Consumer Unit Change Argument - Any suggestions?

(a) Is there anything *wrong* with the current set-up?

(b) If the installation isn't tested he could easily make some

Owain

I'm not saying it's right, but it happens all the time. As an absolute minimum, you need to be sure that you have correctly identified all the circuits, so you can make sure that your new consumer unit is laid out and labelled properly. You can do all the investigation and labelling of existing circuits without removing the existing consumer unit, so ultimately, you only go ahead if you are completely confident you know what's happening. If you realise that there is something "funny" with one of your circuits, you can investigate and fix it at your leisure, before you go for the "big day". Bear in mind that it can take several hours to do the change and your house will be without heating lighting and electric whilst you are doing it, so make sure you start early and have candles and torches available, etc.

As you have probably worked out this is quite a deep question. Much will depend on exactly what you have before, and why you want to change it.

In one sense if everything has been correctly designed and installed to the relevant standards applicable at the time, then the existing installation ought to be ok. There is not any inherent reason why having MCBs is "better" than fuses - in fact there are some downsides such as their propensity to trip when a bulb blows[1].

Some points to consider with regard to fuses:

You mention a split load CU, but neglected to mention if the current CU has any RCD protection. RCD's make a very substantial and worthwhile contribution to the safety of general purpose socket circuits. However if wired to cover the "whole house", then there are also negative factors since they can trip the lights in circumstances where this is itself a more likely serious injury hazard than a that of a serious electric shock. The magnitude of the benefit of having RCD protection will depend again on various factors:

So if you have 1 or 2, then a change from rewireable fuses to MCBs would be worthwhile.

If you don't have a RCD at all and 3 or 4 apply, then rectifying that should be a priority.

If you have 5, then either changing to a split load configuration or installing some emergency lighting should be a priority.

If the answer to 6 is TT[2], and you still have an old voltage operated earth leakage breaker then changing to a RCD should be a priority.

If 7 is not to scratch and there is no RCD then you also have inherent dangers within the installation.

Testing....

One of the attractions of a change to a CU is that the battery of tests one would perform at the time can help locate any number of potential hazards. If you perform the change without carrying out at least the basic tests then you lose this advantage. The possibility that you may end up with a nuisance tripping situation is also (perversely) a benefit in that it forces you to address what may be a underlying fault that may have hitherto gone undetected[3].

So a new CU without tests is less desirable than one with. Note that you can carry out a good selection of fundamental tests with nothing more than a decent multimeter. The specialist gear (insulation resistance tester, earth fault loop impedance tester and RCD tester are only needed for a subset of the tests) the vital ones like ring and CPC continuity and round trip resistance, checking the legs of rings are correctly identified and connected to the correct breakers etc can be done with a multimeter.

Lighting circuits would be better off on rewirable fuses than mcbs, though its a minor point. More info:

NT

I doubt anything is better off on rewireable fuses. Perhaps you meant cartridge fuses rather than MBCs.

Owain

I changed my lighting circuits to the plug in type MCB's when a midnight bulb failure blew the rewireable fuse, necessitating messing around replacing the wire at an inconvenient time. Lots easier to reset the trip!

This was brought home to us last week. The houses here were built with a Wylex four fuse box. My 90 year old MIL moved into one opposite that had been changed to a newer box with just mcbs and an rcd.

Last week at 11pm she was on the landing when a bulb blew and plunged the house into darkness and total power loss. This left a 90 year old woman virtually trapped in a dangerous position with no light or heating. Fortunately she didn't panic and was sorted satisfactorily.

I came in here three years ago to specifically get advice on replacing the four fuse box. After a lot of conflicting advice I settled for reading the advice that unfolded week by week on the topic.

I still have the fuse box and do not intend to do anything further with it for the forseeable future.

I would like to thank people like Mr Rumm and others that do give relatively impartial and balenced advice.

Indeed - I once had a full house rewire to do, and decided that the best way forward was to get a pro in to change the CU and connect that up to the old (but still working) wiring which I would then replace piecemeal at my leisure. However, the RCD in the new meter tripped instantly as clearly there had been some undetected fault in the old wiring. Sparky said this was a common problem, so be aware it might happen to you. (In my case, the sparks left the new CU in place with a single double socket attached for me, as it wasn't worth spending time/effort tracking down the fault).

David

"Whole house" RCDs are deprecated and contrary to regs.

Especially for older/infirm people, a battery backup emergency lighting unit over the stairs can be worthwhile. I also have one in the kitchen.

Owain

It is, but with an mcb you've got several times the probablity of it triping. Fuses are usually unphased by the heavy current surges of blowing filament lamps.

NT

No. Why would I want to make myself beholden to buying unnecessary items, with the inconvenience it entails when all I need is a fuse wire card? A rewirable performs just fine.

I did however overlook saying rewirables are best avoided if the occupant is irresponsible, ie tenants, sheltered housing etc - as they might just rewire them!

NT

The possible exception to that are circuits with GU10 mains halogens. I had these blow a rewireable fuse on a number of occasions, but they have so far not taken out a 6A Type C MCB.

What did she do out of curiosity?

This sounds like a prime example of a system that could do with fixing (either emergency lights in critical places, or a split load CU, or a secondary non RCD CU etc)

That is a valid choice, but you do need to look at the fuller picture to decide if it is the best one.

I have to admit to not being a huge fan of rewireables. Not because there is necessarily wrong with them, but more because of the "baggage" that tends to accompany most systems that have them.

Her stairlift was at the top and she managed to locate it and use it to get downstairs as it is battery powered in a power cut.

Once downstairs there was enough light from the street lamp to get help from us.

As someone else suggested an emergency light over the landing would seem a very sensible addition to the house. I would not consider it DIY territory so will be getting it looked at asap. :)

Fit a dimmer on the circuit. These make excellent MCB protectors. ;-)

When I bought this place I rewired and used a more than adequate number of fused circuits. It's a four bedroom and I installed 5 lighting circuits and 4 rings - as well as the obvious other ones. I can't be exact how many times a fuse blew but it was only a couple of times. Including once when a bulb blew.

I changed to split load with MCBs some time ago and since then haven't had an MCB trip. It might of course depend on the type of bulbs you use - but I have a fair selection.

I know it doesn't help but I've got the hall and landing lights etc on their own circuit. So if that tripped the nearest room should still have lights.

Er no. Ive had the bulb the dimmer AND the MCB all go together. :-)

Thanks for everyone's input, so quickly, and such an extensive response from John Rumm.

Many points are well made, though I'm still struggling to find a practical reason why a diy installed CU should make the installation less electrically safe than it was before. For example, Owain makes the valid point about reversed polarity and it's certainly something to check for but, surely, a careful direct swap would not introduce this issue if it didn't already exist? Similarly, EricP's story about a lighting trip raises some important issues, although the fuse on one of my lighting circuits seems to blow every time a bulb goes anyway (yes, they're mains halogen and need dealing with), and an MCB would be much easier to reset than finding the fusewire and rewiring the fuse in the dark (especially in the tip that is my garage). It certainly makes sense to have the lighting ciruits off the RCD (and on Type C mcbs), as well as the fridge, the freezer and, I suspect, the washing machine and cooker.

My view was, and still is, that getting it done properly would confirm the overall safety of the system, identifying all faults potentially present. My brother takes the view that a proportion of tests (which, as John Rumm says, can be performed with a simple multimeter) would be better than doing nothing at all.

To clarify the reasons this arose: In both cases, the current fusebox is full, so it would be dodgy, at least, to try to add further circuits. In neither case do we have RCD protection to circuits, which must improve safety overall, surely, although I have two RCD sockets for outside use. My earthing is TN-S and I guess his is too. In both cases, we have isolators between the meter and fusebox, so isolating the whole thing should not be an issue.

Anyway, I'm very open to any further suggestions, but I have a feeling that I shall be losing this argument as the consensus seems to be that, whilst not absolutely perfect, a careful diy swap would be unlikely to introduce any problems that weren't already there. At the same time, introducing RCD protection to the socket circuits must make them safer, albeit with the risk of them not working, if there's any significant fault (perhaps not such a bad thing).

Of course, there's more at stake here than just a pint - If he's going to diy it, I have no doubt I'll be roped into giving him a hand, so there goes another weekend! Just don't tell the Part P police (!).

Thanks everyone G