UK Earthing a mechanical coin prepayment meter on a TT

[boulder]

I'm a rusty 16th edition retired spark. At home I'm aiming to send power to my outbuilding for a couple of lights and one 13A socket.

I have a few people that come informally & use my outbuilding to keep & look after their animals. They could do with some light to see by, and maybe the facility to charge a mobile phone or boil a kettle.

There's no requirement for anything else (no heavy machinery etc) as when something like that's required, I tend to just use a portable generator.

I'm on a TNCS supply from a local 11kv 3ph pole transformer which is situated right behind my outbuilding. I can see my house's feed cable run down the pole & pick up 2 connections from the transformer. In the house at the DNO's incomer block, I managed to accost an ENWL van guy who kindly broke the seal for me & remade it once I'd had a look.. I could see the incomer cable come in and split the armour off into N & PE, and one core be used for L.

After much poking around the internet & racking my brains from the past, I figured the best way in my circumstance between exporting the DNO's PE & the house's equipotential zone, and doing a local TT at the outbuilding, was to drive a nice earth rod at the outbuilding & TT it.

I'm ready to be advised otherwise by those more current!?!

But my main question is regards a mechanical "Smith" coin-operated supply meter I have. I notice it has two things that in combination worry me a bit:

1. A metal dial that the user is supposed to insert a coin into a slot then turn a lever
2. No method of attaching a PE that I can see

It's just, Live in, Live out.. Neutral in, Neutral out (Neutrals commoned)

These were extremely commonplace meters for a long time & I bought mine quite recently from a fairly large reputable electrical factors as a 'refurbished' item (they seem to have quite a large stock of them)

I'd just like to ask the forum's opinion on whether this meter is acceptable within regs for my intended use.

If I did use it I suppose I would have to strap the TT rod earth straight past it into the outbuilding's CU? Only way to actually earth it I can see would be to drill and tap the case & attach a lug, even then, there's be no guarantee I'd be earthing all the bits people would actually be not just touching accidentally in passing, but probably mauling with in active frustration

Should I in fact, throw it in the bin and get something different (or just allow free brews)

And am I right with TT'ing the whole shebang..

Thanks

 

[bigspark17]

What size cable is going to the outbuilding? Are there extraneous conductive parts in the outbuilding? You may well be overthinking this

 

[boulder]

2.5 milly 3 core SWA - it's what was there when I moved in. Spent 6 months metering it out & rectifying all the insulation resistance faults, it would be a Royal PITA to have to replace the whole lot with larger CSA, hoping it'll be ok for one socket and about 200W of LED lamps. Cable length fair guess at 100M. The outbuilding's breezeblock to ceiling height then wooden rafters with corrugated tin roofing. No water or gas services. It's in 3 sections - one section's a tackroom, the other 2 are used as stables. Outer walls are the same tinplate the roof's made out of. The same sort of plastic-covered corrugated steel panels most modern factory roofs are made of.

 

[pc1966]

At a rough guess 2.5mm is going to be OK on VD for around 8A at 100m, but if it is almost all LED light use and the occasional kettle I would doubt it is worth changing it. Cheaper to get a 800W camping kettle if any one questions it!

I would assume a metal meter counts as needing earthed, so it ought to be connected to the TT earth (as for any extraneous parts) but you would need the RCD protection before the meter.

It is late and I'm about to go to bed but I think there are special considerations for livestock that means using the PME derived earth is going to be a major PITA (as you need buried mesh on it, etc, to keep the step potential well below what is OK for humans with shoes can tolerate) so certainly it ought to be TT'd there.

 

[bigspark17]

2.5 milly 3 core SWA - it's what was there when I moved in. Spent 6 months metering it out & rectifying all the insulation resistance faults, it would be a Royal PITA to have to replace the whole lot with larger CSA, hoping it'll be ok for one socket and about 200W of LED lamps. Cable length fair guess at 100M. The outbuilding's breezeblock to ceiling height then wooden rafters with corrugated tin roofing. No water or gas services. It's in 3 sections - one section's a tackroom, the other 2 are used as stables. Outer walls are the same tinplate the roof's made out of. The same sort of plastic-covered corrugated steel panels most modern factory roofs are made of.

yes as @pc1966 disconection times and VD will probably limit you to around a 6A mcb so a kettle will be out the question.

being as the building is not of metal construction i could argue that TT the building for livestock is not required as there would be no metal frame or extraneous parts to conduct the fault to earth. So disagree with @pc1966 on that one.

cant get away with the fact the cable is so undersized but its not uncommon to see

 

[pc1966]

As @bigspark17 says, checking the table points to a 5% VD limit of 6.4A for 100m of 2.5mm SWA cable, though if it were a PME supply with Zs of 0.35 ohms the disconnection limit would be met for a 16A B curve MCB.

If nothing extraneous to worry about then you could go either way, but I suspect for all the cost of an extra up-front RCD I would still go TT anyway, just in case something inside (or wall panelling that ends up on the CPC from metal accessories, etc) could just be an issue for livestock.

 

[boulder]

Thanks chaps. I measured the cable properly it's more or less bang on 90 metres.
The meter is one like this -> (not my supplier but same meter)
800W camping kettle.. Yes there's a good idea

I'm going to end up with the supply coming from the pole transformer at 'point A', going the 90-odd metres to the house, then turning back on itself 180 degrees as the submain, and heading 90 metres back again to the tackroom which is just a few metres away from the DNO supply transformer. So my TT earth rod is going to end up really close to whatever earthing the DNO's done at the transformer. Probably within 10 - 15 metres or so. The direct line of sight between DNO pole and my rod will be at the rear of the outbuildings though, in an unused / fenced off area which is rarely accessed & never by livestock.

What effects would the proximity between DNO pole earth and my TT rod have?

 

[pc1966]

For a LV earth at 10+m separation I doubt anything noticeable would happen.

More fundamentally is the need for an equipotential zone - so as long as your local rod reflects the potential of the ground where any contact between the resulting CPC and true Earth can occur that is all that matters.

If the coin meter is metal cased but has no earth terminal (which would be unusual?) you can probably use a ring tag under any fixing screw as a substitute. Of course you like to assume no breach of insulation can occur, but as long as it has a path enough to trip the supply (either OCPD if on TN or RCD if on TT) then folk are protected from shock.

 

[boulder]

There's some possibility the main casing of the meter isn't metal, it's pretty much entirely covered by a 'hammerite' style coating. Only way I could tell for sure would be to scratch the surface.

The part I'm mostly concerned about is the coin tray area & the clockwork user dial / coin slot mechanism. This whole mechanism comes apart for coin removal / rate setting, and is comprehensively bare metal & not just touchable by personnel, it's intended use is to be regularly grasped intentionally by all manner of folks. It does seem unusual to me too, there's no obvious built-in provision for earthing in that area.

Just wondered if there's anyone who's familiar with these Smiths meters, and knows of some special reason why these don't need an earth (or didn't according to regs, at the time they were commonly installed?)

The only thing I can think of is that the exposed metal parts qualify as some form of double-insulated, by the design of the meter. Maybe the mechanism operates the live contacts by some form of long insulative 'pointy stick' inside & that makes the design certifiably safe..

EDIT

Just been reading around MID approval for these meters.. I don't think my setup will qualify as outside the scope of MID, and it certainly isn't MID-approved, so maybe I really am best abandoning this meter? Just limiting everything with a small say 6A breaker and using a low power kettle should be enough to keep usage down to a sensible level with any luck.

 

[pc1966]

I don't know the regulations for electricity billing, but it might be worth checking that as you say. You can get MID approved meters quite cheaply, but not the payment-controlled sort. For example:

1 Phase 100A MID Approved Meter

Provides a compact solution for many metering applications

www.cef.co.uk

If the folk using it are moderately trustworthy you can probably just have a fixed fee and include power in it, but if in doubt then such a meter would at least allow you to check that the usage was "reasonable".

An occasional use kettle and some LED lights won't take much, more so if the lights are automatic where practical (e.g. movement detecting, PIR + photocell, etc). What eats power is someone bringing an electric heater, but as you say a 6A MCB might keep that away!