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Topic Title: Uninsulated Neutral Link on front of LV panel
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Created On: 25 September 2017 01:03 PM
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 25 September 2017 01:03 PM
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stevesmith01

Posts: 14
Joined: 03 July 2013

Did an EICR last weekend in a decrepit old factory the main LV panel is circa 1960s and manufactured by AEI, there's a link on the outside of the panel directly above the main ACB that's mounted on porcelain insulators and carrying 50 odd Amps at zero potential to earth this seems a strange set up to me, Neutral or not its still a Live exposed part.
I've worked on some old installations but haven't come across this before.
 25 September 2017 09:48 PM
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broadgage

Posts: 2386
Joined: 07 August 2007

Not acceptable in my view.
Fairly low risk since the neutral is meant to be very close to earth potential, and moreover is more likely to remain so nearer the origin than on a distant sub circuit.
"fairly low risk" does not make it OK though.
Perhaps it was installed like that to facilitate easy removal when on site generation was in use, with its own earthed neutral ?

Would it be reasonable to fabricate an earthed steel cover, that needs a tool to remove it ?

All this presumes that you are certain that this indeed a neutral link, and not an earth.
50 amps sounds a lot on an earth connection but might be possible with PME.
 25 September 2017 10:30 PM
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mapj1

Posts: 9371
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1960s would be unusual as PME, mid 1970s onward maybe.
Perhaps check it really is N to E, not just In-line N.
Putting it on spacers to allow access suggests it might be a metering link . Presumably removing altogether it is not practical, as broadgage proposes, boxing it in. (maybe possible with a standard box or channel section with the back cut out?) That would be a bit odd, but at least you could argue it was no longer against regs.

-------------------------
regards Mike
 26 September 2017 07:52 AM
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stevesmith01

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Joined: 03 July 2013

The site Transformer is sitting right next to the panel (I probably should have mentioned that). 500MVA oil cooled, manufactured in 1966.
Initially I thought it might be the Neutral earth link but as you say its sitting on insulators, I think it is the in line Neutral.
Would be interested to know if its a modification from the original install.
 26 September 2017 09:45 AM
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OMS

Posts: 22359
Joined: 23 March 2004

Who owns the transformer ?

It sounds very much like a protective neutral bonding link to me

Not sure I'm happy about it being totally exposed - but it should be at or around earth potential actually

When you say you are measuring 50A and zero volts, then that's not physically possible - zero volts to where ?

Be very careful about any attempt to move or open this link - it could be nasty at the LV side and very nasty if you haven't isolated the HV side - you could see significantly elevated voltages to earth

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 26 September 2017 10:07 AM
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stevesmith01

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I'm thinking the customer owns the transformer as its sited within the same cage as the LV panel, the factory owner having the keys to the door and no Network provider signage anywhere to be seen.

Zero potential between this link and Earth.
 26 September 2017 10:08 AM
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leckie

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Could someone give me a link to some information regarding the dangers/explanation of what happens that is causing the danger if a link like this is opened. I am aware that there is a danger regarding opening earth connection to supply transformers but I am not sure why.
 26 September 2017 10:23 AM
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AJJewsbury

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I am aware that there is a danger regarding opening earth connection to supply transformers but I am not sure why.

If it's of any help then this hobbit's head has it that there's capacitive coupling between the HV and LV windings (like between :L and SL that causes low energy lamps to flicker) - without being referenced to earth the LV system is then coupled to the HV system via this capacitance - which tends to drag it up towards a fair fraction of the HV voltage.
- Andy.
 26 September 2017 01:04 PM
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ToniSM

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Originally posted by: leckie

Could someone give me a link to some information regarding the dangers/explanation of what happens that is causing the danger if a link like this is opened. I am aware that there is a danger regarding opening earth connection to supply transformers but I am not sure why.



https://electricalnotes.wordpress.com/2012/01/21/types-of-neutral-earthing-in-power-distribution/

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Could there be a better way?

In theory yes, but in practice?
 26 September 2017 01:56 PM
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mapj1

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The case and core of the transformer will be earthed, and maybe if there is one, an inter-winding shield - this becomes essential if the secondary is to be "IT".
But in a more normal TX, where HV windings and LV windings are alongside without a shield, then even with the earthed core and case, the inter winding capacitance can be quite serious, although with 3 phase HV it more or less cancels, what remains is still enough to bite, and with 11kV phase to phase, that remainder via a few odd nanofarads, is enough to be fatal.
So, don't try to disconnect the link, even if all load side is off, without first isolating the HV side, or more sensibly providing an alternate parallel path. But actually, unless you are really needing to move it, leaving it alone and covering it is more sense, especially as you are not 100% sure of its purpose.

-------------------------
regards Mike
 27 September 2017 10:19 AM
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ToniSM

Posts: 329
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So, don't try to disconnect the link, even if all load side is off, without first isolating the HV side, or more sensibly providing an alternate parallel path. But actually, unless you are really needing to move it, leaving it alone and covering it is more sense, especially as you are not 100% sure of its purpose.



Hear hear.

Unfortunately I've seen the results of someone disconnecting the N?E link. It didn't do the IT equipment a lot of good but the plant equipment survived.

-------------------------
Could there be a better way?

In theory yes, but in practice?
 27 September 2017 09:50 PM
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alancapon

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Originally posted by: mapj1
The case and core of the transformer will be earthed, and maybe if there is one, an inter-winding shield - this becomes essential if the secondary is to be "IT".

But in a more normal TX, where HV windings and LV windings are alongside without a shield . . .

Most U.K. Distribution Transformers have the LV winding wound on the core, then additional paper insulation, then the HV winding wound physically on top of the LV winding, not next to it. This increases the magnetic coupling between the primary and secondary windings, increasing its efficiency.

The downside is that under rare circumstances it is possible to get a flashover directly between the HV and LV windings. I would never advise removing the earth link to the LV winding - it can save your life if you have a flashover - it saved mine a few years ago, in combination with my insulated gloves. I was putting in solid links on the outgoing secondary side of the transformer when the flashover occurred. We are not sure what voltage appeared on the link I was attempting to put in, but it is likely to have been a few kV.

Regards,

Alan.
 28 September 2017 05:30 PM
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leckie

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Joined: 21 November 2008

So, and please forgive my ignorance, are we saying that if we have a dedicated transformer supplying a premises that the main earth should not be disconnected to measure Ze, or have I got that round by neck?
 28 September 2017 06:46 PM
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AJJewsbury

Posts: 15838
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So, and please forgive my ignorance, are we saying that if we have a dedicated transformer supplying a premises that the main earth should not be disconnected to measure Ze, or have I got that round by neck?

I think they're saying that disconnecting the consumer's installation from the supply's earth conductor is fine, what you mustn't do is disconnect the LV supply from the general mass of the earth. Normally you don't get the chance to do that (as it's hidden away in the DNO's substation), but if it's a PNB system or a privately owned transformer and the one and only N-PE link is on the customer's premises you perhaps have to be very careful about exactly what you're disconnecting....
- Andy.
 28 September 2017 08:41 PM
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leckie

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And as I would probably not have that confirmed information to hand it would probably be a good idea not to test for Ze?
 28 September 2017 09:48 PM
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mapj1

Posts: 9371
Joined: 22 July 2004

if there is any risk at all that it is PNB, leave things on the substation side firmly attached.
Actually you can check without disconnecting if you have a one of the fancy earth testers that is a clamp on core that instead of just metering, like a clamp ammeter, also has a setting that injects an AC test current to measure the resistance of the loop it is clamped on onto - beloved of inspectors of lightning rods for finding if the buried wire or tape stops short underground...
If there is an upstream earth bond, then the current clamp indicates a low impedance, as it is transforming into a near short.

-------------------------
regards Mike
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