Is it TT or PME

[JACKC]

I'm new to domestic electrical work and need some advise please.
currently maintainenace engineer in a factory, doing this semi professionally.

1. Line and neutral from overhead wiring comes into house, live connects to CU via suppliers fuse and meter, neutral connects to connector block, on then onto meter , then CU. 16mm earth comes off connector and then connects to CU earthing strip. No RCD protection in CU. I think supply is PME but as it was overhead wires assumed TT. Went into cottage and found no 100mA rcd or rcd protection in CU. Am I right with PME?

2. CU is a Doepke, are there mcb's available for this CU? Have recommended the fitting of rcd to protect cottage sockets. Owner wants supply to garage , told him its notifiable and contact me when approved.

3 CU is in downstairs toilet with shower fitted, not within arms reach of shower. Is this ok, can't find anything in regs book.
Your input will be greatfully received, thanks in anticipation.

 

[Qedelec]

Its TN-C-S, not unusual for supply to be overhead in rural areas though.

 

[james 29]

Do a loop test on the circuit, even at a socket, if its TT you would be very fortunate to gain a reading less than 1 ohm without multiple electrode installation. the other two types of supply should be less than 1 ohm. Check parallel paths before testing though.

If pme the supply neutral should be earthed along the line not just at the incomer. Potentially dangerous if not.

 

[Qedelec]

neutral connects to connector block, 16mm earth comes off connector and then connects to CU earthing strip.

PME!!

 

[james 29]

Qedelec, just because someone connects a 16mm tothe neutral block doesnt make it a pme supply.

In a pme supply the incoming line has to be earthed at several points.
Cant see how you can earth a cable thats 20 foot up unless it has been carried out at each end of the run. I was merely stating he should check this. The DNO often bring in supplies for us which do not have a pme lbel put on them. This is because certain precautionary earths have not been fitted to reduce neutral earth potential in the event of a neutral failure.

The supply authority should be able to advise.

 

[Qedelec]

Qedelec, just because someone connects a 16mm tothe neutral block doesnt make it a pme supply.

In a pme supply the incoming line has to be earthed at several points.
Cant see how you can earth a cable thats 20 foot up unless it has been carried out at each end of the run. I was merely stating he should check this. The DNO often bring in supplies for us which do not have a pme lbel put on them. This is because certain precautionary earths have not been fitted to reduce neutral earth potential in the event of a neutral failure.

The supply authority should be able to advise.

Agree'd but from the information given most would surmise this was the case.
Lets hope Jack has access to a ELI tester.

 

[ban-all-sheds]

Cant see how you can earth a cable thats 20 foot up unless it has been carried out at each end of the run.

By earthing at every pole?

 

[james 29]

Possibly BAS but it is certainly something which needs to be assessed.

The problem is most underground cables are set up for pme. With having seperate conductors for live and neutral which are usually found on overhead TT supplies i would class that as a situation which requires a biy of investigation.

I have never seen a pme from an overhead, and i am not saying it doesnt exist because i havent seen it either. This scenario may well exist but would you assume that pme was ok?

I suggested a loop test to ensure the line loop were below 0.35. This would indicate that when the supply was installed it was adequate for pme. It does not verify the supply is earthed as a pme supply.

 

[Qedelec]

I have never seen a pme from an overhead, and i am not saying it doesnt exist because i havent seen it either.

I have.

 

[PartPapproved]

So have I, I requested 2 weeks ago a TT supply overhead line 2 cores coming in to the house to be upgraded to a PME. The DNO arrived fitted new service head hey presto PME supply (No charge to the customer either !) Took the guys all of 20 mins including them chatting to me.

 

[securespark]

I have seen all sorts of supplies:

"TT" that has been "pme'd"

"TT" that has been converted to TNS

TNS that looks like PME, but isn't - the earth leaves the cutout, but the neutral & earth are not limked.

In the case of a TT - TNCS conversion, the only way to tell is with an EFL tester.

Ze on a TNS should be maximum 0.8 Ohms, and PME 0.35.

 

[JACKC]

Thanks for all the help, I will borrow the 1502 megger from work and test loop impedance as advised, thanks again.

 

[JACKC]

Any help on the mcb for the CU?

 

[securespark]

You only have to look......

http://www.doepke.co.uk/

 

[JACKC]

Thanks for that, tried internet but had so many web pages come up I couldn't find the right one, can now thanks again