High reading on grey phase, all across a large site

[Steve*]

Ive come across an issue on a large site i work on. May be something simple, but im struggling to figure out what the problem could be.
The site has a HV ring and 6 HV-LV sub stations. This is usually split. I have noticed in the last few months that when testing for an earth loop reading on the grey phase(LV) i am repeatedly getting OL or >500Mohm. This is at points in the installation fed from different HV-LV subs. If you repeat the test a number of times eventually you can get a reading similar to the Brown and Black phase.
This concerns me as i can only think of it being an issue at the Hv side of the installation, possibly a neutral link arcing?
Its all a little beyond my knowledge, and my scope of work on the site.

Any one have any ideas how to diagnose the problem or possible causes?
I am not permitted to switch or work ony any HV.

Thanks

 

[Guest55]

i'll go get tony.......



lol ;-)

 

[Steve*]

Should i ring the fire brigade?

 

[Marvo]

Not just yet, keep them on stand-by though. What tester are you using?

 

[Steve*]

Ive used a Kewtech kt62, and a colleague has seen the same result with a Fluke 1652(irc). My first thoughts were the tester,but with the results being fine on L1 and L2 I dismissed it.

 

[Marvo]

What is the earth fault loop impedance reading you finally get when everything settles down? What scale are you using on the tester ie is it a low current test or high current test?

 

[Steve*]

Cant remember off the top of my head but in line with L1 and L2 for example, 0.45 0.46 0.44
but in most cases L3 will be over-limit. AS for low/high current, i don't know if its an option on the tester.
Ps this is not at origin, 2 dbs down from LV MCC.

If i get time tomorrow ill do a few loops elsewhere on site, maybe connecting N and E to see if it still occurs.

 

[Engineer54]

This concerns me as i can only think of it being an issue at the Hv side of the installation, possibly a neutral link arcing?

There is no neutral of any description on the MV (HV) side, neutrals are only present on the LV side of things!!

 

[Richard Burns]

If you are getting a high earth fault loop impedance on a three phase circuit and only getting this high value on the one phase, then this would tend to indicate that you have a loose connection on that phase (or at least a high resistance joint).
The EFLI measurements on the other phases show that the neutral and earth connections are OK.
The only place that a high resistance can occur is, therefore, on the phase.

You say that you get OL or >500 ohms (I assume you are not measuring Mohms!) since on a high current test the tester only goes to 20 ohms, you must be testing on the lowest low current range (2000 ohms) these means of testing are subject to interference, so (so long as there are no RCDs in circuit) it would be better to test at the high current range to get a reliable result, if the result still occurs then you do have a problem.

Since you say that you get this at different supplies this seems to indicate that the problem may be further back, however this sort of problem should have been noticed already. Best to ask the HV side if they have had any overheating connections!