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

 

[John-SJW]

Are you saying ramming in your own earth rod in an outbuilding or garage, not connecting to the house earth solves many problems?
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I was always under the impression one isolator for the whole premises. I may be wrong.

 

[pc1966]

Are you saying ramming in your own earth rod in an outbuilding or garage, not connecting to the house earth solves many problems?

It solves some problems, it gives you others!

If you can use a nice low impedance TN-S / TN-C-S earth then you should use it in most case.

But if you have a situation when the cable cost to meet the far earthing requirements is more than the additional RCDs and time/effort for local earthing, and/or if the situation is one where the risk of a PME fault making external metalwork live is a factor to consider, then going TT it is worth serious consideration.
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I was always under the impression one isolator for the whole premises. I may be wrong.

I suspect that the garage would be seen as separate premises / installation. If it were for some extension to the existing building then I would say yes, it should be on a common isolator.
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Schneider has a useful on-line tool to calculate selectivity in cascaded devices, but it seems only to work properly in Chrome web browsers:
Electrical Calculation Tool - https://hto.power.schneider-electric.com/cbt/app/index.html?code=34a79e11-548d-4e49-8a1e-10f9fa18492b&client_id=cbt#/Dashboard

For other makes of MCB where the precise details of the magnetic trip action are different you would not expect get the same results, but this is still a good general indicator of what you might expect.

 

[AlanSparky]

Great response gents, thanks very much!

To note I have a mains isolator and a dreaded smart meter so there's not much space in my unit for henleys/new tails etc, also changing the main board for an RCBO jobbie would be difficult.

The SWA is buried under a very short section of stone flags so no chance of it ever getting hit.

I'm going to go for a main switch board with MCB's and just have the 30ma RCD protecting the garage circuit from the house. If it ever nuisance trips from the garage (Where all the circuits are going to be new) I can find the fault easy anyway so it's not like it's a customers house who would be moaning

 

[pc1966]

Sounds like a satisfactory arrangement for the customer

 

[AlanSparky]

Sounds like a satisfactory arrangement for the customer

It's my own garage, hence the reason I'm being picky!

 

[John-SJW]

Fuse holders are still available for DIN rail mounting in CUs. One of these with appropriate fuse, swa cable, then mcbs or RCBOs in a garage CU. Selectivity solved.

https://www.bpx.co.uk/dbdocument/109463/DF101.pdf

Fuse Holders | Base Mount Fuse Holders | RS

Browse our extensive range of High-Quality Rail Mount, Panel Mount, Base Mount and Inline Fuse Holders. Order Today!

uk.rs-online.com

 

[pc1966]

Most of those DIN fuse holders are only 32A in the usual single-width spacing, also you need them to match the MCBs exactly for bus-bar location so suddenly you find them largely unusable in a domestic CU.

 

[John-SJW]

DIN rail fuse holders are available that match the size of mcb's, screw in and out of bus bars just like mcb's; Wylex, Schneider, Hager, for three. Not easy to get hold of, usually special order, many fuse rating sizes available, but they are there. They would make a much neater job as well with no separate adjacent switch. Cheaper than a separate switch/fuse holder near the house CU serving the garage, which are running at around £100, the last time I looked.

Wylex:

10 X Consumer Unit 15 Amp Fuse Carrier With 15a Fuses DIN Rail Wylex Nsc15 for sale online | eBay

Find many great new & used options and get the best deals for 10 X Consumer Unit 15 Amp Fuse Carrier With 15a Fuses DIN Rail Wylex Nsc15 at the best online prices at eBay! Free delivery for many products!

www.ebay.co.uk

 

[davesparks]

Fuse holders are still available for DIN rail mounting in CUs. One of these with appropriate fuse, swa cable, then mcbs or RCBOs in a garage CU. Selectivity solved.

https://www.bpx.co.uk/dbdocument/109463/DF101.pdf

Fuse Holders | Base Mount Fuse Holders | RS

Browse our extensive range of High-Quality Rail Mount, Panel Mount, Base Mount and Inline Fuse Holders. Order Today!

uk.rs-online.com

To be fitted in a CU they must be approved by the manufacturer of the CU for use in their CU, which effectively means all parts fitted in a CU need to be from the same manufacturer as the CU.
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If someone drives a metal fork into an SWA, surely an RCD will protect the man?

Yes it will, but so will the armour of the cable. One key purpose of the armour is to ensure that any metal object penetrating the cable makes contact with earth first before making contact with a live conductor, this will cause the OCPD to operate.

This is why the armour must always be earthed whether it is used as the CPC or not.

 

[John-SJW]

The wider point is that supplies to out buildings will become more and more common as more people work from home with home offices being erected in gardens and garages being converted. When offices, they will need sockets, lights as minimum, however many will have ventilation, heating and maybe water heating to a small sink. It has to be done properly.

There is no defacto rule of thumb method of doing it at the moment. Many will be taken off rings from the rear of houses, when in terraced house situations, many using armoured cable from the house CU running down the side of the house. Others using catenary wire (yuk) to reach to the garden office.

However a prime aim must be, where it can be achieved, is to keep the overcurrent & fault protection contained within the outbuilding.

 

[radiohead]

The wider point is that supplies to out buildings will become more and more common as more people work from home with home offices being erected in gardens and garages being converted. When offices, they will need sockets, lights as minimum, however many will have ventilation, heating and maybe water heating to a small sink. It has to be done properly.

There is no defacto rule of thumb method of doing it at the moment. Many will be taken off rings from the rear of houses, when in terraced house situations, many using armoured cable from the house CU running down the side of the house. Others using catenary wire (yuk) to reach to the garden office.

However a prime aim must be, where it can be achieved, is to keep the overcurrent & fault protection contained within the outbuilding.

But really the same applies to all installations, they need to be done properly in accordance with Bs7671.
But judging by the number of questions relating to outbuildings on this forum there seems to be more confusion on this subject than any other. I suppose we should be thankfull that the 'exporting the earth' horror show hasn't unfolded as yet on this one.
Oh booger....I just mentioned it, hopefully I got away with it.

 

[John-SJW]

Yes, outbuilding's supply does cause confusion. It boils down to two:
1) Taking an outbuilding off a 32A ring. I have heard, and read, many variations of this. And many variations on the regs of how spurs can and cannot be used.
2) Taking the supply via a swa cable back to the house CU. Variations on this as well.

I see 1) as being the most used.

 

[pc1966]

Yes, outbuilding's supply does cause confusion. It boils down to two:
1) Taking an outbuilding off a 32A ring. I have heard, and read, many variations of this. And many variations on the regs of how spurs can and cannot be used.
2) Taking the supply via a swa cable back to the house CU. Variations on this as well.

You forgot to add:

3) Taking the supply via a swa cable back to a fused-switch

Also a lot comes down to the big variation in what sort of power is needed. For some it is simply lights and an occasional outdoor power tool, but others want the garage/outbuilding as a workshop (possibly meriting emergency lights for safety) or office (with heating and many sockets for PC supplies, etc), and other plan it as a stopping point to a hot-tub or similar.

So going 1-2-3 is improving the design, but also adding a bit of cost and complexity.

 

[John-SJW]

You forgot to add:

3) Taking the supply via a swa cable back to a fused-switch

That is really rolled into 1), taking it off a ring. OK it could be taken off a radial.