House electrics question

[Chris Hogg]

I'm planning the rewiring of my late mother's 1960's bungalow, that we will eventually move into (it will be done professionally, not by me). At present it has an overhead TT supply and an old CU with wire fuses. I'm proposing to have the CU replaced by a split load unit, with RCBO's on ring mains, and MCB's on lighting, cooker, immersion etc circuits.

A post here from many years ago said that with a TT supply, the CU should be preceded by a 100A 100mA time-delay RCD. Here in our existing TT-supplied bungalow, we have such an RCD fronting a split load CU, although with just a single RCD for the half of the CU with the MCB's on the ring mains (no RCBO's), and a single isolating switch on the half with the MCB's on the lighting, cooker and immersion circuits.

I assume that with RCBO's on the ring mains, the single RCD in the CU isn't required, but what about the time-delayed RCB that fronts up the whole CU. Is that still needed? Is there a better arrangement than that which I'm proposing, now that RCBO's are commonly used? Is it still best practice to have MCB's on the lighting and selected power circuits with a single isolating switch for them all together?

[John Rumm]

With a TT install (note you may find that you have the option to upgrade to PME - check with the supplier), you need RCD protection for all circuits, since you won't be able to rely on a fuse or MCB having adequate trip current on a fault to earth.

Traditionally that was done with a whole house RCD at the front end, and then a cascaded one for the high shock risk circuits. The front end one had a time delay to ensure discrimination, and a high enough trip threshold to make nuisance trips unlikely. However its 100mA trip would offer no shock protection - only fire and equipment protection.

These days the 17th edition rules on cable protection often make it easier to install 30mA trip protection for all circuits. A basic solution being a pair of normal 30mA trip RCDs with the circuits shared between them (i.e. your basic 17th edition CU[1]). The better solution being RCBO protection on most or all circuits. Generally these arrangements will mean you don't need the front end time delayed RCD as well.

[1]

[Chris Green]

Isn't RCD protection for all circuits a general requirement now? That's why CUs come with two RCDs or all RCBOs nowadays.

Ah, you've said what I thought was the case. Not *mandatory* but easier than the alternatives.

[Tim Watts]

Indirectly, but not directly.

eg you could still have your lights on a non RCD circuit, if you can run the cable in way that does not require RCD protection (eg all surface runs, or mechanically protected).

In practise in most houses this means all RCDs :)

[Scott]

I put RCBOs on all the circuits - ring main, lighting, shower, even the alarm system. This seems to me to be much better as any fault is isolated to one circuit (and more easily traced). .

[ARW]

That is, without a doubt, the best way forward if funds allow it.

[Scott]

Well, the electician although initially surprised later conceded that the cost was lower than he had expected.

[ARW]

No doubt cheaper now then when you had the work done.

And ISTR yours was a Crabtree CU and they are not the cheapest of makes.

[Scott]

You must have an amazing memory and/or filing system. You are perfectly correct.

[Chris Hogg]

Many thank for all the replies. RBCO's on all circuits it will be then.

[ARW]

You are probably the first person that posted about having fitting all RCBOs fitted and doing the job properly.

[John Rumm]

No - at least not quite. There is a requirement for general purpose socket circuits, electric showers, and circuits where the cables lack earthed metallic protection or those which are not buried more then 50mm below the surface when concealed.

So generally most circuits will need RCD protection, but there are occasions where its not required or even desirable (e.g. a sub main feed to an outbuilding)

[John Rumm]

Generally the best option unless you have special requirements like feeds to outbuildings or want circuits with reduced trip risk (e.g. freezer, tropical fish tank etc)

[dennis]

Fish aren't a problem unless it trips while you are away. They will be OK for the best part of a day.

[John Rumm]

Same could be said for a freezer or boiler. However if you are away for a week, a trip on any of them could be unwelcome.

[damduck-egg]

Fish and Trips only happens on Fridays.

G.Harman

[John Rumm]

Eat out, or at your plaice?

[ARW]

May I suggest that you fit or at least price up a Hager CU. Hager RCBOs are now only about £22-23.

I have fitted and worked on thousands of CUs and IMHO these have always been the best.

[Scott]

Better than a fire though.

[Tim Watts]

Adam told me the same, 6 years or so ago.

I retested my RCBOs and all RCD parameters are well within spec. Nothing gets hot (well 45A RCBOs under full load for 30 minutes get warm, but that's expected).

Very solid kit.