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Isolator for Sub Main


RussellR

Question

Here's one I would be interested in your views on.

On a three phase board which is not fed directly from the external supply, I always spec a four pole main isolator, my rational being that it provides true isolation. To me if you leave the neutral in there is a potential for the board to be live even when isolated.

Leaving aside isolation procedures etc.

Whats your view guys ?

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25 answers to this question

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They must think that the 3-Phases are always perfectly balanced with no current in the neutral or something, Andy. Lol.

I see it a bit different.

I served my time in large factory (2 x 33kV lines in, DNO had substation on site, you get the idea!)

So many neutrals had been borrowed over the years the last thing you wanted to do was switch a neutral, chances are you would take the whole "isolated" board upto line voltage!

Leave the neutral connected every time in that situation, smaller installtion with discreet zones, then 4 pole every time.

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good point about the shared neutral. although you should be checking its dead before any work is carried out, but it may not be live until something else is switched on later

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Its normal for 3 pole isolation ,so with respect, why make up your own rules.

4 pole isolation is used for TT jobs if memory serves.

Deke

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I see it a bit different.

I served my time in large factory (2 x 33kV lines in, DNO had substation on site, you get the idea!)

So many neutrals had been borrowed over the years the last thing you wanted to do was switch a neutral, chances are you would take the whole "isolated" board upto line voltage!

Leave the neutral connected every time in that situation, smaller installtion with discreet zones, then 4 pole every time.

Hi Riggy,

I dont quite understand how switching the neutral at the board you are working on could cause the whole board to become live. I understand about borrowed neutrals ie on lighting ccts but just cant see sense to your statement. Please can you try to explain, thank you.

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Hi Riggy,

I dont quite understand how switching the neutral at the board you are working on could cause the whole board to become live. I understand about borrowed neutrals ie on lighting ccts but just cant see sense to your statement. Please can you try to explain, thank you.

because the neutral will be live from circuit fed from other DB and borrowed neutral. power will then flow through equipment connected to this neutral, so entire board will become live. along with all equipment connected. leave it unswitched, and all circuits will be safe to work on except the borrowed neutral

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I believe he meant as Neutrals were commonly borrowed. If you switch it by using a 4pole Isolator, then you will be cutting the Neutral to other supplies taken off of it. Ie: The Unit Next Door.

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But AndyC has explained it a lot better, and he did a Pushy Shovey. :(

makes a change from patch!

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Sorry to be a pain but need to understand this properly..

Scenario

You have carried out safe isolation procedure on sub main DB everything fine so far. You now start work on this DB

Seperate cct from another DB is energised but has borrowed neutral with your submain DB that you are working upon.

Current will now obviously flow through the neutral conductor in submain DB that you are working on and potential to earth will rise, but you are saying that it wont try to backfeed through any equipment connected to this submain DB and so the Line conductors will not raise in potential against earth.

Is this right?

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Sorry to be a pain but need to understand this properly..

Scenario

You have carried out safe isolation procedure on sub main DB everything fine so far. You now start work on this DB

Seperate cct from another DB is energised but has borrowed neutral with your submain DB that you are working upon.

Current will now obviously flow through the neutral conductor in submain DB that you are working on and potential to earth will rise, but you are saying that it wont try to backfeed through any equipment connected to this submain DB and so the Line conductors will not raise in potential against earth.

Is this right?

True if a 3 pole isolator is used, ie neutral remains connected.

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No Can't quite get my head round this one, agree with the theory but not the application, if I use a 4 pole isolator it is going to be on a new or replacement board, in both scenarios I am going to make sure that there are no borrowed neutrals, and that the circuits are correctly matched, during my dead tests.

Now if someone comes along after and borrows a neutral then with the greatest of respect they will see that it is isolated on 4 poles.

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No Can't quite get my head round this one, agree with the theory but not the application, if I use a 4 pole isolator it is going to be on a new or replacement board, in both scenarios I am going to make sure that there are no borrowed neutrals, and that the circuits are correctly matched, during my dead tests.

Now if someone comes along after and borrows a neutral then with the greatest of respect they will see that it is isolated on 4 poles.

Ah Ha, That is a completely different kettle of fish and makes sense. :D

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No Can't quite get my head round this one, agree with the theory but not the application, if I use a 4 pole isolator it is going to be on a new or replacement board, in both scenarios I am going to make sure that there are no borrowed neutrals, and that the circuits are correctly matched, during my dead tests.

Now if someone comes along after and borrows a neutral then with the greatest of respect they will see that it is isolated on 4 poles.

If there's only 1 neutral then it must be shared between all 3 phases?

Same way the RCD trips withn the breaker off - neutral still connected.

No?

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If there's only 1 neutral then it must be shared between all 3 phases?

Same way the RCD trips withn the breaker off - neutral still connected.

No?

Sure Understand that but we ar talking about the Main Isolator of a Sub Board, If that Board is isolated then the neutral is still on down stream, just the sub board is isolated, so providing all circuits are fed with there neutral and line conductors from the same boardno problem, problem only occurs if you cross boards, which shouldn't happen.

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I`m backing Deke with the comment r.e. TT supplies. Don`t have BRB to hand; but there is a reg. with respect to TN systems not requiring N to be isolated at main switch; as it can be taken to be at or near CPC potential.

As for the borrowed Neutral scenario, with a cct on your isolated board having it`s N connected to a cct from another board - safest option is, if you have 4 pole disconnect, to put a jumper between board N and casing. Safe all ways round!

KME

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It is at the bottom of 537.1.2

It reads

In a TN-S or TN-C-S system the neutral conductor need not be isolated or switched where it can be regarded as being reliably connected to earth by a suitably low impedance. For supplies which are provided in accordance with the Electricity Safety, Quality and Continuity Regulations 2002, the supply neutral conductor(PEN or N) is considered to be connected to earth by a suitably low impedance.

My view is that 3 Pole is fine at the origin of supply, but where we are dealing with a sub main 4 pole is better, because during isolation even if something fails or is changed upstream, 4 pole isolation will provide protection 3 Pole will not.

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Ah. Well found. 1 scooby snack to you :)

Regarding your last comment. If you isolate the board 4 pole, and a cct with a neutral borrowed from your board is energised; your Neutral bar on your DB will rise to phase potential. Had you only 3 pole disconnected, that wouldn`t happen.

Why does it concern you that it`s a sub-main? What is going to happen to that connected Neutral? It can`t rise to any significant potential, as it`s grounded.

Even if, by some strange happening, it came disconnected elsewhere, it still won`t cause you any issue. Why do you see a need to isolate it?

KME

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Ah. Well found. 1 scooby snack to you :)

Regarding your last comment. If you isolate the board 4 pole, and a cct with a neutral borrowed from your board is energised; your Neutral bar on your DB will rise to phase potential. Had you only 3 pole disconnected, that wouldn`t happen.

Why does it concern you that it`s a sub-main? What is going to happen to that connected Neutral? It can`t rise to any significant potential, as it`s grounded.

Even if, by some strange happening, it came disconnected elsewhere, it still won`t cause you any issue. Why do you see a need to isolate it?

KME

and the power will flow through any connected equipment to that board, so all phases will also be at same potential as neutral

4 pole isolation could be better in this case though - if you only isolate phases, and test dead, all will appear dead. cut the neutral (i.e whilst your doing work needed done), then everything will become live with possible fatal results. with 4 pole isolation, when testing 'dead' after isolation, it should appear live and show something is wrong and further action/isolation taken before going any further

of course borrowed neutrals from another board should never happen, but then we all know the skills of some people

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What I am saying is the borrowed neutral is not a problem in practical terms. As the 4 pole isolation is only likely to be installed during new work or during a board change. Borrowed neutrals (meaning neutrals from other source DB's would be detected and eliminated.

The reason I put emphasis on the sub main, is by being further down the chain potential exists for someone to do something silly between you and the supply, such as a cross connection from phase to neutral (it happens !). To me isolation should mean just that you are completely isolated.

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i agree too - why is it for single phase, isolators always switch both L & N, but in 3 phase its usually only the phases

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They must think that the 3-Phases are always perfectly balanced with no current in the neutral or something, Andy. Lol.

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