An electrician as advised someone I know that before he can change the CU the electrical supply needs to be modified to provide a PME.
cable and there is a 6mm earth wire attached to the outer sheath of the cable and connected into the existing CU. I think that makes it a TN-S system. So why would he want a PME connection..??
No.
That's a good question, and one I think the 'electrician' should be invited to answer.
Owain
That's not always possible with some supplies.
Ask for written evidence supported by test results as to why this isn't satisfactory.
...particularly as a TN-S system cannot be converted to PME (TN-C-S).
agreement with all but to add to Dave's comments it might also be worth asking said electrician does he know what PME is??
CJ
Would the supply company provide an earth connection that was bonded to the Neutral in a situation such as this ? Or would they expect the electrician to simply upgrade the size of the earth connection onto the sheath of the supply cable when he replaces the CU
Well, that would sort of make it TN-C-S without the PME...
Don't think that would be allowed. He would need to get the REC in to upgrade the earth bond.
Strictly, the supply company should do it. In practice, the electrician will do it. With some types of supply cable, you have to be very careful with the earth clamp, or you can damage the cable such that it might short out (particularly if you use a pipe clamp).
Can't it? I'm sure the REC could convert from TN-S to TN-C-S if they wanted to. For any given TN-S final circuit off the substation they would just need to add the PME and change every consumer Henley block from TN-S to TN-C-S.
I've no idea whether they do or not...
Not sure I follow, where do Henley blocks come into this?
Sorry, my bad. I meant house service cutouts.
Actually, looking at the Henley website the REC could maybe changeout some components in the cutout to convert it from SNE to CNE (separate neutral and earth to combined...)
The supply network would need replacing. PME has to have a special high integrity PEN conductor (by law) that TN-S neutral or earth doesn't require. It's not simply a case of changing the service cutout.
I didn't say it was simply a case of changing the service cut out.
I (rather tongue in cheek) said they would need to "add the PME" first. I didn't specify what was involved in "adding the PME" and I agree that in many cases, the relevant substation final circuit ("distributing main") would need to be replaced.
This pdf document ("Distribution Low Voltage Earthing Manual") from Scottish Power gives a good insight:
If I read this document correctly drawing number ME8 shows that existing TN-S circuits can be converted to an PME and the supply company would simply connect the earth sheath of the cable to the Neutral and provide a customer Earth connection.....They also indicate that this would be the preference if a customer requested a new earth connection.
An I right ?
Well, it would seem to be the case, provided that the SNE service line and distributing main meet the requirements of PME as detailed elsewhere in the document.
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