Earthing system??? Concerned

[bartkusal]

Hi guys, need some opinions from big boys. I will try to be short as possible.
I was helping on big rewire the other month for the well-respected electrician mate. On tests before the job the Ze was 6.7 ohmson TNS supply, so the guy called the other mate who works for DNO as the head needed moving anyway.
So when it got moved they just taped the earth from the heads Neutral terminal as you expectto see on TN-C-S systems, and got it down to 0.22 ohms. I asked them how that is compliant and possible,because you are not sure if the PME system is available, as far as I know the PME’s neutral is earthed every say 4[SUP]th[/SUP] overhead pylon or so and on TNS is just that, Neutral goes straight to substation transformers centre point.
The answer from the DNO guy was, “IT’S ALL B*****KS about all these different earthing systems you on about, every supply is now PME’d and that’s how we do it if this sort of problem arises.”
I looked concerned to my mate and, he said yes, don’t worry that’s how it is, we always done that.
So please enlighten me if that’s the case, otherwise what is the point of brain braking if there is such an easy solution to the problem.

Thanks in advance for replies

Al

 

[varilite]

If it looks like tns get ze readings typical of tns then it could be tncs

 

[bartkusal]

If it looks like tns get ze readings typical of tns then it could be tncs

Sorry could not understand your sentance, would you explain futher, thanks

 

[bartkusal]

I know that:
TNS max Ze is 0.8 ohms, and Ze should be different from Ze(L-N)
TNCS max Ze is 0.35 ohms, and Ze should be the same as Ze(L-N)

Is that what you meant Varilite?

 

[johnboy6083]

LV jointers on a HNC course?
Most LV jointers I’ve come across are on a par with pond life.

Skilled yes, theoretical no. Most of them it takes 10 minutes to think “I’m getting a shock.”

yeah, it's part of career development within UK power. There's even a fitter on the course too.

i have heard the earthing of neutrals from the horses mouth. Not even E54 can argue with that.
I agree that they are not doing it on every joint in the street, but when they upgrade anything, it's never to TNS. This suggests to me that at least on non dedicated supplies, there's very few TNS supplies left.

 

[Outspoken]

Whilst what E54 stated is by and large correct, could I also point out that technically all TN-S cables are effectively TN-C-S indirectly because they are stacked wherever a joint is put in, and have been for years (major joints not the silly ones for street lights and Traffic lights etc) and they are all connected to plates, mats and stakes when they get to the location Tx substation as well.

It is impossible to "liven up the neutral" of the supply cable if you connect the earth to it, for a start you would have to be a total dork anyway as your own Earth and neutral would have to be at different potentials anyway, but also the DNO engineers would have had to have separated the neutral on the supply cable and failed to spike it, which they do as a matter of course because otherwise all those connected it lose their neutral and their earth effectively, which would be a sackable offence and likely result in a punitive prosecution by the HSE if anyone was harmed as a result.

You also have to remember that all main LV supplies from Tx's are connected as a ring main (except perhaps in some really old and tiny villages) and thus all services connected to them are connected to Earth via the structures they are part of, lamp posts, traffic lights, street furniture and buildings all have multiple connections via the structure directly or via Lightning protection systems, and this is another reason why DNO engineers will tell you there is effectively no difference.

 

[Knobhead]

Given that they use 3 core wavecon to repair 3½ core SWA they don’t have much choice other than make it TN-C-S.

 

[Engineer54]

Whilst what E54 stated is by and large correct, could I also point out that technically all TN-S cables are effectively TN-C-S indirectly because they are stacked wherever a joint is put in

Only on older cable joints, where and when a fault has occured or part of a converting exercise. There are many network cable distributions that haven't been touched since they were installed from the 60's on housing estates and the like!! All cables connected at TX substations have a neutral earth connection, be they TN-S or TNC-S, on TN-S cables, it will be the only place where it's neutral is connected to earth, unless newer jointing has taken place.

As you say, most of the UK's LV networks have indeed, been effectively converted to TNC-S but certainly not all of them!! What we do need to point out here, is there is a very big difference between the terms TNC-S and PME. A cable that has just one or two N-E joint connections along it's path, cannot be deemed as suitable for a PME connection, otherwise virtually every existing TN-S / TT installation could be converted to a PME connection...

 

[varilite]

I know that:
TNS max Ze is 0.8 ohms, and Ze should be different from Ze(L-N)
TNCS max Ze is 0.35 ohms, and Ze should be the same as Ze(L-N)

Is that what you meant Varilite?

Yes all I meant was as long as disconnection times can be achieved it dosen't matter if its tncs or tns

 

[MDJ]

TNCS max Ze is 0.35 ohms,

Single phase, three phase or both??

 

[Outspoken]

Only on older cable joints, where and when a fault has occured or part of a converting exercise. There are many network cable distributions that haven't been touched since they were installed from the 60's on housing estates and the like!! All cables connected at TX substations have a neutral earth connection, be they TN-S or TNC-S, on TN-S cables, it will be the only place where it's neutral is connected to earth, unless newer jointing has taken place.

As you say, most of the UK's LV networks have indeed, been effectively converted to TNC-S but certainly not all of them!!

E54, in a few cases there may not be major joints that have been staked, however it is safe to say the street furniture will have been upgraded over the years and this process alone is giving the system effective multiple earths via the metalwork buried in the ground, also for a number of years where the local operators (normally in the Board days but it does happen today) have inserted solid isolation points in the ground, these have always been staked into the surrounding ground, so it is rare, though not impossible, to find a buried supply cable in the UK these days that does not have multiple earth paths and thus should give very low resistance readings.

What we do need to point out here, is there is a very big difference between the terms TNC-S and PME. A cable that has just one or two N-E joint connections along it's path, cannot be deemed as suitable for a PME connection, otherwise virtually every existing TN-S / TT installation could be converted to a PME connection...

E54, I think you have fallen into the trap of confusing the origins of the term PME. Originally this was coined by the now defunct London Electricity Board to differentiate a supply from the old single connection types of separated cables (Standard TN-S) from combined N-E type cables that were dedicated to a combined neutral-Earth connection by design.

TN-C-S was never deemed a PME by the LEB because they used the term Protective Multiple Earth to describe any supply cable that had multiple dedicated and deliberate Earth connections along it's supply route. It was later that people began to refer to TN-C-S as PME, which was an incorrect connection because in it's purest form, running from a local sub-station to the street and T-off to homes/businesses it is not actually a PME systems unless it has been multiple spiked.

Most overhead supplies are now deemed PME because at the 11kV Tx on the pole they are spiked into the ground beside the pole, although clearly some are still to do in very rural areas and it is likely wise to assume it is TT unless your DNO specifically states otherwise.

 

[bartkusal]

Single phase, three phase or both??

Sorry, not sure what you mean, always been thought that TNCS is 0.35 ohms, and don’t mater single or 3 phase, just on the 3 phase you measure each phase to earth, that’s all?

 

[MDJ]

YEAH sorry I was mixed between 2 threads one about PSC (ISC) which is 16000 amps at single phase and 25000 amps at 3 phase and got all mixed up, yup your right about the 0.35, sorry again haha

 

[GLENNSPARK]

Sorry, not sure what you mean, always been thought that TNCS is 0.35 ohms, and don’t mater single or 3 phase, just on the 3 phase you measure each phase to earth, that’s all?

each phase to earth for Ze..
for your PFC then if using seperates that cant measure phase to phase & some MFTs the same then its each phase to neutral and X the highest value obtained by 2....or by square route of 3....
phase to neutral is always higher than phase to earth on a PFC with 3 phase....

 

[Engineer54]

=Outspoken;781276]E54, in a few cases there may not be major joints that have been staked, however it is safe to say the street furniture will have been upgraded over the years and this process alone is giving the system effective multiple earths via the metalwork buried in the ground, also for a number of years where the local operators (normally in the Board days but it does happen today) have inserted solid isolation points in the ground, these have always been staked into the surrounding ground, so it is rare, though not impossible, to find a buried supply cable in the UK these days that does not have multiple earth paths and thus should give very low resistance readings.

Internal equipment and actual light fittings may well have been changed, maybe even a couple of times or more, but the street furniture itself is another matter. I know for a fact, that the street furniture on the housing estate that i was brought up on, still has the same street furniture (well it did up to last year on my last visit home) that was originally installed back in 1956. They made things to last in those day's!! lol!! There has always been multiple earth paths, just that they are not always connected to the neutral conductor...

Having a low resistance path is not the be all and end all when it comes to PME. The important feature of the high integrity PME system, is the '''regular'' spaced multiple earthing points, that is going to limit the number and severity to end users affected on the rare occurrence a break/partial break in the supply neutral conductor...


I take it that you live or have lived in the London areas. London like many other major cities in the UK have many extensive reinforced DNO supply systems. That would certainly be the case in central London and the city areas.
I agree that in such areas, it would be very difficult not to say, that most if not all LV distribution have extensive earthed neutral connections.

E54, I think you have fallen into the trap of confusing the origins of the term PME. Originally this was coined by the now defunct London Electricity Board to differentiate a supply from the old single connection types of separated cables (Standard TN-S) from combined N-E type cables that were dedicated to a combined neutral-Earth connection by design.

TN-C-S was never deemed a PME by the LEB because they used the term Protective Multiple Earth to describe any supply cable that had multiple dedicated and deliberate Earth connections along it's supply route. It was later that people began to refer to TN-C-S as PME, which was an incorrect connection because in it's purest form, running from a local sub-station to the street and T-off to homes/businesses it is not actually a PME systems unless it has been multiple spiked.

Most overhead supplies are now deemed PME because at the 11kV Tx on the pole they are spiked into the ground beside the pole, although clearly some are still to do in very rural areas and it is likely wise to assume it is TT unless your DNO specifically states otherwise

I haven't fallen into any trap, the term PME has absolutely nothing to do with the LEB, or any of the other old regional boards, it's a generic term for a High Integrity TNC-S earthing system, that carries it's own stipulated criteria of conformance. It is a term used in just about every Western European country. I do know the difference between PME and TNC-S. As i stated before, in the UK it is far more likely that most of the true PME supplies, will be those found on overhead supplies that have been converted.

Overhead PME supplies, has nothing to do with with the supplying 11KV TX's secondary neutral being spiked at the pole, that would be normal even for a non PME'd overhead supply, ....All TX secondary neutral conductors will be referenced to earth or the neutral will be literary floating!! What makes a poled overhead supply PME, is the neutral line being earthed via a rod/plate at every 3rd or 4th pole along the length of the distributed run(s) No need to assume either, just walk along the pole line, to see the down conductors to the earth electrodes, they should also be clearly labeled on the pole as a PME point too!!