Safe to trip the RCB and fit a new CU?

[Jon Nicoll]

Hello All I am thinking of replacing our existing consumer unit, to be able to add a couple of new circuits (garage lighting/power, and a spur for an electric shower).

Originally I was thinking that I'd have to either get the LEB to do the final fitting of the tails, or do as a few in this newsgroup have advocated and break open the LEB seals to wire this bit up myself.

However, when having a proper look this morning I was pleased to realise that there is a 100mA RCB in circuit just before the CU. It looks unsealed (although I can't quite see yet how to open it up from my cursory inspection).

So - assuming that I can get access to the output terminals of this, it should be safe to trip the RCB and connect up the tails to the CU myself, yes?

A supplementary question - is there any mileage in removing this RCB and fitting one in the new CU? I'm thinking of neatness, really, since I shall be fitting at least one RCB (for the garage circuit) in the new CU anyway.

Thanks for any thoughts

Regards Jon N

[Christian McArdle]

What type of earthing system do you have? If you have TT, I'd advise ensuring that it is a time delay type. If so, keep it. Otherwise, replace it with a time delayed 100mA type. If you have TN, then it is probably unnecessary, unless someone bodged a system together with poor earth loop impedence and fitted it for this reason.

It might be possible to use the new time delay RCD as the main incomer for the new consumer unit.

In any case, consider using 30mA RCBOs for socket (and other high risk) circuits. If you don't want to fork out, fit a split load unit and have high risk circuits on the RCD side. Ensure that lighting, alarm and fridge freezer circuits are on the non-RCD side.

Christian.

[Jon Nicoll]

Hi Christian Thanks for your reply.

AFAICT the earthing systems is a 'standard' TN-S system. At least, the earths appear to be bonded to the sheath of the incoming cable.

Several aspects of work done on the house appear to me to be 'bodged', but I haven't seen any great problems with the wiring so far (apart from the wiring to the old garage, which I've just replaced!).

I'm not sure what you mean by this - can you explain this a little more?

Yep, thanks for the suggestions.

Regards Jon N

[Christian McArdle]

In which case, the RCD is probably not necessary, and may actually reduce the safety of the system due to the possibility that it will trip the lighting circuits in inappropriate circumstances. However, I wouldn't remove it until ALL the circuits are tested for appropriate earth loop impedence, in case it was fitted as a bodge. Also, you need to get appropriate RCD protection for high risk circuits in before it is removed.

As you have TN-S earthing, I would recommend not having a whole house RCD anyway. However, what I meant was that the consumer unit will usually come with an isolation switch incomer. This can normally be replaced by a DP time delay RCD. This acts as the main switch AND a whole house RCD.

Christian.

[Jon Nicoll]

Hi Christian

[...]

OK, got you. I think I will do things in the following order:

1) fit the new CU with the standard isolation switch incomer, using RCDs as you mention for power etc. 2) get the newly-wired circuits checked out 3) remove the old RCD if deemed unnecessary, or get the problem(s) found in (2) fixed first.

Thanks for your advice Jon N