TT or PME?

tudge56 · Jun 20, 2024

Hi folks, I came across this cutout today with an earth cable going directly from the cutout into the ground....is it some sort of hybrid TT system? Ze was 0.39 which seems too low for TT (but a bit in the high side for PME? Armoured cable runs from here to the house about 20m away and I took the reading at the house consumer unit). Thanks in advance for the help

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davesparks · Jun 20, 2024

Could be PNB or just PME with an end of service cable electrode connected at the cutout.

If you tested at the load end of the submain then you have measured Zs not Ze.

SJD · Jun 20, 2024

Where does the E cable from the cut-out go? Possibly it joins the outgoing SWA from the switch fuse to the house, e.g. if the SWA is only 2-core. Possibly it is bonding to somewhere. I don't think you can make any assumptions.

tudge56 · Jun 20, 2024

Just had to Google PNB - very interesting! I was puzzled by the cable because it is connected into a part of the cutout that most normal sparkies don't have access to and therefore thought it must be something the DNO has installed. The cable doesn't seem to go back to the house consumer unit. I think I will remove some of the leaves and see if I can find a rod connection or something. But it's going down on the cert as a TN-C-S and I'll take a proper Ze (davesparks

) at the isolator.

bill01803 · Jun 21, 2024

I had one like that, it was TNCS the DNO had put a mat outside the door because of the distance from the transformer there was only two properties on the cable.

Marvo · Jun 21, 2024

Can't you just phone the DNO? Surely they have records of their load-side supply infrastructure.

R-fur · Jun 21, 2024

Marvo said:
Can't you just phone the DNO? Surely they have records of their load-side supply infrastructure.

Ha ha, you are funny.

R-fur · Jun 21, 2024

It is probably PNB or PME. PNB is usually applied where a singe customer is fed from a single, usually pole mounted transformer.
With PME if the service is over a certain distance the DNO is supposed to fit a separate earth connection, either a rod, a separate earth conductor or by bonding two cut out together. It is quite unusual to see though as it is rarely done.

timhoward · Jun 21, 2024

Marvo said:
Can't you just phone the DNO? Surely they have records of their load-side supply infrastructure.

To give you some idea....with my DNO, the question "Can you see arcing, sparking or smoking?" is 3.5 minutes into the automated call answering system. The call handlers are trained that the default position is "We don't/didn't supply an earth". To get anything sensible/useful you have to either get the call escalated to the bloke (or lady) with a beard, or get an engineer on site.
It was a lovely idea though!

TT or PME?

tudge56

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davesparks

SJD

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tudge56

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bill01803

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Marvo

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R-fur

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R-fur

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timhoward

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