Electrical Question

tony6403

I have noticed that the consumer unit in the house has 16A MCB for the garage circuit.
The CU in the garage has a 32A MCB for the sockets.
Both are on RCD.
Should they both have the same rated MCBs ? and is there any issue with two RCDs?
I would be grateful for advice.

muckybutt

A bit of bad design there !

Do you know what size of cable it is that supplies the garage ? that will dictate what size MCB should be being used to supply the garage board.

As for the RCD's again bad design to have two on one circuit - completely unnecessary. If you have one on the consumer unit then that protects the whole installation including the wiring to the garage.

ValHaller

Whoever did that had not a clue about circuit discrimination - the idea that the protective device which protects the most downstream part of the installation towards the fault should be the one to blow.

The 32A MCB for the garage will never blow, of course, so going on what MB says, all cabling to and in the garage should be adequate for a 16A breaker.

On the RCDs, regardless of current rating ordinary RCD's do not discriminate in any meaningful sense. Even if you have a 100mA RCD feeding a 30mA RCD, any earth fault is likely to exceed 100mA, so both or either could trip.

You can have RCD's in series so that they discriminate, but the upstream one has to be a time delay RCD, which allows 100ms for the downstream RCD to trip. This obviously reduces the protection given to circuits only protected by the upstream RCD and is not good if one of those circuits is feeding an electric lawnmower for example.

tony6403

Thanks for the very helpful replies . It's a 2.5mm cable.

muckybutt

Ideally then dependent upon length of cable etc you want a 20A MCB at your consumer unit, then in the garage the 32A one wants down rating to 16A that way giving you correct discrimination.

As for the RCD's I would leave them as they are unless they prove to be a PITA if they trip, at least with the correct ratings set up any short circuit or over current in the garage will trip that MCB and hopefully the RCD in there as well. Only time will tell.