Garage supply/RCBO Upstream

[AlanSparky]

Hi Gang,

Old member here no idea where my old account went but hey ho!

Anyway, having moved into panel wiring I always like to check that I'm doing the right thing domestically etc, you wouldn't think I've got my 2391 but it's been a while!

So..ressuplying my large garage with a new 2.5 SWA into a Wiska box by the old board (MK DUAL) Currently protected by a 30ma RCD and a 32amp MCB. Buying a 4 way metal clad board for the garage but trying to decide on the protection.

I want to stop the 30ma RCD tripping in the house with a garage fault.

My first idea was to fit a main switch board with 4 x RCBO's, next was having a time delayed or 100ma RCD incomer and MCB's.

Do I need to ramp test my current RCD and see when it trips then decide? Back when I was sparking, I only dealt with a few cases of upstream/downstream discrimination so so just checking

 

[westward10]

If you have additional rcd protection at source it is not required in the garage. You aren't going to achieve selectivity from the 32A device but I wouldn't lose sleep over it. 2.5 swa is undersized for protection by way of a 32A device, personally I would up it to 40A and use 6.0 swa.

 

[AlanSparky]

If you have additional rcd protection at source it is not required in the garage. You aren't going to achieve selectivity from the 32A device but I wouldn't lose sleep over it. 2.5 swa is undersized for protection by way of a 32A device, personally I would up it to 40A and use 6.0 swa.

Thanks, I really just wanted to get around the house not tripping out in the event of a garage fault like a security light getting moisture in etc.

Cable is 4mm swa sorry old cable was 2.5! And is already buried ready. Looks like I'll just get a main switch board with MCB's.

 

[radiohead]

If your house DB is a dual RCD without any non-RCD ways your only option to prevent the house being affected by a garage fault is to Henley block the tails and fit a fused isolator to feed the distribution circuit with RCD protection at the garage end. Not a great deal extra work and a far better arrangement

 

[Spoon]

Thanks, I really just wanted to get around the house not tripping out in the event of a garage fault like a security light getting moisture in etc.

Just a thought to the more intelligent than me... (which is about 95% of people here):
How about fitting a 10mA fused spur for the security light?
Is there enough discrimination for this to go before the house RCD or would you get nuisance tripping?

 

[Pennywise]

If your house DB is a dual RCD without any non-RCD ways your only option to prevent the house being affected by a garage fault is to Henley block the tails and fit a fused isolator to feed the distribution circuit with RCD protection at the garage end. Not a great deal extra work and a far better arrangement

This is what I'd be doing too ?

 

[Ian1981]

Just a thought to the more intelligent than me... (which is about 95% of people here):
How about fitting a 10mA fused spur for the security light?
Is there enough discrimination for this to go before the house RCD or would you get nuisance tripping?

Needs to have a delay to provide selectivity. A 10mA rcd still needs to provide additional protection and be tested at a current of 5x or greater than its Idelta N

 

[pc1966]

As already covered, to get selectivity between RCDs you need two things:

• The upstream RCD has to be at least 3 times higher trip (in practice) so you can be sure that the downstream one on small/growing leakage will not trip the upstream one (that might go at In/2)
• The upstream one must have a delay so the downstream one has a chance to clear a high current earth fault that exceeds both RCD's In values before the upstream one has to trip

As also said, if selectivity on over current is needed (as ideally it should be present) then you won't get very good behaviour with cascaded MCBs for exactly the same sort of reason (the "instantaneous" magnetic trip have no time-selectivity in cascade).

So using a fused-switch to feed the SWA from the meter tails (as radiohead said) is better as typically you will see selectivity to a kA or two fault current, where as for MCBs in series typically it is only a few hundred amps.

 

[pc1966]

My first idea was to fit a main switch board with 4 x RCBO's, next was having a time delayed or 100ma RCD incomer and MCB's.

If you are looking to fit RCBOs in the main board then you could simply have another 30mA RCBO for the garage and for the garage CU just have a switch incomer.

You still won't get good selectivity on over current, and you would have the minor inconvenience of having to go in to the house to reset any RCD trip as well, but it might be less trouble than splitting tails, etc.

Also you would have the benefit that the house circuits are less likely to spurious trips, and when something does trip you know unambiguously which circuit it was!

 

[John-SJW]

How about an mcb at the main house CU supplying only the swa cable. In the garage have an incomer and an RCBO(s). It must be determined that the RCBO trips before the main house mcb, so garage faults & over-current are contained inside the garage.

The only downside is that swa cable is not RCD protected. If RCD protection is required for all cables on the premises then it is a matter of section of two appropriate RCBOs in which the garage RCBO always trips first.

 

[pc1966]

How about an mcb at the main house CU supplying only the swa cable. In the garage have an incomer and an RCBO(s). It must be determined that the RCBO trips before the main house mcb, so garage faults & over-current are contained inside the garage.

You won't get that on a hard fault, it will trip the magnetic release on both at currents of the order of 160A or more for even a couple of milliseconds (even a 13A fuse blowing can do that).

The only downside is that swa cable is not RCD protected. If RCD protection is required for all cables on the premises then it is a matter of section of two appropriate RCBOs in which the garage RCBO always trips first.

The SWA does not need RCD protection as any cut/penetration will trip the over-current protection.
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If they can live with the lack of good selectivity on hard faults, it at least puts the RCD reset(s) locally to where anyone is working.

 

[John-SJW]

The SWA does not need RCD protection as any cut/penetration will trip the over-current protection.

If someone drives a metal fork into an SWA, surely an RCD will protect the man?

 

[pc1966]

If someone drives a metal fork into an SWA, surely an RCD will protect the man?

It will, but to reach the live they have to go through the earthed armour!

So while an RCD would disconnect quickly in any case, if the overall Zs is low enough then the supply over current device would do it anyway.

 

[John-SJW]

So pointless in having an underground swa cable on an RCD/RCBO.
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If having a fused switch taken off henley blocks supplying the swa cable, then a separate mainswitch would also need to be fitted so the whole installation is isolated from one switch. So two switches fitted at the house CU/meter.

 

[pc1966]

So pointless in having an underground swa cable on an RCD/RCBO.

Mostly.

But if you have a long cable where the armour fails to meet an acceptable Zs for the OCPD and you don't actually need it as a load earth (say it will be feeding a TT supply out building) then using something like a 100mA or 300mA delay (S-type) RCD would protect for cable damage and still allow you to have selectivity in the RCD/RCBO at the far end.
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If having a fused switch taken off henley blocks supplying the swa cable, then a separate mainswitch would also need to be fitted so the whole installation is isolated from one switch. So two switches fitted at the house CU/meter.

I don't think it would be necessary if they are separate installations.

But if you have a supply isolator fitted that would do already, and as you are going to the trouble to have the DNO pull the fuse to split the tails you might as well fit an isolator so you don't have to call the DNO for any such work (e.g. CU or fused-switch replacement) in future!