is this TN-S or PME?

[JohnW2]

I am ready to be shot down in flames but my understanding is that if the PME conditions can be met a service which is fed by a SNE cable can be converted to a PME.

What do you mean by "can be converted to PME"? Do you perhaps mean "can be regarded as PME"? If not, what is this 'converting'?

Kind Regards, John

 

[Wontdothatagain]

sorry, i deleted my last post as i misread yours

 

[JohnW2]

sorry, i deleted my last post as i misread yours

No problem - as you say, westie will probably have an answer!

Kind Regards, John

 

[Wontdothatagain]

What do you mean by "can be converted to PME"? Do you perhaps mean "can be regarded as PME"? If not, what is this 'converting'?

If you look at the picture at the beging of the thread again,

An electrician phones the DNO and reports a high Ze. DNO attend and find that the electrician is correct but their existing sweated joint is OK but after some testing they find that it will meet the parameters of a PME supply so they disconnect the sweated earths and run a piece of G/Y from the Neutral block to the MET, supply has been converted.

I have a picture somewhere of this exact situation, will try to dig it out.

 

[Wontdothatagain]

Untitled

• Wontdothatagain
• 11 Mar 2011
• 2

This was originaly a TNS supply running from a comercial unit on the ground floor to a residential in a flat above. In the commercial unit there is a split phase servise head, 1 fuse for the commercial, 1 for the flat above.

One evening, the meter caught fire and damaged the servise head, DNO attended and found that the Earth from the commercial unit was open circuit so it was converted to PME as it was not feasible to get a new supply cable to the flat.

 

[JohnW2]

What do you mean by "can be converted to PME"? Do you perhaps mean "can be regarded as PME"? If not, what is this 'converting'?

If you look at the picture at the beging of the thread again, An electrician phones the DNO and reports a high Ze. DNO attend and find that the electrician is correct but their existing sweated joint is OK but after some testing they find that it will meet the parameters of a PME supply so they disconnect the sweated earths and run a piece of G/Y from the Neutral block to the MET, supply has been converted.

Sure, but that clearly has not been done in this case (unless the 'piece of G/Y' is very well hidden!).

Perhaps I wasn't clear, because I was asking (or intending to ask!) a rather different question ... if there is a CNE main, can that be used to provide a TN-C-S service to a consumer by running an SNE cable from the main to the consumer, using the sheath/whatever of the SNE as the earth (i.e. with the CNE splitting into separate N and E where the service cable joins the main)?

Kind Regards, John

 

[Wontdothatagain]

Sure, but that clearly has not been done in this case (unless the 'piece of G/Y' is very well hidden!).

Hense, i say it is a SNE that has had a repair done with an incorrect lable fitted

 

[RF Lighting]

 

[Wontdothatagain]

Typical yorkshire man, wants best of both worlds

 

[JohnW2]

<a piccie>

Wot's dat - TN-C-S/PME, with the sheath of the cable used as one of the 'M's?

The cables coming out of that Henley (and presumably going to CU/bonding) look a littly puny - is that an optical illusion?

Kind Regards, John

 

[RF Lighting]

I'm guessing it's a failed TN-S that's been converted to TN-C-S, and the sheath of the PILC is bonded / earthed, rather than supplying the earth.

Not hugely unusual around here.

I think the small earths were 6mils to some old boards, now replaced.

Remember this thread?

//www.diynot.com/forums/electrics/things-that-make-you-go-hmmm.332268/#2468518

 

[JohnW2]

I'm guessing it's a failed TN-S that's been converted to TN-C-S, and the sheath of the PILC is bonded / earthed, rather than supplying the earth.

Well, whether you call it bonding, earthing or whatever, it's also unavoidably serving as another 'M' for the PME as well

I think the small earths were 6mils to some old boards, now replaced. Remember this thread? //www.diynot.com/forums/electrics/things-that-make-you-go-hmmm.332268/#2468518[/QUOTE]
Ah, yes - the G/Y spider's web

Kind Regards, ohn

 

[THRIPSTER]

Thanks for the interesting discussion guys. So, contact with the DNO shall be made - lets see (a) if they know (b) how long ig takes.

Regards

 

[THRIPSTER]

Well, back on site today and coincidentally, contractors turn up three doors down to replace overheating, fused neutral, cut-out. He agreed to have a look at the cut-out where I am working and thinks that the supply was intended to be PME'd but they haf forgotten to link the neutral with 16mm G/Y to the MET - just an opinion but seems to confirm what I had thought. Incidentally, they did not complete the job three doors down because the cut-out arrangement is too dangerous. He also said that he thought the supply to the street was run in 70mm (or imperial equivalent - houses built in 30's) with a 6mm 'service' into the cut-out. So, if true, thirty divided by three terraced houses running off a 70mm?

Regards

 

[westie101]

So, if true, thirty divided by three terraced houses running off a 70mm?

No problem.

A 4c .1 sq in cable has a continuous rating of 265A/ph (60kVA @ 230V) and a Cyclic rating of 290A/ph.
Terraced houses would have an ADMD of about 1.5kW so 40/phase is quite likely and seen often (we actually see more than that)