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

 

[Midwest]

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.

Not sure that's an approved way of obtaining discrimination between RCD's, at least as suggested by manufacturers.

 

[Taylortwocities]

You cannot discriminate RCDs on this basis. An RCD has differing trip characteristics - even depending on where in the sine wave the fault occurs.

You can ONLY discriminate RCDs RELIABLY on the basis of TIME.
If there is a standard 30mA RCD downstream, then use a specific time delayed RCD at the origin.
Otherwise its a lottery as to which RCD will trip first.

 

[Midwest]

I think he's just pulling our legs

 

[Stejjsush]

Hi CraigL
Sorry for the late reply. Hopefully it's sorted now. If not, cheapest and most pragmatic solution is to just fit a time delayed rcd ( s type) on the side of the board the 6mm t&e is feeding the circuit from. Your discrimination at that point is achieved. If you have an issue with the garage circuits the garage rcd will go before the rcd at the origin.

 

[Taylortwocities]

He's already got a 30A RCD at the house end. Adding an S type RCD won't fix the issue.

Feeding the garage from a non-RCD protected way or (better) from a separate switch fuse is the way to do it.
But we are now on page 4 of a circular discussion. My Merlot is demanding my attention.

 

[Michaelwgroves]

Easiest/most expensive option is RCBO in house CU and change RCD in garage to main switch.
If this is not possible, MCB in house CU, terminate SWA in plastic box next to garage CU, continue tails only into garage CU as is, install earth rod to create TT system. This means cable is protected by house RCD and garage protected by its own RCD, therefore reducing nuisance tripping.

 

[daveb]

I've a similar scenario next week on a house I changed the mains board on last year. My client now wants a separate DB feeding his garage. The garage is joined onto the house and I know there is one spare way left on the new dual RCD board. So, the BG garage consumer unit I've bought for the job only comes with a 40a RCD mains switch so I've purchased a separate 100a double pole mains switch and will achieve RCD protection from the mains board in the house. Job done.

 

[DPG]

Easiest/most expensive option is RCBO in house CU and change RCD in garage to main switch.
If this is not possible, MCB in house CU, terminate SWA in plastic box next to garage CU, continue tails only into garage CU as is, install earth rod to create TT system. This means cable is protected by house RCD and garage protected by its own RCD, therefore reducing nuisance tripping.

Not convinced TTing is a good option here.

 

[Taylortwocities]

You'll still have two RCDs in series.
Anyway, I've said my bit. Do what you want, change the garage main switch - just don't be surprised when you come back from holiday and find your freezer defrosted because of a fault in the garage.

 

[HandySparks]

...This means cable is protected by house RCD and garage protected by its own RCD, therefore reducing nuisance tripping.

Eh?

 

[DPG]

Eh?

Yeah I'm not sure where he was going with this one!

 

[Taylortwocities]

Its always best to actually read the topic's posts before posting......

 

[Michaelwgroves]

What bit don't you understand, I'm happy to explain?
I assumed the comment on still having 2 RCD's in series was about another post as my solution does not have 2 RCD's in series.

 

[HandySparks]

What bit don't you understand, I'm happy to explain?
I assumed the comment on still having 2 RCD's in series was about another post as my solution does not have 2 RCD's in series.

Isn't this two RCDs in series?:

...If this is not possible, MCB in house CU, terminate SWA in plastic box next to garage CU, continue tails only into garage CU as is, install earth rod to create TT system. This means cable is protected by house RCD and garage protected by its own RCD, therefore reducing nuisance tripping.

 

[Taylortwocities]

What bit don't you understand, I'm happy to explain?
I assumed the comment on still having 2 RCD's in series was about another post as my solution does not have 2 RCD's in series.

I'll try and explain it to you,..........

The house consumer unit is one with two RCDs already.
As far as we can determine, it is not/cannot be adapted as a board with non-RCD protected ways.
So, if you put an RCBO in there you then have two RCDS at the house end .
AND THEN
The garage consumer unit has an RCD already.
So your solution means there would be THREE 30mA protection devices in a row.
And that's better?

Blimey

 

[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?