What earthing system is this?

[Rauer]

Obviously PME but what type?????

 

[Guest123]

Looks to be a TN-S conversion.

The original lead sheath may have corroded at a joint perhaps, and the DNO may then have opted to convert to TN-C-S rather than dig up the road to find the problem and repair it.

The braid yo see attached to the sheath is probably doing nothing more other than acting as a MPB to the incoming sheath of the supply cable.

 

[telectrix]

seconded. if you measure your PSCC and PEFC, if they are the same, then assume it's TNC-S.

 

[Rauer]

I just guessed the meter man was abit confused! Belt n braces n all!!!

 

[Deleted member 26818]

Is that an old Manweb installation?

 

[Rauer]

It was in Warwickshire if that helps!

 

[telectrix]

where is it now?

 

[Rauer]

where is it now?

Scotland !

 

[MarkieSparkie]

where is it now?

Err... 42 Helmsome Rd, Leamington Spa, CV92 7DL:wink_smile:

 

[1shortcircuit]

There appears to be no seal on the cover, so if it was me I would have had a quick peek to see if there is a link between Neutral and Earth

Other than that, testing would prove if similar readings are tested on pefc and pscc

 

[Rauer]

Err... 42 Helmsome Rd, Leamington Spa, CV92 7DL:wink_smile:

Haha that got me for a minute!! Did some minor works , a spark had rewired it and did a right hash job ! Left wooden back boxes in the wall aswell!

 

[Deleted member 26818]

There appears to be no seal on the cover, so if it was me I would have had a quick peek to see if there is a link between Neutral and Earth

Other than that, testing would prove if similar readings are tested on pefc and pscc

Is that the cover with the sticker saying "earthing terminal PME system"?

 

[Rauer]

Think he is on about the neutral cover

 

[i=p/u]

a main fuse states 100A on carrier , does it mean there is 100A fuse inside....????

 

[Jonny66]

Thats true, but a PME sticker and an earthing conductor connected to the neutral suggests that the installation is presented as TNC-S, a call to DNO/supplyer should confirm this if you have the will power.

 

[Marnik]

Defo a TN-S to TNC-S conversion, see them all the time in the Exeter area

 

[Engineer54]

If that is a DNO fix, it leaves a lot to be desired!! You don't mix TN-S and TNC-S. This is especially true in the case of a network supply cable, in the process of being converted to PME!! The TN-S sheath earthing clamp and braid should have been removed and any exposed LEAD sheath insulated with amalgamating tape!!!

 

[Marnik]

Agree with you Engineer54 ,however, while conducting IECR's in student properties in Exeter last week , spotted a dozen or so
left like that by DNO!

 

[Rauer]

Oh I was on the job about 3 months ago and just found the pic on my phone

 

[Mikro]

So are they not duty bound to sort stuff like that out?

 

[mechelec]

If that is a DNO fix, it leaves a lot to be desired!! You don't mix TN-S and TNC-S. This is especially true in the case of a network supply cable, in the process of being converted to PME!! The TN-S sheath earthing clamp and braid should have been removed and any exposed LEAD sheath insulated with amalgamating tape!!!

Well you may be shocked to know this is standard procedure when fitting a new cutout on a paper lead cable.
How are you mixing TN-S and TNC-S? To carry this upgrade out the main must be PME and that was completed on the LV underground network in the 70's in my area so in doing that there was a mix.
Multiple new supplies to steel framed buildings can have a TN-S service made available from a PME main.
So I'm not sure why you have a problem with mixing systems if carried out correctly.

 

[needasparks]

Does this also cure a noisy earth i heard, I might also be talking rubbish.

 

[mechelec]

Does this also cure a noisy earth i heard, I might also be talking rubbish.

Not heard of that before but some else may know.

 

[Engineer54]

Well you may be shocked to know this is standard procedure when fitting a new cutout on a paper lead cable.
How are you mixing TN-S and TNC-S? To carry this upgrade out the main must be PME and that was completed on the LV underground network in the 70's in my area so in doing that there was a mix.

**Multiple new supplies to steel framed buildings can have a TN-S service made available from a PME main.
So I'm not sure why you have a problem with mixing systems if carried out correctly.

Depends on what you are calling the ''Main''. Most underground networks are not yet fully PME'd, they are basically work in progress of becoming a true PME supply. There are not that many network distribution cables, that have been fully PME'd (every cable T joint PME'd) So, on most of these outgoing local network cables you will have both TN-S and TNC-S being supplied from the same cable. So why would you want to introduce the possibility of a neutral fault (however remote it may be) onto a presently sound TN-S section of the distribution cable?? Remembering also that many of the older domestic TN-S installations barely, or doesn't meet current earthing/bonding requirements at all!!

As far as i know, and as explained to me by a DNO area manager, when earthing the neutral at underground cable joints on cables that are supplying both TN-S and TNC-S at service heads, the original cable earthing is left maintained/continuous and forms no connection with the PME, N-E rodded connection.

It's fine to connect a service head as shown in the OP's photo, IF and only if the network distribution cable has been fully PME'd and that the installation connected to the service head is also PME compliant... Fine to mix TN-S with TT or TNC-S with TT, but it's not generally fine to mix TN systems together...

** If carried out correctly is the kicker isn't it, I take it you mean from previously TN-S distribution network cable that's been fully PME'd?? It's not going to happen, if all the other service heads on that supply cable have been connected as in the OP's example is it?? Other than that, i can't see how you can derive a TN-S supply from a TNC-S / PME cable, not unless you take a split concentric cable from a new joint and use earth rods to ground the earth conductor of that cable. Now that could be expensive to get below 0.80 ohms in the wrong soil conditions.

 

[mechelec]

The LV main is the cable originating in the substation that all supplies will be breached from. it is run as a radial configuration sometimes with the option to be backfed or backfeed other LV mains via link disconnector boxes.
Any joint or repair carried out on a paper lead distribution cable will have neutral and earth connected together in that joint. They will already be connected at the substation and every main will have been PME'd at some point in the past. When a paper lead main or service is repaired wavecon CNE cable or hybrid will be used (3 core and copper neutral/earth 3 phase and 1 core and copper neutral/earth for single phase. On new supplies a rod must be installed every 40m (if there are joints) or before the last 4 properties on that main. This is to reduce the amount of customers affected by a neutral fault.
TN-S can be supplied from a PME'd main as previously mentioned, this is current practice on new installs with multiple supplies to steel framed buildings. This is to stop circulating currents in the frame. Alternatively no earth will be supplied and TT will most likely be used.
All the above is based on my knowledge of the DNO in my area and other areas may work to different practices.