An earthing question

[Dave Plowman (News]

Sorry to dilute the posts with a DIY question.

About 20 years ago, having extended the house and with some new circuits, changed the CU to a split load type.

The old was earthed to the incoming water main (lead) and the gas pipe (iron barrel). Not had any problems, but knew that didn't meet the current regs of the time.

Contacted my supplier about them providing an earth. They quoted some ridiculous figure. Several hundred quid.

Talking about it in my local with a just retired sparky (now sadly no longer with us). He offered to sort it out with earth rods. So left him to it, while I got on with the new CU. He made a very tidy job of it all - even although it's in the cellar. And tested it all using some sort of meter he'd brought with him. There is a big earth busbar beside the cutout that everything goes to - IIRC three earth rods plus all the various safety grounds to pipework etc.

Not had any problems with any of it. No rogue RCD trips, or whatever.

Spool forward to a few weeks ago. Smart meter man was unhappy with the cutout - reckoned it had been overheating. He called out UK Power emergency, who arrived the same day. Smart meter man left having done nothing. UK Power bloke said it was safe - but he'd arrange for it to be changed anyway.

The next week this was done. And they've fitted an earth terminal and thick braid alongside presumably connected to the incoming SWA sheathe? And clearly labelled as 'Customer safety earth do not remove'.

It would be a couple of minute's work to link that to the earth busbar - I even have some 16mm earth wire left. But do I leave the earth rods still connected?

ISTR reading somewhere you need a different RCD with an earth rod system? Mine is just the 30mA one that came with the CU.

[Jack Harry Teesdale]

If you use the 'earth'terminal provided by UK Power you could disconnect the earth rod system.

[Robin]

Hard to see why you need disconnect them given regulation 411.4.2 now recommends them in addition to the supplier's earth.

TT earthing often needed a 100ms S-type RCD protecting the whole CU but you now have a TN supply (subject to further developments on the oozing gunk front).

[John Rumm]

If it is connected to the sheath of the incomer, then it is probably a TN-S earth.

You can compare to the photos here:

If you now have a suppliers earth, then you can use that in place of the rods. If it were a PME/TN-C-S earth then you could connect them as well.

One RCD or two or more?

The basic type is the same, however if you have a split load CU, then it was common in the days of the 16th edition to use a pair of RCDs on TT installs. One with a 100mA trip, and a time delay action for the main incomer of the CU, protecting all circuits, and then a normal 30mA trip RCD protecting the socket circuits. That meant you got shock protection on the sockets, and fire and installation protection on the other circuits.

If you have that setup, then it would still be ok with a suppliers earth, although not up to modern standards that require 30mA trip RCDs without a delay on most circuits.

Since it sounds like you only have 30mA trip device(s) then that is ok, although may will give no discrimination if you only have the RCD as the main incoming switch. (if you only have it on the sub section and a normal switch as the main incomer, then it will be safe now (but not compliant), and before would have been potentially dodgy, since you may not have been able to clear an earth fault on a non RCD circuit (depending on the actual earth rods impedance).

[Robin]

I don't have the amendments to the 18th to see the amendment to 411.4.2 and my comment to Dave was based on what I had which seemed to suggest it applied to TN generally rather than just PME. Mea culpa.

I assume then that with earth rods outside the house the connecting cable has to be removed or cut at the point of entry else it'd need to be bonded.

[John Rumm]

To be fair I have not looked at the amendments either and you are correct that both PE and PEN conductors can be connected to a local earth... traditionally it was only installs with PEN.

They are capable of introducing a potential into an equipotential zone, so yup, if leaving them connected you would bond them.

[Roger Mills]

I'm confused! You don't appear to have had a complete new incoming mains cable, but just a new cutout. So, if your incoming cable carries a supplier earth, it's *always* carried a supplier earth, with no need for earth rods.

Am I missing something?

[Robin]

It could be that Dave has - as we do here - an old cable with lead sheathing. That sheathing can be used to provide an earth. It does here: at some time a length of 16mm green & yellow was sweated on.

At other places it can be provided by means of a clamp on the cable - but not just any old clamp and the cable ain't the occupiers to play with.

And at yet others there is nothing and people either fit earth rods or just carry on using the gas and water pipes (despite all the plastic pipes that have replaced the lead and iron).

[Animal]

In short, you're a bit safer relying on the supplier's earth. Leaving the local rod/s connected is either a good thing or immaterial, depending on details. 30mA RCDs are fine with a supplier earth.

[Brian Gaff]

Logic would suggest that any difference between the two earths would be millivolts and an inability to sink any current either, unless there are some special reasons for the way its been done now. I mean many of us out here have not had the earthing changed since before we were born!

Brian

[Brian Gaff]

But really not required if its an issue, I'd have thought. Brian

[Dave Plowman (News]

Third UK Power chap has said ignore it.

So just to clarify. Since I have an earth busbar very close to the cutout, just add a link between that and the new earth tag? I've even found some

16mm earth wire. Don't really want to rip out all the existing earth rods, etc, if not needed.

Other points arising from a pic I put of it on a group where it's easy to do this. There is three core armoured between the cutout in the cellar and the meter which is at the top of the stairs. Installed before I bought this house, likely by the LEB. The armour isn't earthed at all. And the tails into the cutout not double sheathed. The meter end is, though. Would a standard earth clamp for pipes be OK to ground the armour? Looking at TLC, most of their solutions seem to involve a gland of some sort.

And could I remove the second layer of PVC from spare meter tails, split it. and secure with cable ties? Other suggestion was to make a cover between the cutout and SWA.

I'm surprised non of the UK Power chaps mentioned this. They could easily have added sleeving when replacing the cutout.

[Andy Burns]

sounds like it's not your cable to mess with?

[Dave Plowman (News]

Correct. Are you saying I should get UK Power back to sort the points I made? Or does their responsibility end at the cutout? If so who is responsible for that cable, if it's not mine?

[Jack Harry Teesdale]

You don't necessarily need to rip out the earth rods, just disconnect them from the installation

Personally, I would not mess with any of the cables from the cut-out to the meter, they are not your responsibility and if you damaged them in anyway. it could cause more problems than it solves.

The only meter tails you should touch are those after the meter to your fuse box/consumer unit. Altering the tails from the cut-out to the meter in any way might look like an attempt at illegal abstraction.

A non-combustible removable cover as you suggest should be OK.

They won't do any more than they have to.

[Andy Burns]

And IET regs don't apply on their side of the meter.

[John Rumm]

If it is a TN-C-S earth, then yes there will be a negligible difference in potential between the combined earth and neutral conductor at the premises and a local earth rod.

With a TN-S earth it could be more depending on how far away the sub station is, and how many other properties leakage are flowing to it along the way.

[Dave Plowman (News]

I've actually measured the PD between my earth and the unconnected service one. 0.146v

[Dave Plowman (News]

Thanks, Andy. That didn't occur to me. ;-)

[Animal]

sounds good

Double insulation on tails is not something worth doing anything about. Not even vaguely.