Garage Board - achieving discrimination?

[CraigL]

Hi all,

New to the forum so thanks in advance for your help.

I've just fitted a garage board details below:
Main Consumer Unit is Dual RCD - whoever did the re-wire had already fitted 6mm t&e direct from cu into an external jb. I've connected the t&e to swa (house end) and at the garage end the swa is fitted direct to the garage board.

The garage board is a dual board with an rcd main switch. At present I have it connected with rcds on both sides, and it is working fine. Note the garage is only used for storage and rarely used, it's not being used for a workshop. Therefore imo there is no danger caused by having rcds at either end, merely an inconvenience due to any tripping- but I don't want to leave a job if there is a 'realistic' better/safer option.

My understanding of the best option for installing a sub-main in a garage is to have the garage board rcd protected and the feed from the cu from a non-rcd protected breaker - and therefore no inconveniece tripping. However the consumer unit is dual rcd, so how is possible to fit a non-rcd protected breaker?

It's been suggested that I could have changed one block to rcbos, except the garage board breaker - but then it's an additional expense. Alternatively is it possible or even allowed to remove one of the ways from the busbar, and therefore leaving a spareway not connected to any rcd and take a direct feed from the main switch to this breaker - but this would mean 3 lives leaving the main switch, which I don't think would fit? Or could I take a feed from the live side of one of the rcds. In doing this surely I would have to fit another neutral bar and therefore I would be changing the board from it's initial design, implications??

Thinking allowed as I'm typing, even if it is possible to achieve the above scenario, doesn't this mean that the t&e from the cu to the external jb is now non-rcd protected...

Thoughts??

Thanks

 

[Flanders]

The problem you have is that 6mm twin and earth as others have said if its not more than 50mm in the wall and not protected by earthed mechanical prorection then you will nead rcd protection for it. For your situation I would say remove the main RCD in ths garage board and fit a main switch , as you say the garage will hardly be used so it is highly unlikely that the rcd in the house will be triping out. The reason i say to remove the RCD in the garage is because it will be difficult to test due to the house rcd triping and if you cant test it you can't fill out the EIC correctly

 

[westward10]

Do you mean the rcds in series, nothing dangerous about it you just don't have discrimination between rcds.

 

[Wilko]

... I'll be testing tomorrow, so if the garage rcd trips first should I change the current setup? ...

Hi - you may find that both RCDs trip during testing, showing the inconvenience of this set up. Do all the testing and hopefully all will be well. Except ... when both of the RCDs trip, which one was it that the tester recorded ?
If it tests out ok, have a think before breaking out the tools. My place is wired the same as yours and "some day" I'm going to update the house CU and I'll sort it then. But for the 5yrs I've been here it's not been a problem, just an occasional longer walk after I've plugged in something I shouldn't have.

 

[Ian1981]

The ideal scenario has been mentioned. a swa to feed the garage consumer unit with the swa on a mcb not protected by an rcd.
If your that bothered about the discrimination side of things then that's the way to go.
If that means changing your db at the origin it's your shout but if where in my own home and this was the set up I really wouldn't be that bothered.

 

[Karin]

Hi - you may find that both RCDs trip during testing, showing the inconvenience of this set up. Do all the testing and hopefully all will be well. Except ... when both of the RCDs trip, which one was it that the tester recorded ?

as an exercise, yep it's worth seeing which one is likely to go first, but for testing purposes you will only testing the downstream RCD so it's not a problem.
i'd always prefer the distribution circuit not on RCD where possible, but figure if there's an RCD on the dis. circuit at the house either way, it doesn't really matter what the incomer is at the garage for discrimination purposes?

 

[Risteard]

When you have 2 RCD's in series, of the same value, its normally the one in the main house that will trip first...

Not sure that I'd agree. It depends on the individual tripping times of the devices, so could be either (or both).

 

[Rpa07]

this threads getting longer than 'I say you say'!

 

[Deleted member 9648]

Edited:
Thanks for the input all. The general advice seems to be swap out the rcd in the garage board for a main switch - but all this is doing is removing the two rcd's in series which means if the garage trips, we loose circuits on same side in the house - which is what I was trying to avoid by using an rcd protected garage board.

What is the benefit of removing swapping out the rcd in the garage board for a main switch. Is there anything dangerous about two rcds in series. I'll be testing tomorrow, so if the garage rcd trips first should I change the current setup?

For clarity am I correct in saying the ideal setup for a sub-main in the garage for this system would be:
High intergrity board, swa wired directly to breaker in cu, rcd protection fitted in garage board?

Just because the garage RCD might trip first if you test, doesn't mean it will under real fault conditions. Almost certainly both will trip.
There is absolutely no point whatsoever in the garage RCD if there is already one at the source of the circuit. There's no harm either as long as the end user understands they will have two devices to reset.

 

[Rpa07]

Nail on the head @wirepuller
Under fault conditions - safety kicks in you need to investigate and rectify
Under nuisance condition - RCD (s) kick in you need to investigate and rectify (possibly then removing garage RCD)
So no harm as long as end user understands they will have to reset to devices or one whichever.
Difference with this scenario is the original tail CU to adapt box and its protection to that point - SWA after that point wouldn't need the protection.
If no fault or nuisance conditions exist, power will be supplied to garage until the cows come home.
We've all seen MUCH worse scenarios than this install.

 

[Octopus]

Not sure that I'd agree. It depends on the individual tripping times of the devices, so could be either (or both).

I was wondering if he has a MFT!

 

[westward10]

I haven't

 

[Risteard]

I haven't

LOL. You obviously prefer separates.

 

[westward10]

LOL. You obviously prefer separates.

I am the only one at my company who doesn't use an mft. People seem to waste hours fiddling with the leads and my boss thinks I am mad but my 20 year old rcd tester still does the job. Get the usual comments every time at our yearly assessment.

 

[Rpa07]

I have megger separates BMM2000 and LRCD 2000/2 and liked them but did buy a Megger 1552 - that is when the BMM came into its own. It makes a brilliant fault finder with continuity voltage and IR all in a handy little package (just like me) so I do agree with you @westward10 - for once!

 

[westward10]

My ins/cont tester is analogue

 

[Rpa07]

My ins/cont tester is analogue

Respect!

 

[Taylortwocities]

OK people, I'm going to throw another curved ball in here.

Notwithstanding discrimination of two RCD's, the other discrimination factor is the over-current devices. This is a length of 6mm going out to the outbuilding. At the house there is an MCB of what?.... lets say its 40A.
What is in the outbuilding? MCBs of 32A perhaps?

There's no discrimination betweeen the MCBs, so the design is flawed. You'd need a FUSE ate the source end.

Suggest a Henley Block off the supply and a switchfuse of a suitable size that will discriminate with the (whatever they are) MCBs in the outbuilding.

That also solves all the bleating in the above three pages about RCDs in series.

Simples!

 

[Risteard]

There's no discrimination betweeen the MCBs, so the design is flawed. You'd need a FUSE ate the source end.

Absolutely. It's surprising how many people are unaware of this - that circuit breakers to BS EN 60898 of any rating in series will not discriminate.

 

[Midwest]

Absolutely. It's surprising how many people are unaware of this - that circuit breakers to BS EN 60898 of any rating in series will not discriminate.

Of course I'm not as knowledgeable as you chaps, but having done a bit of research I'm led to believe, that such a sweeping statement is not quite true. In truth, from what I've read, it is quite a complicated subject and true discrimination is problematic to achieve. However, in this suggestion of an up stream mcb of 40a, would probably provide no discrimination of a downstream 32a mcb, I don't believe or should I say think that is the case across all mcb ratings?

I link to one piece I've read on the subject;

Guide to discrimination | edmundson-electrical.co.uk - http://news.voltilink.co.uk/articles/guide-discrimination

 

[westward10]

It would discriminate between overcurrent but not necessarily fault current. What about a type B, 2A protected by a type D, 63A would the sweeping statement cover this too.

 

[Midwest]

It would discriminate between overcurrent but not necessarily fault current. What about a type B, 2A protected by a type D, 63A would the sweeping statement cover this too.

You tell me Westward, only too willing to learn.

 

[westward10]

You tell me Westward, only too willing to learn.

I don't know I was responding to #48 but I would put money on discrimination under fault conditions and the 2A would go. It was a rather sweeping statement.

 

[Midwest]

I don't know I was responding to #48 but I would put money on discrimination under fault conditions and the 2A would go. It was a rather sweeping statement.

Ohh I see, thought I'd got it wrong again

 

[Flanders]

There will be no discrimination with mcbs in series except maybe in in westwards case but i think in the situation were it just a supply to a garage with a light and a socket in it i would not be to worried, what the worst that going to happen , you have a fault current on the socket circuit and so it trips the local mcb in the garage and the mcb in the house , not the end of the world , i have seen loads of commersial sites with 3 or 4 mcbs in series and 30ma rcds in series ,and yes a right old bodge in my opinion, but when you speak to the customer they say they have never had any problems so why change it . Even when i have explained the disruption it could course, they would rather put up with that than pay for an mccb board and rewiring

 

[CraigL]

There will be no discrimination with mcbs in series except maybe in in westwards case but i think in the situation were it just a supply to a garage with a light and a socket in it i would not be to worried, what the worst that going to happen , you have a fault current on the socket circuit and so it trips the local mcb in the garage and the mcb in the house , not the end of the world , i have seen loads of commersial sites with 3 or 4 mcbs in series and 30ma rcds in series ,and yes a right old bodge in my opinion, but when you speak to the customer they say they have never had any problems so why change it . Even when i have explained the disruption it could course, they would rather put up with that than pay for an mccb board and rewiring

So if I had a 20A breaker supplying a radial socket circuit from the garage board, and there was a fault on that circuit - are you saying the 20A wouldn't trip independently of the 40A at the house end?

 

[Flanders]

So if I had a 20A breaker supplying a radial socket circuit from the garage board, and there was a fault on that circuit - are you saying the 20A wouldn't trip independently of the 40A at the house end?

No i am saying thay would both trip at the same time if there was a fault current , but if you just overloaded the 20 amp mcb by less that 40 amps then only the 20amp mcb would trip

 

[Taylortwocities]

were it just a supply to a garage with a light and a socket in it i would not be to worried,

He has quite an unecessarily complicated installation.
For some reason he has this

The garage board is a dual board with an rcd main switch. At present I have it connected with rcds on both sides,

While this is an occassional use outbuilding, it has the potential for something much more substantial in the future.

 

[CraigL]

He has quite an unecessarily complicated installation.
For some reason he has this


While this is an occassional use outbuilding, it has the potential for something much more substantial in the future.

Sorry by dual I didn't mean a split board - just two way, i.e. two breakers - purpose garage board.

 

[Taylortwocities]

Ah

 

[Sailbadthesinnr]

2 RCDs in the same circuit is only a problem if tripping of the origin one will be a
nuisance. So it depends what other circuits are on the CU RCD.
Doing away with the RCD at the garage does nothing to help.
You can acheive discrimination if the CU RCD is higher opertaing current and/ or faster.
When I had a similar problem I tested a variety of 30mA RCDs to find a make that would discriminate on speed. I found Hager would always react faster than the existing RCD which was Merlin if my memory serves.