Isolator for Sub Main

[RussellR]

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.

 

[kme]

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

 

[RussellR]

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.

 

[kme]

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

 

[Andy™]

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

 

[RussellR]

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.