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

 

[Michaelwgroves]

No, Garage CU is not connected to house earth. It's now a separate system, hence TT.
Only the cable supplying the garage is protected by the house CU.
Any faults on either the garage lights or sockets will only trip the garage RCD.
If however you put a pick axe through the SWA this will trip in the house only.
You may not need to protect the SWA with RCD, more detail would be required for this debate. But I always would unless good reason not to.

 

[davesparks]

No, Garage CU is not connected to house earth. It's now a separate system, hence TT.
Only the cable supplying the garage is protected by the house CU.
Any faults on either the garage lights or sockets will only trip the garage RCD.
If however you put a pick axe through the SWA this will trip in the house only.
You may not need to protect the SWA with RCD, more detail would be required for this debate. But I always would unless good reason not to.

No, a fault in the garage will trip either or both RCDs, separating the earthing systems will not prevent the RCD at the house me from tripping.
An RCD looks at the balance of current flowing out through the live and returning through the neutral, An imbalance of more than 30mA will cause it to trip.
It does not matter how or where this imbalance has occurred or which earth reference the current has returned through, it will see the imbalance and trip.

To achieve what you have suggested you would need to install an isolating transformer to completely separate the two installations. This would then need a N-E link via earth electrodes on its output side to create a TNS system.

 

[Michaelwgroves]

Dave, thanks for input. I clearly need to update my understanding.

 

[Michaelwgroves]

No, a fault in the garage will trip either or both RCDs, separating the earthing systems will not prevent the RCD at the house me from tripping.
An RCD looks at the balance of current flowing out through the live and returning through the neutral, An imbalance of more than 30mA will cause it to trip.
It does not matter how or where this imbalance has occurred or which earth reference the current has returned through, it will see the imbalance and trip.

To achieve what you have suggested you would need to install an isolating transformer to completely separate the two installations. This would then need a N-E link via earth electrodes on its output side to create a TNS system.

Dave, unsurprisingly you are correct. Had time to check my understanding this morning. Obvious when you take time to think about it.

So best solution would be pull tails from house CU into service block. Then new tails into house CU and new RCD enclosure with time delay RCD. Then SWA to garage CU with RCD.
Cheap simples and best soloution. Any fault in garage will only trip garage RCD, with SWA cable also protected by time delay RCD.

 

[Wilko]

Dave, unsurprisingly you are correct. Had time to check my understanding this morning. Obvious when you take time to think about it.

So best solution would be pull tails from house CU into service block. Then new tails into house CU and new RCD enclosure with time delay RCD. Then SWA to garage CU with RCD.
Cheap simples and best soloution. Any fault in garage will only trip garage RCD, with SWA cable also protected by time delay RCD.

Wellll, what protects the SWA from overcurrent fault ?

 

[Taylortwocities]

Michael, you have missed some essential points.

1. The new tails (heading towards the garage) MUST pass through a current limiting device. RCDs do not limit the current.
So a switch/fuse is needed at that point (I think I detailed this in this thread about 6 months ago. I CBA to read all the way back).
The fuse needs to be appropriate for the SWA.

2. A time delay RCD (or any RCD) is NOT necessary at the house end.

 

[Michaelwgroves]

Michael, you have missed some essential points.

1. The new tails (heading towards the garage) MUST pass through a current limiting device. RCDs do not limit the current.
So a switch/fuse is needed at that point (I think I detailed this in this thread about 6 months ago. I CBA to read all the way back).
The fuse needs to be appropriate for the SWA.

2. A time delay RCD (or any RCD) is NOT necessary at the house end.

Absolutely agree about switch fuse, schoolboy error.

I agree time delay "might" not be necessary, but it gives another level of protection I think is worth the extra small cost.

 

[Taylortwocities]

I think is worth the extra small cost

Time delay RCDs come in at £80+

Imo, bit more than a "small" cost! plus an enclosure.

 

[davesparks]

Dave, unsurprisingly you are correct. Had time to check my understanding this morning. Obvious when you take time to think about it.

So best solution would be pull tails from house CU into service block. Then new tails into house CU and new RCD enclosure with time delay RCD. Then SWA to garage CU with RCD.
Cheap simples and best soloution. Any fault in garage will only trip garage RCD, with SWA cable also protected by time delay RCD.

Why the time delay RCD?

 

[davesparks]

Re-reading the OP there is T&E for part of the run so it may need 30mA protection for that.

 

[davesparks]

I agree time delay "might" not be necessary, but it gives another level of protection I think is worth the extra small cost.

Level of protection against what? It does not provide shock protection and the fuse is more than adequate for fault protection.

 

[Midwest]

Level of protection against what? It does not provide shock protection and the fuse is more than adequate for fault protection.

Don't you start, had a similar long debate, a while ago.

 

[Michaelwgroves]

No, please do start as this thread has improved my knowledge which has got to be a good thing. And hopefully for others.

Agreed time delay RCD does not provide shock protection, but it does still provide earth leakage protection. So I'm still thinking it's a good thing ?

 

[davesparks]

No, please do start as this thread has improved my knowledge which has got to be a good thing. And hopefully for others.

Agreed time delay RCD does not provide shock protection, but it does still provide earth leakage protection. So I'm still thinking it's a good thing ?

Why does it need earth leakage protection?

 

[Michaelwgroves]

If someone working in the garden accidentally hits it, it will trip early. Rather than not at all if it was an Earth fault?