TT to PME?

[xr4x4]

Hello,

We have a TT earth system and am wondering how easy it would be to upgrade to PME.

The pole that supply's us and next door, the overhead line is in our garden.

Now there's what looks like a earth cable running down the pole, then under a cover into the earth. At the top it's connected to a loop of wires that come in and serve us and next door.

Can anyone point out why this is? Is this how PME is connected to neutral via earth?

Thanks.

 

[RF Lighting]

The neutral will be tied to earth regardless of the supply type.

The M in PME stands for protective MULTIPLE earthing, so a single connection to earth is not enough. There needs to be several connections to earth on the network, and the only people who can tell you if that is the case with your supply is your DNO. Just contact your DNO and ask them.

<edit> typo

 

[ricicle]

The neutral of a PME system is earthed at various places along it's length on overhead lines (hence the Multiple Earthing of pME) Then your earth is derived from the neutral at the cut-out. It could quite well be that PME is available but only the DNO will know for sure. They are sometimes unwilling to give you PME if your neighbour has not got it or does not want it.
Best check is my advice.

 

[PrenticeBoyofDerry]

A TT arrangement would normally be connected to your earth bar at your board or via MET block to earth bar at board, from either an earth rod or electrode often external of property.
A PME would normally be connected to the neutral at your incoming services and as ricicle states earthed at various points.
The DNO will change to PME at a charge, normally about £200 but could be regional variations. Confirm with them!

 

[xr4x4]

Hmm ok thanks,

Bearing in mind that the house next door, (that is connected to the same pole) is TT, would this cause problems if we where converted to PME and they werent?

 

[ericmark]

Hmm ok thanks,

Bearing in mind that the house next door, (that is connected to the same pole) is TT, would this cause problems if we where converted to PME and they werent?

Theory yes. The problem in theory is your earth and next doors earth could be at different voltages so touching your earthed soil pipe and their earthed soil pipe could give you a shock.

In practice it is realised that converting houses to PME can't be done in one hit and it is accepted there can be work in progress even if it takes 10 years between both houses being converted.

End of day down to DNO. When I wanted a PME supply I lied and told guys next door was PME so should not I be the same and they measured the ELI and said yes. I got it FOC. I had not a clue what next door had but did I really care?

So much it in how you ask.

 

[bernardgreen]

Having just installed a new supply ( PME ) into a cottage that was previously supplied from the adjacent shop (TT) I can assure you that PME and TT in adjacent buildings is prone to problems. For a start I had to ensure the water supply ( also commoned ) has isolation gaps to ensure my "earth" cannot conect to the "earth" in the adjacent shop. If at some time in the future steel re-inforcements are needed in the roof then special arrangements will also be needed to ensure these do not and cannot link the "earths" when they are bonded.

I am seriously considering converting from PME to TT for my supply. That decision is influenced by experience of a bouncing neutral at my previous house where the "earth" was recorded as being 30 volts above ground during network "problems" ( Problem = fault and/or un-balanced loading on the phases )

 

[JohnW2]

Having just installed a new supply ( PME ) into a cottage that was previously supplied from the adjacent shop (TT) I can assure you that PME and TT in adjacent buildings is prone to problems. For a start I had to ensure the water supply ( also commoned ) has isolation gaps to ensure my "earth" cannot conect to the "earth" in the adjacent shop.

I take it that you mean that the 'insulating section' completely spans the connection between the two properties? If any metal pipe entered one building from the other, the required main bonding would obviously defeat your objective.

Situations similar to that you describe are not that uncommon. As discussed before, I have a TT supply and my nearest neighbour has a TN-C-S/PME supply. Although TT, my installation's Ze is about 0.25&#937; with main bonding present (and only about 0.35&#937; without main bonding - presumably due to incidental connections of CPCs to pipework). I presume this is due to main bonding to the water supply pipe of the neighbour's PME supply, since I find it hard to believe that the underground pipe, per se, can explain a Ze as low as I get. If I'm right, then the bottom line is that there is probably a pretty intimate connection between my MET and the neighbour's PME 'earth'.

There really is little I can do about that which is compliant with the regs. The water supply enters my cellar in a large iron pipe several feet below ground level, so there is no practical way of inserting any 'isolating section' between the outside world and the interior of my property - and the regs require any metal pipe entering my property to be main bonded.

Having I am seriously considering converting from PME to TT for my supply.

As discussed here before, I am being 'threatened with' a PME supply. Were it not for the above, I would be definitely tempted not to use the PME 'earth', even if supplied - although, as things are, I'm not sure that I would actually avoid 'PME issues' by so doing.

Kind Regards, John.

 

[bernardgreen]

I take it that you mean that the 'insulating section' completely spans the connection between the two properties? If any metal pipe entered one building from the other, the required main bonding would obviously defeat your objective.

The pipe on my side of the isolation gap is connected to the "earth" of my PME system and the pipe in the shop on the other side of the isolation gap is ( I assume ) connected to the earth that is the ground rod of the TT supply.

If a metallic pipe did cross from one to the other without an isolation gap then bonding (*) it would create a hazard. ( source:- the DNO technical office when I was arranging for the new supply to be installed. )

(*) bonding or any other form of connection of the pipe to the PME "earth". One example of while the terms "bonding" and "earthing" are totally different functions the end result is the same connection to the "earth" through the bonds and pipe work.

 

[JohnW2]

The pipe on my side of the isolation gap is connected to the "earth" of my PME system and the pipe in the shop on the other side of the isolation gap is ( I assume ) connected to the earth that is the ground rod of the TT supply.

Yes, I realise (or, at least, was assuming!) that, but what I was asking was for you to confirm my assumption that the 'insulating gap' encompassed (at least) all of the pipe run between the two properties (i.e. that the 'insulating gap' was not contained entirely within one or other of the properties) - since, as I said, you would otherwise not be able to achieve your objective with a BS7671-complaint installation.

Kind Regards, John.

 

[bernardgreen]

(i.e. that the 'insulating gap' was not contained entirely within one or other of the properties)

The pipe to the shop comes out of my outside toilet at ceiling level, crosses under the ceiling of my large rear porch and goes across the top of the door frame of the back door of the shop, down the side of the door and then through the door frame into the shop. There is an "isolation gap" in my toilet created accidently by use of a flexible coupling. ( previously both properties were on the same TT supply with the cut out and meter in the shop ).

To be 100% compliant there should be an isolation gap inside the shop so the exposed pipe in my yard is NOT connected to the TT rod and an isolation gap before the pipe leaves my toilet to ensure the exposed pipe is not connected to my PME "earth" in case someon in my yard touches to exposed pipe and the exported PME "earth" is far enough from the ground they are standing one to give them a shock.

- since, as I said, you would otherwise not be able to achieve your objective with a BS7671-complaint installation

Compliance with BS7671 is maybe possible for one or the other but difficult for both. It gets worse as 75% of one of my bedrooms is over the shop so in the void between their ceiling and my floor boards there has to be ensured separation of any and all conductive items that may become earthed by earthing wire or bonding wires. That relatively easy as I am confining all fixed under floor wiring to the 25% that is over my hallway.

 

[JohnW2]

There is an "isolation gap" in my toilet created accidently by use of a flexible coupling. ( previously both properties were on the same TT supply with the cut out and meter in the shop ).

I presume you must mean a non-conducting flexible coupling, not the standard hose surrounded by SS braiding.

To be 100% compliant there should be an isolation gap inside the shop ....

I'm not sure that even that would be (BS7671) compliant. I have a feeling that the regs' definition of an extraneous-conductive-part could well be taken to apply to any conductor which enters your property, even if, at present, you know that it is not 'liable to introduce a potential' (because of an 'insulationg gap' within the shop) - on the basis that no-one knows what might get connected (explicitly or implicitly) to the metal pipe (in the shop) on your side of the shop's 'insulating gap' in the future.

Compliance with BS7671 is maybe possible for one or the other but difficult for both.

As I implied, it would be very easy to achieve compliance (in the respect we are discussing) for both if the length of pipe which passed from within your property to within the neighbouring property were entirely plastic - but that would, at the least, require the co-operation of both property owners.

It gets worse as 75% of one of my bedrooms is over the shop so in the void between their ceiling and my floor boards there has to be ensured separation of any and all conductive items that may become earthed by earthing wire or bonding wires.

That's one approach. As you know, BS7671's primary approach is generally not to 'ensure separation' but to accept that extraneous-conductive-parts exist and to bond them (with whatever 'problems' that could potentially create).

Kind Regards, John.

 

[xr4x4]

I think im gonna stick with TT, its just easier!

 

[JohnW2]

I think im gonna stick with TT, its just easier!

As above, same here - except that, in my case, it probably isn't quite that, given that I probably am 'enjoying' a PME 'earth' via a water supply pipe and (my and my neighbour's) main bonding, whether I want/like it or not!! Something similar could, of course, possibly be true of your installation.

Kind Regards. John.

 

[xr4x4]

we have no external metal work, incoming water pipe is plastic, gas is bottled for hob.