Extra RCD's when there is one behind the Consumer Unit?
- Thread starter JAmmmmm
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It would depend on the rating of the 'upfront' rcd, this setup however contrvenes the regs.
It has a big downside in that if your pond pump leaks water, all the food in your freezer goes off.
One fault in a minor, non - essential device (pond pump?) would trip the 'front end' RCD, cutting power to everything in the house(including your freezer).
'Front end' RCDs are usually set up to protect your entire consumer unit from damage if a serious fault occurs, not to protect people against potentially fatal shocks from faulty appliances. That's the job of seperate RCDs installed on individual circuits or groups of circuits.
If an accident should occur, an individual RCD should trip whilst you are still alive, but a front end RCD may not.
One fault in a minor, non - essential device (pond pump?) would trip the 'front end' RCD, cutting power to everything in the house(including your freezer).
'Front end' RCDs are usually set up to protect your entire consumer unit from damage if a serious fault occurs, not to protect people against potentially fatal shocks from faulty appliances. That's the job of seperate RCDs installed on individual circuits or groups of circuits.
If an accident should occur, an individual RCD should trip whilst you are still alive, but a front end RCD may not.
Sounds like a TT earth system, have you an earth rod ?
TT systems require front end rcd's before the fuse board. They tend to be 300 mA which means they are 10 x less sensitive than the rcd(s) you need in a board, which should be 30mA.
So on a TT system with requirement for rcds to cover the supply and the fuse board circuits you would have:-
main supply- main 300 mA rcd- feed to fuse board.
At the fuse board, to prevent losing all services 2 x 30mA rcd's with the services (house circuits) fanned out between the 2 x rcd's. This then means if something does go pop and 1 rcd trips then you still have live circuits on the other rcd.
So you would have say 1st floor lights on rcd A and grd floor lights on rcd B. A pops and the house won't be in darkness because B is still energised.
RCBO's are better still, although expensive (circa £35 each) they do the job of both a rcd and an mcb. An RCBO protects only the circuit that uses it. If you have 8 circuits, you'd need 8 RCBO's. The benefit is that if a circuit pops the rcbo goes off, but it will only affect that circuit-no others,
TT systems require front end rcd's before the fuse board. They tend to be 300 mA which means they are 10 x less sensitive than the rcd(s) you need in a board, which should be 30mA.
So on a TT system with requirement for rcds to cover the supply and the fuse board circuits you would have:-
main supply- main 300 mA rcd- feed to fuse board.
At the fuse board, to prevent losing all services 2 x 30mA rcd's with the services (house circuits) fanned out between the 2 x rcd's. This then means if something does go pop and 1 rcd trips then you still have live circuits on the other rcd.
So you would have say 1st floor lights on rcd A and grd floor lights on rcd B. A pops and the house won't be in darkness because B is still energised.
RCBO's are better still, although expensive (circa £35 each) they do the job of both a rcd and an mcb. An RCBO protects only the circuit that uses it. If you have 8 circuits, you'd need 8 RCBO's. The benefit is that if a circuit pops the rcbo goes off, but it will only affect that circuit-no others,
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RCD's in the main are designed to trip between half and full rating so where a number of RCD's are used then each one is selected at a third of the rating of preceding one.
So with a TT system having a main RCD of 100ma as main isolator that then could feed a few MCB's for cooker and freezer etc without any further RCD plus two 30ma RCD's these would then feed MCB's and for outside electric's one could fit a 10ma RCD socket.
This would mean three RCD's supply the electrics outside. However there is no requirement to set up in that way.
As to a 30ma RCD feeding a 30ma RCD there is only one place that is done and that is the supply to caravans. In the case of caravans I think the regulations require both the site supply and the caravan to have RCD protection in case one of them does not follow the rules. Since the caravan site RCD has to disconnect all live conductors including the neutral it is better than the caravan one anyway so there should be no need for a caravan RCD but they are required by regulations.
So returning to question yes fit a 10ma RCD but no point in fitting an extra 30ma RCD. The Plug in type of RCD are far cheaper than the fixed type and since you are still protected even if some one did unplug and remove it I would use that type.
PS If a major fault happens the 30ma and 10ma RCD's both are set to trip at 40ms so it will take out both together however if an item is slowly getting worse then 10ma will trip first. The 100ma if fitted is likely an S type which has a delayed trip time and that may hold. Having said that a worker who knocked a nail in cabin wall to hang up his coat took out 30ma in cabin then 100ma in board feeding cabin then 500ma at 1/2 second in our main distribution board and the 1amp at 1 second in suppliers board so a major fault can trip the RCD's all the way back.
So with a TT system having a main RCD of 100ma as main isolator that then could feed a few MCB's for cooker and freezer etc without any further RCD plus two 30ma RCD's these would then feed MCB's and for outside electric's one could fit a 10ma RCD socket.
This would mean three RCD's supply the electrics outside. However there is no requirement to set up in that way.
As to a 30ma RCD feeding a 30ma RCD there is only one place that is done and that is the supply to caravans. In the case of caravans I think the regulations require both the site supply and the caravan to have RCD protection in case one of them does not follow the rules. Since the caravan site RCD has to disconnect all live conductors including the neutral it is better than the caravan one anyway so there should be no need for a caravan RCD but they are required by regulations.
So returning to question yes fit a 10ma RCD but no point in fitting an extra 30ma RCD. The Plug in type of RCD are far cheaper than the fixed type and since you are still protected even if some one did unplug and remove it I would use that type.
PS If a major fault happens the 30ma and 10ma RCD's both are set to trip at 40ms so it will take out both together however if an item is slowly getting worse then 10ma will trip first. The 100ma if fitted is likely an S type which has a delayed trip time and that may hold. Having said that a worker who knocked a nail in cabin wall to hang up his coat took out 30ma in cabin then 100ma in board feeding cabin then 500ma at 1/2 second in our main distribution board and the 1amp at 1 second in suppliers board so a major fault can trip the RCD's all the way back.
Hi thanks for the replies. The way it works is:
Meter -> 60A Singe Fuse Box -> RCD -> Fuse Box (not mcb consumer unit)
We had a light once with developed a short (live touched earth) and it was plugged into a plug in RCD but the one at the fusebox tripped first, does that mean it's more sensitive?
PS don't have an earth rod
Thanksss
Meter -> 60A Singe Fuse Box -> RCD -> Fuse Box (not mcb consumer unit)
We had a light once with developed a short (live touched earth) and it was plugged into a plug in RCD but the one at the fusebox tripped first, does that mean it's more sensitive?
PS don't have an earth rod
Thanksss
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