Isolator for Sub Main

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RussellR

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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 ?

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

 
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.

 
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

 
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.

 
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

 
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.

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

 
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?

 
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.

 
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.

 
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

 
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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