I've just fitted an RCBO (in Wylex consumer unit) into a circuit previously only protected by an MCB in the consumer unit and plug-in RCD adaptors.
Is it possible to have too much protection on a ring circuit (protected by RCBO in consumer unit) by adding plug-in RCD adaptors for individual appliances? Will it simply be a case of the more sensitive item tripping first in the event of a fault or will there be any unpleasant consequences?
Cic.
unpleasant no. Both rcds will be the same sensitivity, so the one more preloaded will generally trip first, ie the one in the CU. In short, the plugin rcd will be useless, but not harmful.
NT
Cicero used his keyboard to write :
The idea should be that the more local the circuit, the more sensitive the tripping should be, so that they are first to trip rather than the main ones.
However there will be no unpleasant consequences even if the trip current is the same - other than uncertainty over which will trip. The only consequence might be your fridge or freezer circuit tripping whilst you are out and the contents defrosting - something to be aware of.
Harry Bloomfield wibbled on Saturday 13 March 2010 19:30
I think in practice, if the fault exceeds the trip current of both devices, both will almost certainly trip as they will have "committed" to the action.
This is the reason upstream whole house RCDs for use when you have a TT system must be a time delayed type. The fact it may have a trip current of
100mA won't necessarily guaranteed discrimination in itself for faults > 100mA.But it is harmless, in the sense that the user will be no worse than if they didn't have an RCD in the plug, although arguably worse off than if they didn't have a whole circuit RCD. But no-one will die as a result, just be a little more inconvenienced at worst.
That doesn't work with RCDs. Usually, they'll both trip if the trip current of both of them was exceeded, as the trip time is not related to the extent of the excess current over the tripping current, unlike a fuse.
If you want discrimination with RCDs, you have to use a time delayed RCD on the supply side, but there's no p[oint here.
I agree. The OP has is no use for the plug in adaptors anymore on that circuit (assuming that all RCDs are 30mA)
Adam
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Many thanks all for the informative and reassuring replies.
Just a supplementary question related to the new situation. My CH boiler is currently connected via an FCU (switched fused spur) which is on the same ring as the newly installed RCBO. If I've understood correctly this FCU is now redundant in the same way that the plug-in RCD adapters are redundant. Would it be correct to remove the FCU and replace with a simple connection box?
Cic.
Nope, keep the FCU - the RCBO will ensure safety as in human to earth for the ring plus the spur, the FCU will deal with overloads *on the spur* - the spur fuse is likely to be 5A, the ring's RCBO will be 32A - the FCU will disconnect the boiler spur in the event of a short , leaving (one hopes) the rest of the ring operational.
HTH, Dave H.
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The FCU's not redundant, keep it. An RCBO is simply a combination of an MCB and an RCD in one lump.
Definately not.
The FCU will supply power to the boiler with a 3A fuse and allow double pole isolation of the boiler and all of it's controls.
The 3A fuse will protect the boiler in the event of a live-neutral short on the boilers cables. A 32A RCBO cannot do that as it is "looking" for a small live earth fault (>16mA) or a large (>>32A) live neutral fault.
Adam
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Thanks again, all, for further information and advice. I'll be keeping the FCU which makes one less item to worry about.
Cic.
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