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Topic Title: Generator changeover switch domestic
Topic Summary: Correct order
Created On: 31 October 2017 12:53 PM
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 31 October 2017 12:53 PM
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Baldyhugh

Posts: 358
Joined: 01 June 2007

Hi
Following on from my post about earthing in a changeover switch I was wondering about the correct order that things should go in. The job I'm on is a domestic property with the meter inside (tt system). I plan to bring the tails from the meter Into the insulated changeover switch, then out of the switch to an upfront time delay rcd which will then supply a new metal consumer unit which I have yet to fit. My theory is that when on the generator the upfront rcd will still be in use. My question is should I have the upfront rcd between the meter and the changeover switch instead? Is there a particular order that this should be done in?

Cheers in advance
 31 October 2017 12:59 PM
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alancapon

Posts: 6917
Joined: 27 December 2005

Originally posted by: Baldyhugh
. . . My question is should I have the upfront rcd between the meter and the changeover switch instead? . . .

In my opinion, yes.

Regards,

Alan.
 31 October 2017 01:05 PM
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Baldyhugh

Posts: 358
Joined: 01 June 2007

What's your reasons for having it this way Alan?

Peter
 31 October 2017 01:58 PM
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AJJewsbury

Posts: 16008
Joined: 13 August 2003

Usually you'd have the N-PE link and RCD as close to the generator as possible so to minimise what didn't have ADS protection - hence for a typical TN system you'd normally have the link and RCD immediately after the generator, before the change over switch.

With a TT system I can see the temptation to extend the "double/reinforced insulation" system through the change-over switch to the existing S-type RCD - saves the cost of having two identical (and expensive) RCDs, only one of which will ever be in use at a time, and less clutter and confusion for the customer too. I can't see there's anything physically wrong with that approach - if done properly double/reinforced insulation should be just as safe as ADS (safer in some respects). It's not the traditional UK way of doing things though and BS 7671's insistence of a c.p.c. at every point and accessory in unsupervised situations does rather fly in the face of such an approach (although that requirement is typically, and correctly, ignored at the front end of TT installations anyway). I might be tempted to claim that "big things" before the CU aren't likely to be messed about with by DIYers or otherwise unsupervised in the manner envisaged of final circuit accessories so the requirement for c.p.c.s doesn't apply. Definitely no c.p.c. to the change over switch in that case and I might even be tempted by a couple of those 'crossed-out earth symbol' stickers too.

- Andy.
 31 October 2017 02:19 PM
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Baldyhugh

Posts: 358
Joined: 01 June 2007

Thanks Andy
Is there any guidance or diagram of the 'typical uk way' of connecting such a single phase domestic setup as I have seen many versions with changeover switch directly after the meter, meter then rcd then changeover switch and so on. It could be argued that if the rcd was first then this gave you isolation to work on the changeover switch if necessary but by having what I proposed, meter- changeover switch-rcd consumer unit, this kept the rcd in circuit when on generator aswell as mains adding an extra level of protection.

Peter
 31 October 2017 06:14 PM
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mapj1

Posts: 9571
Joined: 22 July 2004

I agree with Andy - you almost need two RCDs, one for the mains, one for the genset, whose outputs join at the switch.
However, if you only have one, downstream of the switching, then all of the wiring before it, in both branches, so including that to the genset, needs treating as double insulated, like the meter tails, as a fault there would trip nothing.
Depending on the genset location and mechanical arrangements that may be hard.
Where is the genset side NE bond in this set up ?

The saving grace is if you can consider the genset part to be TNS, as then you can come in from the generator in SWA, and need no RCD.

-------------------------
regards Mike
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