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Topic Title: 11KV Transformer Earthing
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Created On: 09 October 2011 08:03 PM
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 09 October 2011 08:03 PM
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flashtestdummy

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Hi folks, bit of an issue on a site last week, I took a "Ze" (with all bonds still connected) at the origin which was 0.98ohm.

The installation is 3 years old an the original reading was 0.06ohms, system is TNS

The site is fed from an 11kv private transformer and 2x600 singles with 3x copper tapes onto 30metre steel piles.
1x tape is to the transformer, 1x tape to the Main Distribution Panel, 1x Tape to the standby transformer all interconnected with eachother.

About 5 months ago I was on site testing and on my return last week noticed my Zs at the same Dist Board had rose by 0.70ohms. On the first visit I had no access to the Intake so cannot compare my Ze

Are you all still awake.......

Work has been carried out to these 3x piles which meant temporary disconnecting the earth tapes and and reconnecting (by a sub contractor who is sure they were replaced as was)
I have been able to inspect these connections and have 0.02ohm between them and the MET, with an earth loop off them of 1.00ohm

Any ideas? I know very little about Transformers by the way!!

Thanks in advance
 09 October 2011 09:45 PM
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alancapon

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Close in to the transformer, your earth loop reading will be meaningless, as standard test equipment is not designed to measure reactance, only resistance. If you do a search, you should be able to find plenty of threads on this subject.

Regards,

Alan.
 10 October 2011 05:52 PM
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flashtestdummy

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I did a few searches but I started this thread for specific advice on this particular issue
 10 October 2011 07:04 PM
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John Peckham

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The earth electrodes are probably (should not be) in your earth fault path when measuring Ze.

As Alan says you cannot get a reliable, consistant or accurate reading with a loop tester close to a transformer.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 10 October 2011 07:27 PM
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flashtestdummy

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John, the electrodes seem to actually be the earth fault path as I dont appear to have any reference between Supply Neutral and Earth. Effectivley a TT system, is this normal from a transformer?

As for the meter inaccuracy, yes I have come across this before with a reading of <0.01ohm ZE but the 0.06ohm original reading appears to tie in with sub main circuit readings
 10 October 2011 07:36 PM
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flashtestdummy

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Just to clarify the difference of 0.70ohms I got between the two dates was not at the origin
 10 October 2011 07:47 PM
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JonSteward

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Just out of interest. Can you get a Zs from a downstream DB. This should reduce the reactance issue.
 10 October 2011 08:19 PM
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flashtestdummy

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Jon i have done that which was what drew my attention to the issue. I couldnt gain access to the main switchroom first time round so cant compare Ze
 10 October 2011 08:54 PM
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John Peckham

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You need to establish if there is a separate earth coming from the transformer star point. Alternatively the installtion may be PNB with the neutral connected to earth at the main panel. It may of course be a TT installation.

Is this a private transformer or a DNO transformer?

You big change in Ze (with bonds connected) is suspicious and requires further investigation.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 10 October 2011 09:03 PM
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flashtestdummy

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Originally posted by: John Peckham

You need to establish if there is a separate earth coming from the transformer star point. Alternatively the installtion may be PNB with the neutral connected to earth at the main panel. It may of course be a TT installation.

There is no connection between Neutral & Earth



Is this a private transformer or a DNO transformer?

Private

You big change in Ze (with bonds connected) is suspicious and requires further investigation.

Well above the Maximum value for all MCCBs fitted for a start

 10 October 2011 09:23 PM
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John Peckham

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I assume there is no neutral earth connection at the main panel?

Is there a neutral earth connection at the transformer?

Is there a separate earth run out of the transformer to the panel?


It is beginning to sound like an un-earthed installation to me unless it is a TT system with a means of earth fault protection.

Are there MCCB/ACBs with electronic trips on the main panel?

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John Peckham

http://www.astutetechnicalservices.co.uk/
 10 October 2011 09:34 PM
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flashtestdummy

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Originally posted by: John Peckham

I assume there is no neutral earth connection at the main panel?

No connection



Is there a neutral earth connection at the transformer?

Earth tape from electrode is onto the transformer frame, not in the connection box



Is there a separate earth run out of the transformer to the panel?

Yes 300mm2


It is beginning to sound like an un-earthed installation to me unless it is a TT system with a means of earth fault protection.

The general concensus on site is to improve the earthing but as there was no earth/neutral connection I was thinking regardless of the Ze reading it needs an RCD upfront?

Are there MCCB/ACBs with electronic trips on the main panel?

ABB ACB and Merlin Gerin MCCBs ive no more information on them
 11 October 2011 08:19 AM
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Jobbo

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As John and Alan has mentioned above, it appears you have lost the neutral/earth link from the system, possibly as a result of the works carried out by the sub contractor that you mentioned (assuming a TN system as you indicated)

What measurements are you obtaining between line and neutral?

Can you confirm that the installation was not designed for a TT system (i.e. Hazardous Area)?

What measurement are you obtaining between earth and neutral?

I have on a few occasions inspected electrical installations with similar readings and following further investigations, was found to be the result of the earth/neutral link being removed, or perhaps never installed

Regards

Jobbo
 11 October 2011 10:54 AM
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AJJewsbury

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Hang on, if there's no N-PE link, I don't see how you're getting an earth loop reading <<1 ohm. It's not like it's a TT derrived from a TN distribution system where main bonding can parallel path to a TN earth. Unless you've got a N-PE fault somewhere of course...
- Andy.
 11 October 2011 11:00 AM
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Jaymack

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Originally posted by: Jobbo
Can you confirm that the installation was not designed for a TT system (i.e. Hazardous Area)?

Why not TNS as stated by the OP? (even if it was a hazardous area).

If there is no Neutral connection to earth at the transformer, why is a Ze reading being recorded at all?

Regards
 11 October 2011 12:46 PM
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Jobbo

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Before I would considered further investigation I would consult with the relevant people responsible for the installation (i.e. building maintenance/facilities) to ascertain the earthing arrangement for the site, in case it had been altered or changed from a previous visit.

As Andy has pointed out a neutral/earth fault would give you a reading, even a good one, as was found on one of the job I had investigated with a similar defect. Another gave a reading due to the large amount of filters and capacitors used throughout the site although this was much higher

Regards

Jobbo
 12 October 2011 03:40 PM
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HarryJMacdonald

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How many times did you take each reading?

I have had this problem and believe that it is caused by small voltage fluctuations. The meter takes a short sharp current pulse, which on domestic wiring, is a substantial proportion of the max load current (60Amp?) and there is a measurable drop in voltage

If your supply is 2 x 300mm², then your load current is large and the meter pulse is just a small increase so any random voltage spikes will mask the dip caused by the test current pulse. I had a huge variation in readings when testing this sort of supply (some even -ve) and resorted to averaging them.
 12 October 2011 08:19 PM
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flashtestdummy

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Thanks for the replies everyone, information has been requested from the owner of the Transformer as to the supply characteristics. Information has been requested from the client as to the original supply proposal from the Transformer owner

We are trying to determine has any work been carried out on the Transformer in the past 6 months as it seems the neutral/earth connection has only been lost since April

Another issue raised here, could there have been a N-E fault on site from day one and what work has been carried out on site on the past 6 months which may have cleared it, this information has also been requested

Previous certificates all show a much lower reading of "Ze" between 0.06 and 0.15ohms
 12 October 2011 09:51 PM
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Jobbo

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It seems you are taking the correct procedure by consulting the relevant people.

Previous records could be incorrect or even falsified. I'm sure enough of us on here have come across a few suspicious EIC/PIR in our time

Regards

Jobbo
 13 October 2011 08:42 AM
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OMS

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Previous certificates all show a much lower reading of "Ze" between 0.06 and 0.15ohms


That's a hell of a variation for a site owned transformer with one consumer. Almost certainly instrument error.

To be definitive, I guess you need to first calculate what you think Ze is and then further calculate what you think Zs will be at some point well into the installation. You can then test for this Zs at that point with a degree of accuracy.

It can happen, but it's unlikely that you have lost the N-E link (they are usually "robust" - you may have lost a PNB termination (possibly due to previous persons fiddling) but again unlikley as you are reading a loop.

Where exactly are you testing in relation to the N-E link position - I've seen a few brave souls decide to test at transformer spill boxes after having first also opened the main 4 pole ACB (ie main switch off with consequent removal of the N-E link))

Regards

OMS

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