Upgrading TT to PME

[tom91]

I recently had a new CU installed (but pre 17th Ed). It is split load, with all circuits RCD protected except the two lighting rings and a 16A radial that supplies a few sockets and 2 security lights through FCUs.
Our supply used to be overhead, but it was changed to underground a few years ago. But, we still have a TT earth system (about 107 ohms impedance).
AIUI, on a TT system all circuits must be RCD protected (or ELCB) as if there is a live-earth fault, there is not enough current to trip an MCB. Our old CU was fully RCD protected. When the electrician was here doing the job, he said we should get the supplier to upgrade the earth to PME (TNCS). I called EDF and they wanted £150, which seems a bit steep.
So, what do other DNOs charge, and what would be the implications be of keeping the TT earth ? I'm guessing the non-RCD circuits are going to need to be RCD protected.

 

[hairyben]

You really shouldn't have been left with unprotected circuits.

I've done work in simular situations and left a 30mA RCD on the main tails, not in itself exactly compliant but IMO okay as a temporary measure.

As for EDF, they consider themselves as having a licence to charge what they like and there's little you can do about it or them. If they're putting in a PME earth they may require 16mm earth bond to gas & water, and may charge cancellation fee's if this is not in place.

 

[ericmark]

I agree with hairyben you should not have been left without RCD protection on all circuits.
Anything after 2004 I think will be controlled by Part P and a consumer unit change is one of the notifiable items so you should as well as the installation cert have also been given a completion cert. I can't see how an electrician could issue one and leave the work as he has and would have thought you have some re-dress. I would hope some of the others will comment on this.
As to TT and TN systems the supply authority is not allowed to mix and match either all houses in an area are TT or TN which does leave the electrician in a fix in a way as he can't decide on type of supply it has to be supply authority.
I have in the past phoned up the supply authority and asked them what type of supply should I have. They normally then tell me they don't have to supply an earth. I then say that was not what I asked I want to know supply type (hear I often lie and say seems next door is TN) and it is your responsibility to tell me what type of supply I should have. They then normally have to send out someone to check and 9 times out of 10 they will then provide an earth FOC.
It has to be you to phone not the electrician but if right questions are asked normally you will get it.
But if not sorted when I leave a job then I have always used a 100ma RCD as the main switch. Under 17th Edition this is not so much a problem. And I am told other areas may work in different ways I am in old MANWEB area.
Eric

 

[tom91]

Thanks for the replies.

We also recently had a letter saying we needed to have our meter changed. Is it possible they would change the earth when they came to do the meter, possibly avoiding the charge?

The spark did issue a certificate. Under the 'Comments on existing installation' section he's entered 'Awaiting system being converted to T-CNS'. It also notes that the main earth to CU is only 6mm, as is the water bond. The gas bond is 10mm. Is this going to need to be upgraded for PME?

One final question, when the come to do the work, will the connect the new earth to the main earth terminal, or do I need to do that myself?

 

[Spark123]

He should never have issued a certificate nor energized the system without it being RCD protected as it is dangerous, even if it is awaiting a TN-CS update. I take it the certificate which he has signed says it complies to BS7671:2001? If so then he committed fraud too.

 

[hairyben]

Thanks for the replies.

We also recently had a letter saying we needed to have our meter changed. Is it possible they would change the earth when they came to do the meter, possibly avoiding the charge?

The spark did issue a certificate. Under the 'Comments on existing installation' section he's entered 'Awaiting system being converted to T-CNS'. It also notes that the main earth to CU is only 6mm, as is the water bond. The gas bond is 10mm. Is this going to need to be upgraded for PME?

One final question, when the come to do the work, will the connect the new earth to the main earth terminal, or do I need to do that myself?

1. Meter monkey won't do anything but swap the meter.

2. While a certain amount of existing non-compliace can be discharged with the "comments" section, it's not a licence to do what you like.

The leccy board may insist on 16mm earthing. You need to clarify this with them before the work commences. so long as you have a installation cert they'll do the work, they can't read it anyway.

3. If you main earth terminal is outside the consumer unit they should connect, make sure you have a bit of 16mm earth, they often specify this is client-provided.

 

[hairyben]

As to TT and TN systems the supply authority is not allowed to mix and match either all houses in an area are TT or TN which does leave the electrician in a fix in a way as he can't decide on type of supply it has to be supply authority.

I dunno if it's a recent thing but when I worked for them overhead supplies were often converted/renewed to have PME avaliable, so you would have houses with alternate TN-CS/TT supplies

 

[RF Lighting]

The DNO round here are quite happy to mix TT, TN-S, and TN-C-S all on the same street.

I can't see a problem with it either.

As for leaving a TT supply without an RCD is one of the worst things he could have done.

It really is serious, and needs sorting immediatly.

I would not switch that installation on.

 

[JohnD]

If EDF want £150 to convert to PME, it is not very cheap, but still money well spent. Once done they will be responsible for maintaining it so you will not have to pay again.

You will need 10mm main bonding to gas and electricity in place before they arrive, which you can do yourself, and 16mm to the CU, plus a 16mm length ready for them to connect to the new point. A convenient way to do this is with an earth block that should cost you about a pound, which you can mount close to or on the meter board, and you can marshall all the earth wires in here, including your old earth spike for the moment. It should have a durable "safety electrical connection do not remove" label and the cheapest way I know is to buy an extra Pipe Bonding Clamp, they come with these metal labels and have a screw hole..

A 4-way block is usually enough but an 8-way costs little more and has room for expansion if you need it in future. You are allowed to do the bonding yourself.