Earthing - yes yet another electrical earthing question or two

[tom.harrigan]

I've got a TT installation (overhead wires), but it still comes into the house via a "PME" cable.

T

[John Rumm]

By "PME cable" I presume you mean a concentric one? (they seem to use that type of cable quite commonly for TT and TN-C-S supplies these days).

I think we have got the same here - overhead, concentric cable through the house to the main cutout, and then TT from there.

[John Rumm]

Indeed, which raises the original question again about adequate earthing. Looking doubtful with what we can see.

No, they are unsealed. To seal them the tags bend up and round and are wired together such that they block access to the screw.

Well that does depend a bit on what is on the end of the tails that romp off the top of the photo! ;-)

The other question that springs to mind is why the three (what look like) 16A MCBs, when the label suggest 2 ring circuits.

[The Wanderer]

You say 'tapped off'. How exactly was that done? Was it a loop from the cutout? Was there a joint on the cable? Was it a separate service jointed on outside?

Sorry, but on what you've said, it's difficult to offer any comment.

[The Wanderer]

I have Google posters KF'd, but....

when an overhead system is made PME, all that happens[1] is the neutral integrity is strengthened by ensuring all joints are compression fittings as distinct from mechanical connectors - line taps - and some additional earth electrodes are scattered around at the end of radial legs.[2] The system is now PME and services can be connected as PME.[3] For an (armoured) underground service off an underground network, it's just a case of bonding the armouring to the neutral. The armouring is considered to be the additional earth electrode at the end of the system, IYSWIM.

[1] Well it was in my day - I've been out of the industry for a few years. Perhaps Colin Wilson can confirm current practice. [2] Of the distribution system, not at the end of individual services. [3] But only if you ask the relevant company - the distribution neutral may have been compressed, but individual services may not have been.

[tom.harrigan]

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I suspect they may have been using that sort of cable to bring TT into a building since the late 70s. My house used to be a post office and had its supply updated then. I've got the armoured cable, while all my neighbours have two separate wires.

T

[tom.harrigan]

My system certainly hasn't been made PME even though it is connected via that type of cable. I think the important point is whether the DNO (or whoever it is nowadays) provides an MOE. When I asked what that was I was told it means Means Of Earthing. Round here at least, I get the feeling that if they provide an MOE, you can use it. Otherwise your earthing is your responsibility.

T

[Geo]

They are marked "Live to storage heater meter" in the photo...

[John Rumm]

Doh!, yup well spotted - I missed that completely. ;-)