CU replacement - specs?

[Lobster]

I'm shortly going to be renting out a property which I'm buying, and before I do I'll be getting the electrics checked over.

Before that - pre-empting the results of the PIR - I think I should probably upgrade the CU to a modern RCD-protected job, even if only a butt-covering exercise (and I'll also first need to install main equipotential bonding twixt gas and water pipes).

I'm aware I may be opening a can of worms by upgrading the CU and then finding the RCD won't set owing to a hitherto undetected issue with the existing wiring, but I'm accepting that. I'm not intending to touch the existing wiring as such, unless necessary - it looks OK - ie I'm not into adding any new circuits etc.

I would be very grateful for advice as to what specification of CU to go for, especially as I'm not familar with the 17th edn regs.

Existing syatem seems to have 1 lighting circuit and 1 ring main (not sure why there are apparently 3 circuit breakers though?):

What would be most appropriate as a like-for-like replacement - a 2-way (3-way if needed?) CU, with full RCD protection? Or split-load with the lights not on RCD? Something else?

Here's the current configuration, which no doubt will prompt other questions!:

Thanks David

[Owain]

15A for an immersion heater, probably.

Under 17th Edition

- all circuits need RCD protection (with limited exceptions for specific cable types/mounting methods)

- whole-house RCD is not permitted

Presumably the property is on the small size, but it would be wise to allow for future needs (and the special offer packs of CU+MCBs are usually about 10-way) which usually work out cheaper than buying a smaller CU with main switch and using RCBOs for every circuit.

MK 16 module, 10 way, populated Dual RCD Consumer Unit including MCBs is =A380 on scrooficks. Gives plenty of room for expansion for an electric cooker, shower, additional lighting and power circuits etc.

Bear in mind if you are renting it out and all the lights fail, that needs an emergency response, if the ceiling lights fail but the wall lights still work that gives you more breathing space to get it fixed.

Don't forget mains smoke alarms.

Owain

[ARWadsworth]

I just don't fall for this "the lights have gone out so I will fall down the stars" routine.

We do not turn into lemmings when the lights go out.

[ARWadsworth]

I would use a 5 way CU and 3 RCBOs. The 15A MCB would probably have been for an immersion heater.

It looks like some bonding is already done.

[A.Lee]

Would you do a Periodic before starting a CU change on something like this? My parents want theirs changing, but being as most of the wiring is 50 years old, I'm not going to be changing anything until I get it all tested. They really need a full rewire, but they dont want the re-decoration that would involve, so it'll be a compromise of fitting a CU with proper bonding. Ta Alan,

[ARWadsworth]

If I was at work then yes as a) I could then warn the customer that there will be extra costs as the RCD is going to trip and b) I can use the results on the certificate I need to issue.

If it was for a property that I owned then no. I'd do the PIR after the CU swap and fix any faults as required. A PIR for 3 circuits should not take long.

You need to do the test and fix any faults first. You know that your mother will complain if the sockets trip:-)

Are you intending to fit a 17th edition compliant CU? And

applys to your parents.

Cheers

[Owain]

They're tenants.

A wise landlord will already have put a 'no candles' clause in the tenancy anyway.

Owain

[Lobster]

Thanks for the advice.

Thinking on this further a bit - the reason for making the change is purely to make it safer for tenants and to cover my butt (see above!) - I have no plans at all to extend the wiring. I'd like to provide RCD protection if possible, for safety reasons.

Looking at the current CU, it looks as if original wired fuses have been replaced by MCBs - is it possible to get hold of plug-in RCBOs in the same way? If so, it seems that would meet my requirements in full?

If not... then how important does the panel reckon it is that I do upgrade the CU? I'll certainly be getting the house wiring checked anyway, and one improvement I could easily do is replace the double socket by the back door (which could be used for electrical appliances in the back yard) with an RCD-protected item.

Indeed, but AFAICR not to the water main (on the opposite side of the house!)

Thanks David

[Andy Burns]

Yes.

No, because they need neutral connections too.

[ARWadsworth]

I am afraid not. And if I could invent one I would be very rich.

Under the 16th edition RCDs were required on sockets that could reasonably be expected to supply portable equipment outdoors.

Now that is a wide definition. Could you reasonably expect someone trying to cut the grass with a faulty lawnmower that trips the RCD socket to then try it in another non RCD protected socket in the house?

The 17th edition is a little tighter in it's rules - it requires all sockets to have RCD protection apart from a very few selective sockets that almost certainly do not apply to your installation.

Assuming that the place is a long term investment and you can change the CU yourself for less than £100 (and still have the electrician check it all out and issue a PIR for the same price as checking out the old fuse box and issuing a PIR) I would say swap it. Your arse is coverered and apart from the PIR it is DIY.

Bugger! Never mind.

[george [dicegeorge]]

You could put an RCD before the consumber unbit which would then trip everything if theres a fault- so maybe also put a few battery maintained emergency lights at the top of the stairs.

If you rewire you legally have to put in mains powered fire alarms, i believe. [g]

[ARWadsworth]

No, you don't have to install fire alarms or smoke alarms with a rewire.

[Lobster]

Am revisiting this thread as am now near the time when I need to take the plunge, plus I've done a bit of reading up on the 17th edn jungle.

Having looked at the old CU again, there are clearly only actually 2 active circuits - one lights, one ring main. While I have no plans to rectify that far-from-ideal situation, I can imagine it might prove necessary to divvy up the current circuits or add new ones at some point in the future.

So - am wondering: can't I achieve the same (and equally compliant) level of protection by using a 17th edn dual-RCD CU with standard MCBs, instead of 1 RCBO per circuit? - ie, lights on one RCD and ringmain on the other? Or am I missing something? AFAICS it would be a slightly cheaper way of doing it anyway; and it certainly would be if I did ever want to add new circuits in the future.

Thanks David

[ARWadsworth]

No reason why not.

The price difference between a 5 way CU with 2 RCBOs and a 17th edition CU is minor, but the cost to add circuits on the latter is much cheaper.

[Lobster]

Great, thanks for the reassurance Adam: glad I've apparently correctly got my head round it!

David

[Lobster]

Well that was lucky! Because having bought the 17th edn CU I've just discovered that actually there are three circuits, in that what was evidently formerly a radial for an immersion heater is now used by the combi boiler instead.

So my next question - given that I will have one ring main and one lighting circuit, each on separate RCDs - which side should I wire in the radial for the boiler, with the ring main or lights? Worried about loss of frost protection in the winter when an RCD trips...

Thanks David

[ARWadsworth]

You have never said what the property is used for. Is it occupied? If so then stick the boiler on the same RCD as the lights. Most people tend to notice when it is cold and the lights are not working.

[Lobster]

Just a normal residential home... so in terms of frost protection I suppose the main concern would be when nobody's home, eg the classic going away for the Xmas holidays?

So thinking about what might cause a trip while the home is empty... security light bulb (or inside light left on) blowing? faulty appliance left plugged in? Dunno. I suppose it's best on the lighting circuit (though from personal experience in my home, every time a bulb blows, my RCD pops)

David

[ARWadsworth]

Faulty fridge or freezer, faulty CH pump:-)

I suppose if we have a full week below zero degrees that you can pop in and check if the place is left empty for that week. Of course you will have to check the gas and electric meters are not pre payment meters with no credit left on them

There is not really much to over engineer on your setup. And don't forget there are thousands of people out there with the CH on RCD protected circuits that have no problems.