New CU & Supply type

[FredFlintstone]

Hey all,

Been lurking for a while and this is my first post asking for help.

I'm refurbing my delapidated new house. As the electrics are a disgrace and I'm fitting a new kitchen I figure it would be sensible (and in any event compliant) to install a new, separate ring in the kitchen.

Because of this, and because I'm fitting extra showers and a new circuit for outside lights among other stuff I need to fit a larger CU which, if my supply is right, will be split load and I'd like a bit of advice because I haven't got any money and am doing as much as I can myself.

I know it's notifiable - BC have been informed and all will be inspected and certified before being covered up. I also know I'm not a pro spark but would be grateful if people could refrain from telling me to pay a pro to do it. I'd love to but can't afford to. Having said that, while I don't intend on killing myself, if something I say raises concern then please tell me so (but without telling me to get a pro in - I'll only do that if I seriously mess something up and can't figure out what).

I've attached a couple of pics of my incoming supply and the current CU. Although it doesn't appear too clear from the pics, can anyone confirm whether my supply is, as I think, TNS?

You'll also note there's no isolator switch but if you look at the main fuse closely you'll see the security tag doesn't actually pass through the fuse. To save me waiting for the installation of an isolator is it safe enough to replace the CU if I switch off the supply at the existing CU and then the main fuse happens to fall out?

Any advice appreciated other than the pay a pro advice.

IMG]http://i159.photobucket.com/albums/t157/FredFlintstone_album/02_Incoming.jpg[/IMG]

 

[luminaire]

It looks like it is T-N-S, but now would be the time to have your main fuse upgraded to 100A and at the same time changed to PME system. You could also ask for an isolator to be installed at the same time.
Seems rather a lot of work to take on if you're completely confident.
Best of luck.

 

[ELZ4742]

if the main earth comes from the sheath of the main incoming cable, you have t-n-s.

are you saying the CU doesnt have an isolator switch or there isnt 1 before the CU?

 

[ayen]

do some over time and get a expert in! lol

 

[DESL]

because I'm fitting extra showers

Just how many showers are you fitting as this will have a bearing on the size of the supply

 

[FredFlintstone]

Cheers folks.

CU has a main isolator switch but there's no isolator before CU to cut the incoming supply.

I figure removing the main fuse will cut the supply but also figure switching off the Cu main switch first would be a good idea - I don't know if just pulling the main fuse without switching of CU would do any harm but think it would be safer to have no load on the CU when the fuse is pulled.

Are there any hazards involved with pulling the fuse or disconnecting and reconnecting the meter tails with the main fuse out?

Luminaire, it is a lot of work but unfortunately I'm stoney broke. I've already done the work on the existing ring (because that wasn't notifiable) and have fitted back boxes and wiring between said boxes in the kitchen ring. Just gotta run cable from meter cupboard to the first and last socket on the kitchen ring now and connect . Need to get the outside circuit wired (but this may have to wait meaning another BC nitification and test / inspection.

Really need to get this CU changed asap so I can connect up new bits, get BC out to test / inspect and then cover up all work. Can't cover up though until BC have inspected hence urgency of new CU install - I really need to plaster my kitchen before units arrive.

 

[Spark123]

I can't see it very well, is there an earth wire coming out of the DNO cutout?

 

[ELZ4742]

its very common for the CU to be the main point of isolation, you only tend to find them where an rcd has been installed on a T.T. system. its not necesarry in your case, just pull the fuse.

 

[FredFlintstone]

because I'm fitting extra showers

Just how many showers are you fitting as this will have a bearing on the size of the supply

Sorry, replacing an existing electric shower, but with a 10.5kw one instead of the rubbish 7kw thing that's in. Because cable needs changing anyway I'm gonna dig out the diagonal one (6mm) and install a 10mm one via a bit of a safer route.

Incidentally, I found this behind the plaster in the kitchen.

Notice where the cable goes? Into the wall and up the cavity! Isn't that wrong? Also, it spurs of an upstairs socket on the ring. No problem with that you say. Well said socket also provides power to another spur. Presumably the idiot thought that two singles were acceptable instead of one double!

 

[FredFlintstone]

I can't see it very well, is there an earth wire coming out of the DNO cutout?

Earth comes out of the sheath of the supply rising outta the ground. From what folk here say it sounds like I figured correctly in TNS. Needed to mae sure though - don't wanna go fitting a CU to a TT supply and then have BC out and tell me to change it!

 

[pcboffinuk]

Hi Fred,

As others have said it appears to be T-N-S, although it is very hard to tell without looking at it properly as sometimes things have been altered and the original cabling left in place to mislead you.

A call to your current electricity supplier is definitely worth a try as they may be prepared to upgrade the fuse and earthing for a small cost, or even for free.

If you are careful, confident (but not over confident) and can read and follow instructions from a book there is no reason why you couldn't handle changing a CU, although the sticking point will be doing the testing.

If you say BC will be inspecting it then they may well send in a spark to test things, and possibly even provide a certificate, although the problem will come if anything fails.

Because changing a CU affects all the existing circuits it means that ideally all the tests should be done on all the circuits, and if your existing cabling is poor then there may be problems there.

I'm not trying to put you off, just warn you of the pitfalls.

Regarding the main fuse, although of course you should never pull it out, if it was to 'fall out' then that would (from what I can see on the photos) completely isolate the CU and allow you to replace it.

However, do not leave the cover on the floor where it has 'fallen' as the contacts inside will still be live and easily accessible. If the fuse was to fall out of its holder then the holder would fit back on to cover it nicely

I don't know how much you know about electrics, but the fact you have identified it as a T-N-S shows you know more than the average DIYer. If you don't have one already, the IEE On Site Guide would be a very worthwhile purchase and should tell you almost everything you need to know and at about £17 shouldn't break the bank.

If you do go ahead, good luck and take it slowly. Feel free to ask further questions here, but if people think you are taking undue risks they will always recommend getting a professional in, if only to cover themselves.

One other thought - If you haven't bought the parts already, find a local small electrical wholesaler and buy from them - they will likely give you some good free advice to make sure you buy the right things - don't bank on the local B&Q to know what you need!

Regards

Gavin

 

[Spark123]

It does sound like TN-S, can you borrow an earth fault loop impedance tester and check the external loop impedance?

 

[Teahead]

Hi Fred, hows Wilma.

Have been reading your post and am wondering how the Local building control inspection business works.

Am looking at doing a CU change in my house in the future and would like to do as much as possible myself.

My dads mate is a time served spark but is not Part P certified. He knows what he's doing and would be able to do anything i wasn't to keen or sure of doing. I had thought that you had to get a part P certified spark to do the works or can you do all the work and get it tested/certifed by Building control before getting the Lectric board to move the meter tails.

 

[securespark]

Are there any hazards involved with pulling the fuse or disconnecting and reconnecting the meter tails with the main fuse out?

Yeah, electrocution & facing the wrath of the DNO.

 

[pcboffinuk]

Cheers folks.


I figure removing the main fuse will cut the supply but also figure switching off the Cu main switch first would be a good idea - I don't know if just pulling the main fuse without switching of CU would do any harm but think it would be safer to have no load on the CU when the fuse is pulled.

Are there any hazards involved with pulling the fuse or disconnecting and reconnecting the meter tails with the main fuse out?

Just to add, the fact you've already considered this shows you should be able to cope with sorting out the rest...

It is a good idea to isolate the CU first, that will reduce any possibility of arcing when you remove or replace the fuse.

It is best to remove or replace the fuse quickly and firmly, rather than levering it out gently - again to reduce any risk of arcing.

Good luck

Gavin