PME protective conductor sizing?

drew35 · Jul 12, 2011

Need a little advice on this one.

I rewired a house last year, all sorted and signed off, but I was told I would be coming back as there was a new extension planned. Unfortunately the building contractor would only use their own electrician so I lost the extension work, no worries. The customer has put lots of other people my way so he's worth his weight. He asked me just to cast an eye over the electrical work, and certificate as he wasn't one hundred percent with the finished job. Its not the tidiest job but it ok, except.

The main cut out had to be moved by the supplier to a new meter cabinet on an outside wall. The csu was then reconnected via a switch fuse using three core 16mm swa cable. The 16mm line and neutral obviously go straight into the csu. The earth cable runs to a main earth terminal block, but then steps down to 10mm before entering the csu.

This was done, according to the certificate, to satisfy 542.3.1, earthing conductor sized in relation to the neutral as its PME. I've not seen this done before, is it sized properly seeing as for most of its run the earth conductor is 16mm, the same as the line and neutral, all but for the last 500mm?

Cheers.

Electro-tech · Jul 12, 2011

I would say 16 mm all the way to the Consumer unit.

malcolmsanford · Jul 12, 2011

Technically they have wired it to the regulations and table 54.8 and so less than 35mm tails is 10mm. I would say if you did the adiabatic equation you could check to make sure that 10mm is fine, but I'm sure it would be.

Engineer54 · Jul 12, 2011

drew35 said:
Need a little advice on this one.

I rewired a house last year, all sorted and signed off, but I was told I would be coming back as there was a new extension planned. Unfortunately the building contractor would only use their own electrician so I lost the extension work, no worries. The customer has put lots of other people my way so he's worth his weight. He asked me just to cast an eye over the electrical work, and certificate as he wasn't one hundred percent with the finished job. Its not the tidiest job but it ok, except.

The main cut out had to be moved by the supplier to a new meter cabinet on an outside wall. The csu was then reconnected via a switch fuse using three core 16mm swa cable. The 16mm line and neutral obviously go straight into the csu. The earth cable runs to a main earth terminal block, but then steps down to 10mm before entering the csu.

This was done, according to the certificate, to satisfy 542.3.1, earthing conductor sized in relation to the neutral as its PME. I've not seen this done before, is it sized properly seeing as for most of its run the earth conductor is 16mm, the same as the line and neutral, all but for the last 500mm?

Cheers.

More like it , that his length of 16mm earth cable was short, and had a bit of 10mm single in the van, which means there will be a joint in the line (hopefully crimp a joint). ...No-one reduces a cable run size, along the way, and least not for the sake of half a metre!! ...lol!!!!

Electro-tech · Jul 12, 2011

Onsite Guide page 15......on , General Layout of equipment does show 16mm for all Types of supply.

IQ Electrical · Jul 12, 2011

malcolmsanford said:
Technically they have wired it to the regulations and table 54.8 and so less than 35mm tails is 10mm. I would say if you did the adiabatic equation you could check to make sure that 10mm is fine, but I'm sure it would be.

Remember that it's not 'tails' size that you are using for PME but the SUPPLIERS neutral conductor.

Stick with 54.8 as Malcolm says and you can't really go wrong.

The On-Site Guide is 'idiot proof' in many cases and sticking to BS7671:2008 can often save you a few quid while still being fully compliant.

malcolmsanford · Jul 12, 2011

IQ Electrical said:
Remember that it's not 'tails' size that you are using for PME but the SUPPLIERS neutral conductor.

Stick with 54.8 as Malcolm says and you can't really go wrong.

The On-Site Guide is 'idiot proof' in many cases and sticking to BS7671:2008 can often save you a few quid while still being fully compliant.

Quite right mate it is the Suppliers Neutral conductor on the main cable, but as I'm not the type to open the head to check the neutral size, I wrongly I agree, just look at the tails coming out of the head to the meter. ...........

drew35 · Jul 13, 2011

Ok thanks everyone but I don't think I quite explained it enough. What I was saying is, according to what's written on the certificate, the main earthing conductor, NOT THE BONDING, has been reduced in size to comply with regulations.

I was confused by this but thought there may be something I was missing.

What I think has happened is that the electrician who did the job has confused bonding conductor sizing, with earthing conductor sizing for PME. He must have thought that the earthing conductor had to be sized to the neutral, and so has reduced it to 10mm?

It does still comply. I just thought someone was quoting a reg that I had never heard of.

malcolmsanford · Jul 13, 2011

Did you read 542.3.1 for a PME system you do size the main earth conductor to regulation 544.1.1 which deasl with bonding conductors.

544.1.1 tells us PME bonding conductors except for highway power and street furniture must be sized to table 54.8 and for a domestic situation the chances are it will be 10mm as the supply conductor from the DNO will have a neutral smaller than 35mm

So your bonding conductor will be 10mm so will your earthing conductor.

IQ Electrical · Jul 13, 2011

malcolmsanford said:
Did you read 542.3.1 for a PME system you do size the main earth conductor to regulation 544.1.1 which deasl with bonding conductors. 544.1.1 tells us PME bonding conductors except for highway power and street furniture must be sized to table 54.8 and for a domestic situation the chances are it will be 10mm as the supply conductor from the DNO will have a neutral smaller than 35mm

So your bonding conductor will be 10mm so will your earthing conductor.

Exactly, I had this discussion on another thread last week but I think I failed to convince!