Wiring a second workshop (Advice on consumer units and armoured cable)

EFLImpudence said:
I think it is accepted that such circuits should not be fed from RCDs if possible - SWA is used for that reason.
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Is what "is accepted" not that such circuits should, ideally, not be fed from RCDs which are protecting other circuits? Do you also see a problem if, as was suggested, it is fed from an RCBO or dedicated RCD (theoretically removing the need for further RCD protection downstream, unless one wants redundancy)?

There are, of course, also reasons other than the absence of residual current protection for using SWA for the feed to outbuildings.

Kind Regards, John
 
JohnW2 said:
Is what "is accepted" not that such circuits should, ideally, not be fed from RCDs which are protecting other circuits?
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I suppose, but that would be foolish for two workshops.

Do you also see a problem if, as was suggested, it is fed from an RCBO or dedicated RCD (theoretically removing the need for further RCD protection downstream, unless one wants redundancy)?
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That would be a problem in itself.

There are, of course, also reasons other than the absence of residual current protection for using SWA for the feed to outbuildings.
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Of course there are but as that is one then take advantage of it.
 
Well, wiring two workshops with only one RCD at the house would be foolish.

Do you also see a problem if, as was suggested, it is fed from an RCBO or dedicated RCD (theoretically removing the need for further RCD protection downstream, unless one wants redundancy)?
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I think that would be foolish to do because there are better ways, and could be a problem in the usage of that circuit.
If you want to install two RCDs especially and solely for redundancy then you can double-up wherever normally there would be just one.
 
You'd want local RCBO/RCD protection in the Sheds.

I'd not even bother with an RCD in the house protecting the SWA - imagine an RCD tripping in the house and being reset when someone was inside the shed working away and it popped and inadvertently got powered up again.
 
EFLImpudence said:
Well, wiring two workshops with only one RCD at the house would be foolish.
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Because a fault in one workshop would take out the supply to the other? Maybe I misunderstood, but I got te impression that both of these workshops were just for the OP's personal use (and he may well largely stop using the small one once he has built the larger one), in which case he couldn't be in both at once.
EFLImpudence said:
I think that would be foolish to do because there are better ways, and could be a problem in the usage of that circuit. If you want to install two RCDs especially and solely for redundancy then you can double-up wherever normally there would be just one.
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Do you think it foolish just because of my comment about redundancy? If so, I only mentioned that as being the only reason for ('unnecessarily') having an RCD in the outhouses if the circuit already had upstream RCD protection.

Having said all that, I agree that to have RCD(s) only in the outhouse(s) (and SWA) would be the standard way of doing it - but was making the point that I thought the only real downside of having RCD protection (only) at the house end arose if that RCD also protected other house circuits (i.e. resulting in the lack of a hot dinner to which muffking referred).

Kind Regards, John
 
Personally I would prefer to supply the SWA from a time delay RCD and MCB in the house to protect the cable. Then each shed or work shop would have its own local rapid operating RCD to protect items and people in the shed / workshop.

To run SWA without residual current protection may be acceptable in the regulations but I would consider the possibility of a buried cable being penetrated by a sharp non-conductive item ( a sharp edged stone ) that reached the Live conductor. Moisture ingress could then occur creating a resistive fault Live to SWA and/or Live to Ground. Without residual current protection on the SWA cable this fault could over time develop with the only protection being the MCB operating when the fault current was high enough to trip the MCB. Given the MCB rating has to be enough to supply two workshop then the fault current could be several amps with localised heating at the fault location.
 
bernardgreen said:
Personally I would prefer to supply the SWA from a time delay RCD and MCB in the house to protect the cable. Then each shed or work shop would have its own local rapid operating RCD to protect items and people in the shed / workshop.
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Yes, I did think of that as an option. In fact, that's equivalent to what I have in my (TT) house, with (very long) distribution cables (not SWA) protected by 100mA TD RCDs and 30mA RCD protection on all the final circuits fed by that cable.
bernardgreen said:
To run SWA without residual current protection may be acceptable in the regulations but I would consider the possibility of a buried cable being penetrated by a sharp non-conductive item ( a sharp edged stone ) that reached the Live conductor. Moisture ingress could then occur creating a resistive fault Live to SWA and/or Live to Ground. Without residual current protection on the SWA cable this fault could over time develop with the only protection being the MCB operating when the fault current was high enough to trip the MCB. Given the MCB rating has to be enough to supply two workshop then the fault current could be several amps with localised heating at the fault location.
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Whilst I do not disagree that what you suggest could conceivably happen (although extremely rarely), I'm not sure that it is necessarily anything to worry about - what do you perceive as being the problem with "localised heating at the fault location" in an underground cable? (By the time the scenario you envisage had arisen, the cable would have to be replaced, or at least repaired). In any event, you are essentially talking about an 'overload current' and, provided only that the MCB was appropriately rated for the size of the cable, it would, by definition, operate before the cable was at risk of thermal damage (which implies not enough heating to damage anything else, either).

Kind Regards, John
 
bernardgreen said:
progressive deteriation of the cable leading possibly to a catastrophic cable failure.
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As I said, by the time the scenario you envisaged had arisen, at least part of the cable would already have 'catastrophically failed' - and, as I also said, the MCB, if appropriately rated would protect the cable in general from any harmful effects of the current through the fault (which, as I said, is, in magnitude, equivalent to an 'overload' current).

Kind Regards, John
 
Got sparky out, needs a new run from house (10 SWA) as current workshop on 2.5 SWA and can't run from that supply.
 
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