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

 

[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?