolution of your test instrument. it may not go down low enough to give you a reading.
 04 February 2013 01:45 PM
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daveparry1

Posts: 6210
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Also make sure you're not inadvertently measuring L-N instead of L-E! I've done that on my Fluke a time or two,

Dave.
 04 February 2013 03:08 PM
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ennisteve

Posts: 10
Joined: 10 December 2012

Temperature of the tester can be a problem make sure your tester is not left in a cold van over night.

Hope this helps
 04 February 2013 09:09 PM
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Martynduerden

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Joined: 13 July 2008

Why don't you add a known resistance - common practice on extremely low Ze.

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 04 February 2013 09:41 PM
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Legh

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Originally posted by: Martynduerden

Why don't you add a known resistance - common practice on extremely low Ze.


Yes , I'd agree with that.
A couple of 0.2 Ohm 200W resistors in series with the leads.
Stick them in a box and use them to check the instrument from time to time. and against known supplies.

I remember a few years ago having problems with a brand new Ethos MFT, although this was on the low reading Ohmmeter range .

Legh

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 04 February 2013 09:43 PM
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PETER555

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Joined: 13 July 2011

Originally posted by: ennisteve

Temperature of the tester can be a problem make sure your tester is not left in a cold van over night.



Hope this helps


i have had the same problem cold weather as a bad effect on testers
 06 February 2013 12:47 PM
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Approvedspark

Posts: 107
Joined: 09 March 2008

Hi, thanks for the replies. Sorry for the mate reply had to check a few things. Not sure on the resolution, how would I check this? I'm not testing between L-N. I am aware of cold meters giving funny readings but this doesn't appear to be the case as most times it well warmed up. I'll have to have a look into the resistor.

So when I get a reading of 0.0 ohms for Ze. Would it be acceptable to record Ze as the enquiry reading (i.e. 0.35 ohms for a TNC-S) as my meter has proved there is a good earth path there but I can't write down a recorded reading of 0.0 ohms.

Cheers

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 06 February 2013 02:02 PM
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ennisteve

Posts: 10
Joined: 10 December 2012

I once had an NICEIC inspector show me a trick with a fluke tester which I was suprised at. He said that I should calibrate the leads on the low reading ohmeter setting before performing an earth fault loop impedance test. I did what he said and it made a considerable difference to the earth fault loop reading. Maybe worth a try
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