Garage Electrics

[jeff oakley]

I am rewiring my Garage, which has a separate fuse box at present. This is an old wire type not RCD one.

I want to replace that with a new consumer unit with 4 circuits, one for lights, one for a ring main, one for the compressor and lathe. One will be spare.

The question is the house has a RCD protected type unit which feeds the garage, but all the new ones I see have an RCD built in these days. Can you run with two RCD unit one in the house feeding the garage and one in the garage?

Might seem a dumb question and if you cannot does anyone know of a 3 or 4 way consumer unit that si available

Thanks in advance

[Deanspoors]

There is no problem in having two RCD's in series, it's called discrimination, this adds redundancy, and allows a sub-circuit to trip without affecting the parent circuit. You may find theres a 50% chance that the RCD in the house will trip first, meaning that anything else on that RCD will also be tripped. this could be a problem if you have a beer fridge, fish tank, etc on the same circuit. The only way you can assure the that the garage RCD will trip first would be to use an S type RCD upstream and a general RCD downstream BUT you would have to make sure you can use an S type RCD in your house Consumer Unit, i'm not sure you can, it would probably have to be individual to the rest of the circuits. i.e. only supply your garage. 

When you're speccing your MCB's make sure you get the correct type current inrush, e.g a lighting circuit would be more typical a type B (low inrush) (unless it's fluorescent lighting which would be type C. A motor circuit may need a little extra current surge time so i.e. the lathe maybe more suited to a type C MCB. 

[CraigHew]

I've got an RCD in the garage feeding my external garden lights. unfortunately when a bulb fails (admittedly not that often as they're LEDs) the main house RCD trips when the garage one does.

Will investigate the S type as a better option.