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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.
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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.
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.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.
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?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
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 tripSo 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?
He has quite an unecessarily complicated installation.were it just a supply to a garage with a light and a socket in it i would not be to worried,
The garage board is a dual board with an rcd main switch. At present I have it connected with rcds on both sides,
Sorry by dual I didn't mean a split board - just two way, i.e. two breakers - purpose garage board.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.
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.
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.
Eh?...This means cable is protected by house RCD and garage protected by its own RCD, therefore reducing nuisance tripping.
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.
...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.
I'll try and explain it to you,..........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.
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.
Wellll, what protects the SWA from overcurrent fault ?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.
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.
Time delay RCDs come in at £80+I think is worth the extra small cost
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.
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.
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 ?
If someone working in the garden accidentally hits it, it will trip early. Rather than not at all if it was an Earth fault?
So that begs the question; what use does a time delayed RCD have?
We know it must be used for RCD's in series.
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