Changed CU - questions

[sm_jamieson]

After 5 hours crouched up in the cupboard under the stairs, I finally finished the CU change on Saturday. Many hassles, plaster falling off wall etc. At the end I had to drag myself out by my hands, since my knees had seized up ! Anyway, put in MK Sentry unit, temporarily with no RCDs (rewire and new circuits pending). A couple of questions.

1. I removed several of the scored openings for immediate and future needs. So there are various square openings in the case top, side and bottom, some of which are used. All have enough space to poke in something. Must I do anything to seal the openings and around the cable entry in order for the thing to be passed by an electrician (certifcate for building regs) ? No grommets or anything were supplied. But anyway I cannot see how a grommet could be used since some openings have up to

3 cables entering from various angles.

1. I extended some cables using choc blocks then wrapped in insulating tape. Looks crap, but is it OK for regs ?. I had made up a project box with choc blocks inside but in the end this was not usable due to cable locations and spare wall space etc.

Simon.

[John Rumm]

One of the joys of doing these things in the real world - you can always guarantee the cupboard is just to narrow, or low or in some other way impossible to get comfy in! ;-)

Depends a bit on what the something is... Its generally not considered good practice to leave holes big enough for a finger to poke in - especially if they could make contact with live parts. You don't really want holes big enough for mice etc. However something you could slip a screwdriver into would be much harder to prevent.

Note sure what certification an electrician would be able to give in the circs. If its a part P job then it ought to be down to building control to organise any testing etc (although the reality on this varies with the LA in question).

Personally I would blank off any completely unused knockouts.

Where are the chock blocks? Inside the CU itself would be OK, outside the CU this would not really be acceptable. (insulated crimps are usually the easiest way of extending wires in the CU, or crimps and heatshrink outside it)

[sm_jamieson]

I know an electrician who will check my work and sign it off if I leave it exposed enough for him to test and be sure he's not dropping himself in it ! The current cabling of the CU is temporary and will be changed when I rewire (lack of sockets and one ring for house currently) and wire up the extension when it is finished later this year. Just wondered if he could fail to certify the work due to lack of grommets and open knockouts etc ?

Since I will need these in the future, what temporary method of blanking off is recommended ? Some strong tape over the holes ? The knock outs are square - I guess they would be a standard size and I may be able to get some square grommets to use that would fit. If the type that you pierce, I could use these to blank off the unused knockouts.

This was partly because I was in a rush to get power back on (wife sitting in dark staring crossly at me) and also it is temporary. Choc blocks are 30A for ring circuits and

15A for the lights etc. They are outside the CU and covered in insulating tape. I guess having them inside an insulated box would be OK.

I had heard that crimps on solid cable are not reliable, plus I'd have to buy a ratchet tool. But I'd be happy to do this if its the best solution. Recommended crimping tools ?

Any photos of professionaly fitted CUs showing wiring entry etc. would be useful I guess. I was thinking about mounting on a board and wiring into the back, but was not practical due to cable lengths etc. And the CU at my last house was screwed direct to the wall.

Thanks, Simon.

[Andrew Gabriel]

If it's the type of CU I'm thinking of, I don't think you'll be able to repair it acceptably. One option would be to buy another empty one to get a new cover, and then cut that to fit accurately around the cables.

Here's how I did this when replacing a CU where the cables would not reach...

[John Rumm]

Is there any point in getting a temporary install certified?

The open holes, yes. Lack of chafe protection would matter on metal boxes more than plastic.

For big holes, probably a suitable bit of flat rigid material to use as an oversized patch, and some small self tapping/drilling screws or silicone to fix it. (trimmed from a bit of large trunking for example).

For gaps round cables you can use things like silicone sealant. Note that the least desirable gaps are those on the top horizontal surface (IIRC you are meant to be IP4x or better here), whereas one on the side or underside only need to be to IP2x or better.

Maybe - not seen square ones... you can get grommet strip that is used for covering the edge of irregular holes etc but that does not fill the hole, it only protects things from the sharp edge.

How could you tell if it was dark? (no need to answer that - we all know exactly how!)

Yup, in a box would be fine.

If made properly then crimps are fine on solid cable. They are generally industry standard practice for extending wires in tricky situations since they don't suffer from the limitations of the other permissible options.

Something like the one in the pictures here:

or CPC will flog you one for £12 probably.

Rear entry is certainly simpler in terms of knock outs (and can make wiring inside slightly less chaotic), but it is not always usable in every case.

[sm_jamieson]

They are not gaping holes, just enough space around the cables to push in, say, a screwdriver. To be honest, I think the knockouts are there to be used ! If you tried to hacksaw a smaller hole, the knockouts would snap out in the process. I wonder what MK would say ? I may send them an email. Surprising this issue is not covered in the installation guide.

Where did you get those connector blocks - they look a bit better than plastic choc blocks ??

Simon.

[sm_jamieson]

No there's not. It will be certified after the extension wiring is complete - later this year. And the installation will be tidied up a bit.

Well maybe I got the wrong idea about the knockouts. Maybe the correct method was to just use one knockout, and squeeze everything through. However, doing that would more likely result in strain and chafing. I have wires entering through various holes which also makes layout inside much tidier.

Simon.

[Andrew Gabriel]

They are DIN rail mounting connectors. Any electrical wholesaler will have them and the DIN rail box. Also CPC. B&Q used to too, but might not do now they've dropped lots of electrical stuff in order to fit in more soft furnishings, etc.

Note that they are very accurately sized for cable capacity, e.g. you probably won't fit two 1.5mm conductors into a 2.5mm terminal. I would suggest always getting at least one size larger than you think you might need. The earth ones also make an electrical connection between the terminals and the DIN rail (so I didn't really need several parallel earths back to the CU, but it does no harm, and you probably don't want to rely on just one).

Regarding crimps -- I usually only use them where the join will be inaccessible afterwards. If the join will be accessible, I use screw terminals, as someone might later want to disassemble for testing, or to connect in another conductor, etc.

[sm_jamieson]

Looks like you've combined the 2 ends of your ring circuits in the top box, and only a single cable to the CU, i.e. ring not right back to a single MCD. Did you use a wiring size larger for the single cable section, e.g

2.5mm ring joined in top box and single 4mm cable back to CU ? Simnon.

[sm_jamieson]

There seems to be a general issue here, and its not covered by the UK.DIY wiki. The MK info (from TLC site) says: Precautions must be taken to maintain the IP rating, e.g. use of cable glands and knockouts.

So, using their square / rectangular knockouts supplied, what cable glands fit ? I am not aware of any cable glands that fit a square hole. Anyone know the answer, or do I need to email them ? Why is it you always find these things out afterwards, however much research you do ?

Simon.

[RobertL]

What happens about the house insurance in the meantime?

Robert

[John Rumm]

The thing with knock outs is that you frequently don't need the full amount knocked out (and in some cases want more, and in other want a hole where is no knock out at all!). So it is often a case of use then a a guideline for what a hole will be, but machine carefully to match what you require.

Obviously with a metal box this is far harder.

[Andrew Gabriel]

Yes, although I used 6mm, not 4mm. IIRC, the lighting circuits use 2.5mm between the connector and CU. Mostly this was down to what sized singles I happened to have. (4mm is not a particularly useful size for most purposes.) You could strip singles out of T&E to make up these, given they aren't exposed (at least in my case).

[sm_jamieson]

Its a good question of what happens when building work is in progress, and things temporarily don't comply. Like if an RSJ is put in a house and the builder has left it exposed without fire protection for a weekend, and the house burns and a wall collapses. I guess his insurance pays out. What about a DIYer ? I had self-build insurance to cover my extension foundations etc, since there was a small risk of the neighbours walls being affected. But this would not cover wiring. Does not really answer the question ... Simon.

[Andrew Gabriel]

There are none.

Another option might be to put a piece of large-ish trunking along the sides where you've passed cables in, to route the cables and conceal the over-sized holes.

Although it's obviously important to not be able to stick fingers in, you also don't want any masonary dust (brick, plaster, sand, etc) getting in. That quickly wrecks MCBs if it gets inside. Some manufacturers say it's the main cause of MCB failure.

[sm_jamieson]

Wot i propose to do (idea partly from John Rumm), is to cut some rectangular pieces of plastic (I may buy a cheap small MK CU case to chop up for matching colour), and silicone on to cover the knockouts. The holes that have some wires in, I'll cut a keyhole type of shape out of the plastic and "bend it" around the cables to a fairly tight tolerance, then silicone it to the case, and later silicone around the cables. That way, I can easily slice off the covers when I want to use a certain knockout at a later date. After all the whole point was to get the CU install started when its too cold to work outside. I need to be able to add circuits later without having to drill or chop the case when full with wires. Simon.

[John Rumm]

House insurance is not written on the understanding that the building will be maintained to current building regs. Most buildings in the country would not meet that requirement anyway. Also DIY mishaps *are* usually covered by you household policy.

[Mike Tomlinson]

In article , Andrew Gabriel writes

However, you clearly had the advantage of space to work in and a well- lit environment. Most of us have to grovel round in the cupboard under the stairs or in a grotty boxed-in cupboard with minimal space.

[js.b1]

Top surface entry to CU must be IP4x. No object >1mm may pass.

Bottom surface entry to CU must be IP2x. No object >12.5mm may pass.

If you are entering the CU from above, use the rear entry and stick the CU on batons.

Get an RCD on there a.s.a.p. even if whole-house because accidents DO happen. The ideal setup is a 17th with 3 RCD or lots of RCBO - they don't cost that much.

[sm_jamieson]

too late for that now.

alright don't panic !

Simon.