One for the electricians...

[Tim Lamb]

Meter tails.

Back when this house had storage radiators and a 60 amp 3 ph supply, the load was spread across 3 switch fused outlets and connected with what appears to be 16mm tails.

During a house rebuild and professional rewire, a new consumer unit was fitted using what looks to be 35mm and the old heater supplies swung over to a 32 amp TP switch fuse to provide power for some outbuildings.

Somewhere along the way the main fuses were changed to 100 amp. The incoming tails (fuses to meter) appear to be 25mm.

Is this OK? Earthing is via the supply cable sheath.

From previous articles discussing supplies to outbuildings it appears this set up would not meet current regulations regarding earth provision but what about the mix of tails?

regards

[meow2222]

There is no requirement for old installs to meet current regs.

NT

[Lurch]

On Wed, 28 Mar 2007 09:42:14 +0100, Tim Lamb mused:

Possibly, but are you actually 100% sure that the only earthg is via the armouring?

Mix of tails? 25mm tails would be fine for 100A, anyhting after that larger than 25mm would be fine as well, or have I misunderstood?

[Tim Lamb]

In message , Lurch writes

Yes:-) Incoming gas is plastic as is the water.

Lead sheathed cable with a new head. Originally installed around 1938. Leaks pitch!

Ah. I need to lose the 16s then. Pity as it would involve breaking the meter seal.

regards

[Lurch]

On Wed, 28 Mar 2007 21:46:08 +0100, Tim Lamb mused:

Erm, I don't remember asking about that.

I'm lost, your original post makes no sense to me, and your reply further confuses me I'm afraid.

Pictures would help.

[meow2222]

Earth outer is normal, but... one cant get a reliably gastight connection to lead, its too soft, so if you're sure its lead, if it were mine I'd check earth impedance. A lot of lead sheathed cable was used in the 30s.

why do you need to bring this to current regs? Does your 1938 incomer meet latest regs? Unlikely. And not a problem in itself.

NT

[Tim Lamb]

Actually, on closer examination, in addition to the main earth terminals linked to the incomer sheathing, there is an inconspicuous bit of 6mm disappearing under the hall floor tiles.

As you say, a check would be informative.

I want to connect a second 3ph supply to another outbuilding and in the near future have some electrical alterations to new bathroom/kitchen areas.

Time to spend some money on professional help:-)

regards

[Tim Lamb]

In message , Lurch writes

No. I was trying to anticipate future questions.

As said elsewhere, there is a 6mm earth cable disappearing under the floor screed. This may just be bonding for the ground floor bathroom or... I was on site during most of the house rebuild and believe I would have noticed earth rods being installed.

Sorry. This is a dog's breakfast installation having been adapted several times.

You'll be on your pension by the time I get a viewable photo published.

ASCII probably won't help but I'll try some more verbals.

Three phase incomer feeding meter via 100 amp fuses and 25mm tails.

L/N supply from meter to consumer unit via Henley blocks in 35mm.

3ph supply to 32 amp switch fuse in 16mm from meter terminals and Henley blocks.

The only known earth is the incoming cable sheath which is connected in

10mm to a strip of terminals.

Initially I want to connect a second TP switch fuse to supply another outbuilding.

I can install whatever Screwfix supply by way of additional Henley blocks but have only existing 16mm tails to use. Alternatively I break the meter seals and fit 25 or 35mm.

As this is *new work* presumably it has to meet current regs?

regards

[Roger R]

Oh go on, try. It's not so difficult and you sound like a reasonably competant person. You will also be able to use the process again for future issues.

Roger R

[Andy Wade]

OK - but don't call those "tails", this term being reserved for cabling on the _output_ side of the meter (or output side of any isolator provided by the supplier).

I presume this is a 3-ph meter(?)

Meaning single-phase, I presume. OK, provided tail length complies with the supplier's requirements (< 2 or 3 m).

Tail size here is OK on current rating, assuming that the switch-fuse is fitted with fuses not greater than 32 A, and this is OK for the assessed MD of the installation it feeds. However with 100 A main fuses the supplier (DNO) will usually specify 25 mm^2 min. tail size to ensure adequate short-circuit rating. Were the 100 A fuses fitted by the DNO, or has someone (unauthorised) been cutting seals? If you can measure the supply prospective s/c current and Ze and show by means of adiabatic equation calculations that the 16 mm^2 tails are protected by the 100 A fuses for phase-neutral, phase-earth and phase-phase faults then there's not much need to worry further. If not then consider upping the tails to 25 mm^2. Don't even think about cutting meter seals - arrange a supply disconnection and connection of new tails if necessary.

TN-S supply. Quite alright provided the Ze reading is OK (< 0.8 ohm) and the connection to the cable sheath is secure. If it isn't get the DNO out on an emergency call to sort it out.

Why would you need more blocks? You only have a 2-way split now and that's only one one phase (or are there no blocks on 2 of the phases?).

See above.

Yes. Min. 16 mm^2 earthing conductor required, and 10 mm^2 main bonding (assuming normal service cable sizing). Old 6 mm^2 main bonding will need to be be upgraded. Also bear in mind that the earth in any cable exporting the TN-S earth to an outbuilding where further main bonding is needed (e.g. an outbuilding with a water pipe) acts as both a CPC and a main bonding conductor and needs needs to be at least 10 mm^2 (Cu equivalent) to fulfil the latter function.

[Tim Lamb]

OK.

Yes.

OK Less than two metres total.

Seals are intact. I think the fuses were upgraded for the off peak heaters.

Hmm... Easy enough to change the main earthing conductor. Less easy to change the bonding. This is an old house with ply stapled over wide board flooring.

Exported earthing is by cable armouring. The current project is to connect up a timber frame barn having no soil engaging metal parts. (I may use 2"x2" steel trunking internally). An existing 3ph supply feeds a steel framed barn (unbonded at present).

Food for thought. Ta.

regards

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