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

 

[LankyWill]

Could Henley block the tails and install a small 2 way cu inside next to existing cu. Then supply the garage cu from the new 2 way board. This would only cost £50 tops and negate the need for rcbo's

 

[westward10]

Could Henley block the tails and install a small 2 way cu inside next to existing cu. Then supply the garage cu from the new 2 way board. This would only cost £50 tops and negate the need for rcbo's

Removing the single means of isolation.
Is the 6.0 T&E concealed in the wall it may require the rcd protection. Ditch the rcd in the garage.

 

[Jay Sparks]

Is the dual cu in the house a high integrity one? If so it should have 3 neutral bars so you can fit a new circuit from the main switch and not on any of the rcd. However, you need to make sure the 6mm twin in the house doesn't require rcd protection.

Jay

Sorry just realised you've said some of this in your OP. I shouldn't skim read!!

 

[Wilko]

Is the 6.0 T&E concealed in the wall it may require the rcd protection.

Yes, this is the question. If you change what you've got now without understanding this part you risk making an inconvenience a non compliance ...

 

[westward10]

The way I look at it is the OP has created this situation by fitting the rcd in the garage and is now looking for a solution. The correct thing to do was to fit a non rcd board in the garage as opposed to doctoring a perfectly good CU to resolve the error he has made.

 

[CraigL]

Thanks for replies

Westward - I was under the impression that rcds needed to be at the origin of the installation, and in the case of a sub-main in my case the garage board being the origin? Therefore tripping can be resolved in the garage. Obviously the original setup of the consumer unit has thrown a spanner in the works.

Wilko - Regarding the 6mm t&e I didn't install it and without ripping up the house I'd be guessing. Is my current setup compliant althought possibly an inconvenience depending which rcd trips?

Jay - 5+5 board pal.

 

[westward10]

The board in the garage is not the origin. What earthing system do you have.

 

[CraigL]

The board in the garage is not the origin. What earthing system do you have.

TN-S

 

[westward10]

The garage board does not require its own RCD protection because it has it at the point of source.

 

[Deleted member 9648]

Big time over thinking and over complication going on here. As already suggested, change the garage RCD for a main switch and the whole lot will be protected by the RCD at the house DB. There's no perfect scenario with a lot of this sort of thing, you can always find a snag whichever option is taken. Just use the simplest option in compliance with 7671 and head for the next job.

 

[LankyWill]

Removing the single means of isolation.
Is the 6.0 T&E concealed in the wall it may require the rcd protection. Ditch the rcd in the garage.

The new board would feed the garage by a 2 way rcd unit. The 6mm would still have rcd protection, you could then take the rcd protection out of the garage cu. As for the sole means of isolation the distributors fuse is still there Or you could fit a isolator.

 

[Jay Sparks]

By changing the cu in the garage to a none rcd one the only annoying thing would be if you were working in the garage and the rcd tripped, you would have to go to the house cu to turn it back on. And it would knock out all the other circuits in the house on that rcd.

Jay

 

[Wilko]

If I have understood correctly, you've got 6mm T&E from an MCB under an RCD in the house CU. Unless you can see how that cable is installed, I would assume the original installation Electrician put it under an RCD because it needed it. Having a 2nd RCD in series (in the garage) is not required but doesn't go against regs (in my opinion).

 

[westward10]

By changing the cu in the garage to a none rcd one the only annoying thing would be if you were working in the garage and the rcd tripped, you would have to go to the house cu to turn it back on. And it would knock out all the other circuits in the house on that rcd.

Jay

And by having an rcd in the garage would prevent this?

 

[CraigL]

Westward/Wirepuller - thanks for your advice. For some reason I thought in the event of tripping I needed to provide the ability to restore power to the garage, from the garage.

 

[westward10]

The new board would feed the garage by a 2 way rcd unit. The 6mm would still have rcd protection, you could then take the rcd protection out of the garage cu. As for the sole means of isolation the distributors fuse is still there Or you could fit a isolator.

Why would you lash in another CU to a perfectly good install removing the single means of isolation.

 

[LankyWill]

Why would you lash in another CU to a perfectly good install removing the single means of isolation.

Nothing stopping you going- meter-isolator-henley block

 

[Deleted member 9648]

Westward/Wirepuller - thanks for your advice. For some reason I thought in the event of tripping I needed to provide the ability to restore power to the garage, from the garage.

No requirement to do that, if that was the case any overcurrent or additional protection device at the origin of supply to an outbuilding would not comply.

 

[Pete999]

The board in the garage is not the origin. What earthing system do you have.

Westward/Wirepuller - thanks for your advice. For some reason I thought in the event of tripping I needed to provide the ability to restore power to the garage, from the garage.

2 RCDs in the same run asking for problems no discrimination whatsoever

 

[westward10]

Nothing stopping you going- meter-isolator-henley block

The obvious thing would be a non rcd garage board, why anyone thinks lashing in another board to be a good idea is quite frankly beyond me.

 

[Deleted member 9648]

The obvious thing would be a non rcd garage board, why anyone thinks lashing in another board to be a good idea is quite frankly beyond me.

That's what I'd do....but I can see that if you want to cover every eventuality then avoidance of the garage affecting any of the house circuits might seem a good idea. If there was a load of garden lighting for example on the circuit then you'd defo want to keep it away from the house RCD, but as the OP states a garage used for storage then I'd be quite happy to use the house DB RCD for any required additional protection.

 

[Jay Sparks]

And by having an rcd in the garage would prevent this?

No, I was making the remark based on the OP changing the cu in the garage to a none rcd one as was suggested earlier in the thread. But tbh there would be no difference in leaving it as it is now.

Jay

 

[westward10]

That's what I'd do....but I can see that if you want to cover every eventuality then avoidance of the garage affecting any of the house circuits might seem a good idea. If there was a load of garden lighting for example on the circuit then you'd defo want to keep it away from the house RCD, but as the OP states a garage used for storage then I'd be quite happy to use the house DB RCD for any required additional protection.

I see your point and it would have been better to fit a high integrity board with the garage on a separate rcbo, if required.

 

[LankyWill]

That's what I'd do....but I can see that if you want to cover every eventuality then avoidance of the garage affecting any of the house circuits might seem a good idea. If there was a load of garden lighting for example on the circuit then you'd defo want to keep it away from the house RCD, but as the OP states a garage used for storage then I'd be quite happy to use the house DB RCD for any required additional protection.

I was thinking the same thing with a separate cu, it would avoid nuisance tripping on the main house cu.

 

[Octopus]

I was under the impression that rcds needed to be at the origin of the installation, and in the case of a sub-main in my case the garage board being the origin? Therefore tripping can be resolved in the garage.

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

Connect up your MFT in the garage and test the RCD - then see which one will trip first.

 

[CraigL]

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?

 

[Octopus]

A better solution is a RCBO board....

 

[westward10]

Thanks for the input all. The simple solution seems to be swap out the rcd in the garage board for a main switch.

However 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?

That would be one way or use any cable but install to a method which doesn't need additional rcd protection.

 

[CraigL]

That would be one way or use any cable but install to a method which doesn't need additional rcd protection.

Sorry I edited my post during your reply - would appreciate your feedback on my revisions pal?

 

[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.