Meter remote from CU in a flat

[Nige F]

couple of queries, guys.....situation is a 1st floor flat (my son+gf`s first home)Needs an electric shower and therefore a new Cons. unit. Have read here that the meter tails could be disconnected with a 100a.D/P switch.....but have also been told that ONLY the company supply fuse and meter should be in the wall box on the outside of the building Also the "tails" are a length of armoured cable from the meter running hidden in the fabric of the building and appearing in the fusebox cupboard in the flat....the armour wire is the earth back to the company supply.......will this be adequate I`ve a feeling that a separate earth wire should be run........this I can do before I re fit the bathroom.....would the 100a isolator be ok. in the fuseboard/cons. unit cupd...or is it necessary at all. can`t even get hold of my regular sparks to ask him let alone get him to come and change the CU from old fuse box. so any advice would be appreciated..before I build things in that he will need to get behind etc

 

[Adam_151]

You need to ask your supply company what length of cabling to the CU they allow their service fuse to protect, usually to simpify things for themselves they'll say something like 3m and make sure that they have left enough lea-way that the Z's will pass by a mile...

Ideally you'd have a switch fuse near the meter, be aware though, that as you allude to, the electrity company can be a bit possesive of their piece of chipboard/meter cabinet.

Normally armour is more than sufficent for earth as long as its terminated properly and Z's are ok and thermal withstand checks out ok (chances are that it will, but a calculation might be necessary if the tables don't immediatly say yes), but with yours, I'm assuming the start of the 'submain' is quite a bit out of the equipotenial zone, which means your bonding will be running from the end of it where it enters the zone, which makes your earth in the cable a MEC (main earth conductor) as opposed to a CPC (circuit protective conductor) and this might shake things up a little, sorry, my head ain't working properly atm, got a nasty cold (again!)

What size is the SWA submain OOI? and how is it terminated at the meter end?

 

[didthathurt]

Have read here that the meter tails could be disconnected with a 100a.D/P switch

Practically -tho Adam is spot on - the DP isolator is there to isolate the CU to allow work to be done without having to remove the board fuse or switch live tails around. You don't need an isolator as you describe, but it is useful to have one on the few occassions it is used.

I do disagree that ideally a switch fuse should be near the meter - the whole of the system from the incoming main is protected by the board fuse. The Isolator is a DP switch not a switch fuse, which is installed for the above reason, therefore, putting it at the CU end, in your case, is the logical site (and it doesn't add significantly to the length of the meter tails) which some companies will come and install for as little as £15.00. If someone needs to work on the CU it's a lot safer if the isolator is where they can see and control it.

the armour wire is the earth back to the company supply

I presume you are describing a TN-S earth (see the wiki/sticky or search Wikipedia for info on that). If so, your MET should connect to the boards earth terminal with 16mm earth wire. Your CPC's and MEB conductor should connect to the MET as well and that will leave just the Supp Equi Bond to deal with at the bathroom end. The new CU should have a 100A isolator, too, but, in the case of a flat, does not strictly need an RCD for the ring finals (as, I assume, the sockets are unlikely to be used to power outdoor equipment) so you could use a non-split board and an RCBO for the shower (and any other circuit you wished to so protect).

 

[Nige F]

OK had a look@ the meter etc. there is a small sign half obscured by the co. fuse ....says protective mutiple earthing ( multiple is the obscured bit) The strands of sheathing wire from the armoured have been twisted and green taped and then put into an earth terminal on the side of the co. fuse. The overall size of the armoured is 22-ish O.D.....and the conductors look to be about 3-4 mm across .slightly oval shape....I`m now thinking that the "submain " may not be up to the job of a 9KW shower....3 KW immersion heater x-KW electric cooker.....2 KW undersink water heater .........Thanks for the advice for the Non-rcd board/RCBO for shower......that pre empted another Q. ......Y`see this electric shower lark is all because the flat is a) all electric.....b) although a top floor with roofspace ,it`s got a fortic combined hot cylinder/feed cistern..............Now being a plumber I could fit a "coffin" tank in the trussed rafter roof ....and a new eco7 double immersion cylinder...and a shower pump ........and gas heating when they can afford to get the gas from the main road BUT I`m trying to keep it all electric/simple .....PS the storage rads are fed in the same way with same armoured submain back to the board and separate metalclad Wylex fuse board.............Build. is c.1976

 

[didthathurt]

The PME note refers to the fact that this is a TN-C-S sysytem, so the earth actually routes through the supply neutral line. The Local supplier decides what their supply is capable of doing and fit a fuse accordingly - normally 80A or 100A. Which it is may be marked somewhere or you may have to contact them to find out. Even if it's 80A, if you ask, they normally say it will take 100A and come and fit a new fuse of that rating.

If it will take 100A, you need to apply diversity to the loads, where you can, to work out what you can and can't install. Your 3 water heaters would be, 9kW, 3kW and 2kW. The diversity rules mean you need to account for 12.5kW of that load (about 54.5A). For a cooker you account for 10A + 30% of the load above 10A + 5A if there's a socket outlet in the cooker switch. For the other ccts, you take 100% of the largest and 40% of the rest + an allowance of 100W per light fitting. It will be pretty tight, you'll have to look at the ratings of the cooker etc and work it out.

 

[Nige F]

Thanks for the info guys.......I`ve now got another sparx.via my son ...so I`ll work out the loads and quiz him too.will probably run a new armoured cable anyway as he suggested this and showed an easy route for it...again, many thanks