Electricians – Garage consumer unit not tripping?

My garage has a power supply with it’s own RCB on the house consumer unit.

It used to just have a switched fuse spur box thing.

I replaced it with garage unit which has separate breakers for the lights and sockets.

It tripped last night but it doesn’t trip at the unit in the garage but in the house.

Why didn’t it trip in the garage unit? Is it an earth problem?

Do I need a better earth connection in the garage.

Does it have RCBO’s? They are more resistant to tripping than RCD’s so the house unit might be more sensitive. Or is it just a basic trip switch with no RCD protection (i.e it will only trip when overloaded or shorted out, whereas the RCDs in the house will monitor for leakage to earth/human)

Have you tried the test buttons on the unit in the garage?

The garage unit is one of these http://www.screwfix.com/p/bg-garage-kit-enclosure-5-module-ip55-40a-rcd-dual-6a-32a-mcb/68849

So I think they are RCD’s?

The breaker in the house unit is marked as a “RCBO”.

I have tried the test switch in the garage and it’s fine.

Thats the limit of my knowledge then! There must be a tolerance as my extension lead RCDs will always trip before the house.

You only need one rcd on any given circuit. As you have 2 it’ll be pretty random which one trips. You need to get rid of one. If you want to be able to reset it locally the swap the rcbo for an mcb, although if you do this you will lose rcd protection for the cable so it needs to correctly specced and located. You haven’t added any extra protection with your new garage consumer unit, what where you hoping to achieve?

You haven’t added any extra protection with your new garage consumer unit, what where you hoping to achieve?

Just tidying things up a bit. It was only £20. Also I can isolate the lighting circuit or the sockets if I want to.

You need to get rid of one.

Why? If the main circuit is protected anyway by the RCBO is there any harm in having extra/redundant RCD’s on the circuit?

Is it any different from an extension lead with a RCD?

It doesn’t do any harm having 2, it just means one of them will trip but you won’t know which one. If you just wanted to isolate the lights then it would’ve been easier to have them run from a fused spur from a socket (As you had before) Again no problem with an overkill solution but it doesn’t add much.

Have you tested it with proper kit? I’d be a bit wary if it was too cheap as there are a few fake rcds out there. It probably isn’t, these unit are cheap now as the rules change next month making a lot of them not suitable for new installations

I’d be a bit wary if it was too cheap as there are a few fake rcds out there.

Even legit ones can fail, I had a brand new British General RCD fail the 30mA test – it tripped at about half that every time.

As for two RCDs in series, ideally you want to change the one in the house to be the delayed type, but you should only do this if the only circuit it protects is the garage Consumer unit.

Vaguely related mini thread hijack!

I replaced a light fixing yesterday; in doing so I think I touched the earth to the neutral while pulling the cable through the hole in the fixing and tripped the RCD, even though everything was off at the consumer unit (I checked with a multimeter and everythink).

I’ve done the same thing before doing a different light a while back.

Is this right, that the RCD trips even with the circuit off? If not, what can cause it, given everything works and the RCD is happy the rest of the time? In short, am I going to die?

That’s normal phiiiiiil. Even with the mcb off the neutral is still connected. Nominally the neutral and and earth are both at zero but there might be a tiny potential difference.

Is this right, that the RCD trips even with the circuit off?

A neutral to Earth fault will trip the RCD.

The isolation switch (in the UK) is single pole and only isolates Live.

That makes sense, thanks. I’m surprised isolators are only single pole, though, I assumed they isolated both. Every day is a schoolday…

I’m surprised isolators are only single pole

In France they’re normally dual pole. Single Pole is a PITA as you isolate a circuit to work on it then still trip the whole house with a N-E fault.