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Topic Title: Isolation on changeover generator sets
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Created On: 16 February 2018 02:05 pm
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 16 February 2018 02:05 pm
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leckie

Posts: 4705
Joined: 21 November 2008

If you have a changeover generator set for a TPN TN system, does just the change over switch need to be 4-pole to split the neutrals, or does the main switch fuse and any isolators downstream of the changeover switch also require to be 4-pole?
 16 February 2018 02:48 pm
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

In general you can't have two N-PE links in place at the same time on the same system - BS 7671 doesn't like it (444.4.6) and (because you're effectively creating PEN conductors) the ESQCR says it's illegal for consumer installations.

There can be a number of way of achieving that (including smart control systems and a contactor in the generator's N-PE link), but the simplest approach is to use 4-pole change-over switch (as per 444.4.7) - ideally one that opens grid Ls first, then grid N, then closes generator N and then generator Ls (and same in reverse when switching the other way).

That said, I gather that some big installations (typically those with their own HV:LV transformers) do cheat a little and either have multiple N-PE links or combine everything into a single one - especially when they have downstream equipment (e.g. UPSs) that are sensitive to loosing the N-PE link even for a moment - but others know more about that than me.

- Andy.
 16 February 2018 03:53 pm
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leckie

Posts: 4705
Joined: 21 November 2008

I am not certain what the supply is yet, but at 400A I imagine a standard 400A cut-out and coming in as TN-C-S.

So as I said, if the changeover switch is 4-pole, that would keep the neutral from the generator and the standard supply apart. So if that is the case I presume there is no requirement to have the incoming fused switch as 4-pole, just 3-pole?

There is also an isolator after the changeover switch, so again I presume this can also be 3-pole?

Just trying to make sure my thinking is joined up!
 17 February 2018 12:00 am
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mapj1

Posts: 12039
Joined: 22 July 2004

I think you are correct it I have understood it properly. You only need one 4 pole C/O device the rest can be 3 pole. Do note that you need an earth to the genset that still works if the incoming main is severed, so the genset has its own electrode and NE link, so it is like a small TNS substation as far as the building is concerned.

-------------------------
regards Mike
 19 February 2018 10:09 am
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leckie

Posts: 4705
Joined: 21 November 2008

Thanks for that Mike. Good point about the generator earth. I will double check who is responsible for that as the control wiring after the connection point is by the specialist generator supplier.
 19 February 2018 12:13 pm
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

Agreed - both upstream and downstream of the change-over switch normal switching/isolation rules apply - so 3P is usually fine for TN (but might need 4P for TT or for some odd situations).
- Andy.
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