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Topic Title: Neutral Link in LV main board - To disconnect or not?
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Created On: 10 April 2013 07:53 PM
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 10 April 2013 07:53 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

I am after some advice on the disconnection of a neutral link and the benefits and logic of doing so.

Scenario.
11 kV - 400 V secondary substation transformer is faulty and need to replace.

TX feeds LV bus-bars through an ACB. Two other TXs in the same substation feed the bars. Two bus-section switches stop the bars being fed in parallel in normal operation.

ie

- o - o -

So each bus bar section - has a TX connected, and the bus section switches, o , are normally open. The outer of the two bus-sections, the left and right hand sides feed LV customer. The middle section is live but with no customers.



Method
Isolate incoming HV feed, isolate main LV 3-phase ACB. Earth incoming feed. Permit. Swap TX. Switch back. (for the sake of clarity, ingoring testing etc here)

Issue
I have many years experience working for a DNO and I have never once isolated the LV neutral link doing such a job, only the three phases.

The LV neutral-earth link is IN. The HV cable sheaths are bonded to the HV steelwork earth, which is bonded to the LV neutral-earth at the TX star points. All the three TXs are in the same configuration.


I have been asked to remove the neutral link for the permit to be issued.


I cannot see the point because all the steelwork of the entire substation is bonded to the earth, and if the neutral link stayed in, any conductors connected to it remain at earth potential (even if the earth potential rises)


Any clarification on why I am being asked to remove the neutral link?

cheers
W
 11 April 2013 08:28 AM
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dlane

Posts: 690
Joined: 28 September 2007

I would say you are being asked to remove it as the neutral conductor is classified as a live conductor and to issue the permit they want all live conductors isolated but that is just my opinion.

I too have carried out transformer replacements and have not isolated the neutral earth link. The main issue that we have is that the link is housed within the LV board and cannot be disconnected unless the board is isolated to gain access, which kind of defeats the objective of having 2 feeds into it in the first place.

I agree with you that if it does become live then as it remains connected t earth the neutral and all of the surrounding steel work will be at the same potential.

Even when we want to test a transformer we will disconnect the HV terminals and then usually just the neutral link in the LV cable box of the transformer so it still remains connected in the LV Board.

Kind regards

Donald Lane
 11 April 2013 10:23 AM
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Zuiko

Posts: 521
Joined: 14 September 2010

Thanks for the reply Donald,

Just to confirm, I am asked to remove the neutral link, not the neutral-earth link.

kind regards
W
 12 April 2013 08:22 AM
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Zuiko

Posts: 521
Joined: 14 September 2010

Found my answer to this question

DSR 5.7.1 (c)
 13 April 2013 11:55 PM
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simongallagher

Posts: 148
Joined: 28 July 2005

My understanding is that that rule applies to things like 2 primary transformers earthed through a comm ASC or NER, in your case there is solid earthing, which that rule excludes.

I see no reson for this.

Regards,

Simon
 21 April 2013 07:27 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

Simon,
I am of the same opinion as you; I see no reason to remove the neutral from solid earth.

cheers
W
 21 April 2013 09:17 PM
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alancapon

Posts: 5810
Joined: 27 December 2005

Originally posted by: Zuiko
I am after some advice on the disconnection of a neutral link and the benefits and logic of doing so. . .

. . . I have been asked to remove the neutral link for the permit to be issued. . .

. . . Any clarification on why I am being asked to remove the neutral link?

It could be something in their safety rules. If the removal of the link is a condition of the permit, then you have few options other than removing it. Have you tried asking the person issuing the permit for an explanation?

Regards,

Alan.
 22 April 2013 05:27 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

Alan,
I will be issuing the permit.

I have never come across this issue before: as far as I can tell from the DSRs, solidly earthed neutrals are an exception to isolation when isolating a transformer.

It comes down to the issue of whether it is safer to leave in the link or remove it. The DSRs state it stays in; there may be a local procedure to say it is removed, but I cannot locate that and have asked for it.

Essentially, the issue is one of safety - is there a logical reason to disconnect a solidly earthed conductor? ie can it become live from a fault on one of the other transformers? If it can, how? (considering it is earthed)

cheers
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