High Ze

[rainydays]

Hi all, investigating a high Zs, so I measured Ze on a PME system and got a reading of 0.53Ω

I contacted the DNO who informed me that they would accept a value of up to 0.8Ω, I repeated that it was a PME and they said that it did not matter.

Anyone had a similar conversation with them?

 

[steptoe]

no, but the reading of 0.35 and 0.8 in OSG etc are only what the calcs are worked out to AFAIK , hopefully someone else can come along and say if they are required max or not,

they could if they want, I suppose, just withdraw the earth altogether.

 

[Megaohm7]

A Ze of 0.53 ohm on a pme is on the high side which would give you a pfc of 434 A. It's all down to what the circuit Zs's come out as and what the CPD's are and if the disconnection times can be met. The regs say a max of 0.35 ohm. You've enquired about this but the DNO say it's ok? . It seems to me that they don't want to be responsible for supplying a suitable earth. You might have to fit a rod/rods to provide a decent earth path.

 

[Chr!s]

Hi all, investigating a high Zs, so I measured Ze on a PME system and got a reading of 0.53ΩI contacted the DNO who informed me that they would accept a value of up to 0.8Ω, I repeated that it was a PME and they said that it did not matter.

Anyone had a similar conversation with them?

Many people i suspect.

The installation needs to meet the requirements of BS7671, so a higher ZE may not be an issue if the dno decide that its down to network parameters. All they would do is ensure the main fuse is sized appropriately.

 

[Larnacaman]

I think Plumber has it right here. This supply cable may have been on a previous TN-S network that has subsequently having it's lead sheath breaking down. This particular area of the network, may be undergoing conversion to PME, but not yet, have been fully PME'd!!! There are numerous other reasons why the DNO are not being particually worried about the above normal 0.35 ohms value..

Meanwhile, I'm still trying to fathom out how on earth (excuse the pun) anyone can think that this comment ''You might have to fit a rod/rods to provide a decent earth path. can better a supplied earth system of 0.53 ohms?? Hell, it's better value than a supplied TN-S system!!! lol!!

 

[rainydays]

Thank`s for the replies guys.

From first impressions i would say that it used to be a TT. You can see the cables coming in overhead. Looks to have been jointed a few times aswell

 

[Manator]

I complained once about an overhead supply, they sent the high level guys up to the job and found a couple of loose joints.

 

[Megaohm7]

I think Plumber has it right here. This supply cable may have been on a previous TN-S network that has subsequently having it's lead sheath breaking down. This particular area of the network, may be undergoing conversion to PME, but not yet, have been fully PME'd!!! There are numerous other reasons why the DNO are not being particually worried about the above normal 0.35 ohms value..Meanwhile, I'm still trying to fathom out how on earth (excuse the pun) anyone can think that this comment ''You might have to fit a rod/rods to provide a decent earth path. can better a supplied earth system of 0.53 ohms?? Hell, it's better value than a supplied TN-S system!!! lol!!

Twas me!! Tongue in cheek!! I know that this value is better than a TN-S max value but the original post stated that it was a TN-C-S which has a max Ze of 0.35 of an ohm. I would have queried this myself to the DNO. If the DNO are not concerned about it and then I would still make a note on the certificate about the high reading, besides I don't like banging rods in really..lol

 

[Larnacaman]

I complained once about an overhead supply, they sent the high level guys up to the job and found a couple of loose joints.

Yep, ...That'll do it too!!!

---------- Post Auto-Merged at 22:34 ---------- Previous post was made at 22:23 ----------

Twas me!! Tongue in cheek!! I know that this value is better than a TN-S max value but the original post stated that it was a TN-C-S which has a max Ze of 0.35 of an ohm. I would have queried this myself to the DNO. If the DNO are not concerned about it and then I would still make a note on the certificate about the high reading, besides I don't like banging rods in really..lol

He didn't, he quoted a ''PME'' system, there IS a difference!!

As we now know that it's an overhead supply system, i would contact the DNO again, and hopefully get hold of an Engineer. Overhead systems used to be a priority call out, especially on the ''then'' newly converted PME overheads...

Like myself Megaohm, not for a domestic installation anyway. Rod(s) would always be the last possible choice...

 

[Megaohm7]

Yep, ...That'll do it too!!!---------- Post Auto-Merged at 22:34 ---------- Previous post was made at 22:23 ----------

He didn't, he quoted a ''PME'' system, there IS a difference!!

As we now know that it's an overhead supply system, i would contact the DNO again, and hopefully get hold of an Engineer. Overhead systems used to be a priority call out, especially on the ''then'' newly converted PME overheads...

Like myself Megaohm, not for a domestic installation anyway. Rod(s) would always be the last possible choice...

What is the difference?? What do you put down on a certificate when you have to decide what type of earthing system the installation has. I'm confused now.

 

[steptoe]

Yep, ...That'll do it too!!!---------- Post Auto-Merged at 22:34 ---------- Previous post was made at 22:23 ----------

He didn't, he quoted a ''PME'' system, there IS a difference!!

As we now know that it's an overhead supply system, i would contact the DNO again, and hopefully get hold of an Engineer. Overhead systems used to be a priority call out, especially on the ''then'' newly converted PME overheads...

Like myself Megaohm, not for a domestic installation anyway. Rod(s) would always be the last possible choice...

tell me then larnacaman,?

you are the one always thumping on about how good PME is,,

doesnt matter how it is supplied, PME is PME,

so now you are back-pedalling and saying this is a priority cos perhaps its not safe.?

you cannot have PME without it being TNCS, that is fact!

so you have now stated it is different,

along with your scaremongering about TT not being a safe reliable earthing system how many other myths are you going to spout?

we now have a PME of >0,35 ohms and you think this is OK? it really doesnt matter how much over 0.35 it is, we have no control of this,

I much prefer my rod at 57 ohms that I control and I know it will save me when a fault occurs,

rather than this PME of undeterminate value we have here,

who knows, it might be 268 ohms by next tuesday, never mind its still only just over 0.35 , but who will know, no-one is going to be there to measure it until the coroner arrives.

 

[Larnacaman]

I believe it was You that quoted that TNC-S is not a true PME system Steptoe!!! Not all TNC-S systems, meet PME requirements, as well you know!! Yes they are both TNC-S systems, but PME has a higher/specified requirement. As far as scaremongering goes, you would know far more about that, concerning PME systems than I. TT systems for domestic installations is, and will always remain, my absolute last choice, give me a PME system every time, ...and i have a TT system here at a much lower value than yours!!!

Oh, and I don't thump on about how good PME is or isn't, as you put it. I just continue to maintain that PME is a dammed sight better than an ill installed unstable roded TT system, for domestic installations, as many of them are these days...

You are twisting what i have posted above, as well you know. The very reason i advised OP to contact the DNO again, is that on overhead systems, joints can have a tendency to weaken over time. And as stated, were always a priority call in the past. It's always preferable to confirm a statement given by an undetermined DNO operative over the phone, with a named Engineer at the DNO...

Just because anyone mentions PME, you seem to think and spout disaster is just round the corner, which is total nonsense!!. As for not having control over a supplied earthing system, think about it, ...how many would want control, let alone know how to control it?? I've told you before, it's fine for someone like yourself that knows how to maintain a roded system, but i'm afraid the vast majority of the population doesn't, and that includes a good many electricians

The only myth i have seen, is this ongoing crap about about losing a neutral, and the catastrophic consequences therein. When in reality, the chances of this catastrophic senerio is rarer than being knocked down by a bus in your lifetime.

As for the this present senerio of 0.53 ohms being present, It could well be that this part of the DNO network is being up-graded and are quite aware of the lower than normal value!! As long as the installation meets BS 7671 then there surely isn't a problem, disconnections times are maintained. It's only your own rhetoric of all the ''what ifs'' and ''maybes'' that cloud the actual reality, ...This is the Real scaremongery...

 

[steptoe]

I believe it was You that quoted that TNC-S is not a true PME system Steptoe!!! Not all TNC-S systems, meet PME requirements,ESQCR STATES THEY MUST as well you know!! Yes they are both TNC-S systems, but PME has a higher/specified requirement. NOT ACCORDING TO THE CURRENT REGS afaiaa As far as scaremongering goes, you would know far more about that, concerning PME systems than I. I only state that in no way can you rely on a 3rd party earth via a dubious system which has now been shown to be so TT systems for domestic installations is, and will always remain, my absolute last choiceyour and others inadequeacies in installing an earthing system should not and can not be taken into account for a perfectly sound and acceptable earthing system, your poor expoeriences and inability to properly install a TT system is not an issue I feel an internet forum is capable of dealing with., give me a PME system every time, ...and i have a TT system here at a much lower value than yours!!!Oh, and I don't thump on about how good PME is or isn't, as you put it. I just continue to maintain that PME is a dammed sight better than an ill installed unstable roded TT system, for domestic installations, as many of them are these days...

NO, you just continually say how good PME is over TT, you are being proved wrong yet again

You are twisting what i have posted above, as well you know. The very reason i advised OP to contact the DNO again, is that on overhead systems, joints can have a tendency to weaken over time. And as stated, were always a priority call in the past. It's always preferable to confirm a statement given by an undetermined DNO operative over the phone, with a named Engineer at the DNO...

Just because anyone mentions PME, you seem to think and spout disaster is just round the corner, which is total nonsense!!. As for not having control over a supplied earthing system, think about it, ...how many would want control, let alone know how to control it?? I've told you before, it's fine for someone like yourself that knows how to maintain a roded system, but i'm afraid the vast majority of the population doesn't, and that includes a good many electricians

The only myth i have seen, is this ongoing crap about about losing a neutral, and the catastrophic consequences therein. When in reality, the chances of this catastrophic senerio is rarer than being knocked down by a bus in your lifetime.

As for the this present senerio of 0.53 ohms being present, It could well be that this part of the DNO network is being up-graded and are quite aware of the lower than normal value!! As long as the installation meets BS 7671 then there surely isn't a problem, disconnections times are maintained. It's only your own rhetoric of all the ''what ifs'' and ''maybes'' that cloud the actual reality, ...This is the Real scaremongery...

Im in RED

and to be quite honest,

for a short time I started to like you Larnacaman,

I thought you to be a proper spark,

but no,

yet again you go all out and prove me right as to being a total nobheafd and not having a clue,

go on , is it any wonder you had to leave the UK to stay in a job with ethics like yours.

 

[Larnacaman]

Sorry you feel that way Steptoe, but i can assure you, I'm nobodies fool and certainly no knobhead!!!

My ethics are sound mate, Your the one making assumptions on this present system, and then go on as if it's ''fact'' your talking about, when it's still, just your assumption!! I don't go around seeing and spouting disaster at the very mention of PME and the like.

Yes i have also made assumptions here, but i don't, and haven't tried to make them out to be facts, because there not!!!

It seems that you have changed stand mate, because i can remember you quite clearly stating that not all TNC-S meets PME standards!!!

So How have i been ''proved wrong yet again'' as you say?? You have proved absolutly nothing!!!

What do you mean by ''my inadequacies in installing an earthing system should not and can not be taken into account for a perfectly sound and acceptable earthing system, your poor experiences and inability to properly install a TT system is not an issue'' ....hahaha!!!

I've been involved in the installation of more and bigger TT systems, than you would imagine. Even my own lowly TT system, has a much lower value than yours, and in a probably less favorable soil type too, ...so i must be doing something right!!! lol!!! Your just being insulting now mate..... Getting personal like this, just tells me your in a corner, when there really is no corner to be in, grow -up man...

Unlike your statements about myself, i can tell you, that apart from your views on TT/PME, ... i've always respected your views and comments on our industry here.

 

[Manator]

Lets keep it all civil gentlemen, there are always two sides to any opinion a personal attack on the opposing veiw is just belittleing the debate, however good the points are.

 

[Larnacaman]

Calm down gentlemen, perhaps count to ten and agree to disagree :run

I have no problem whatsoever with this suggestion. It is exactly the same solution that we came too, in our last confrontation/debate on this issue!!!

 

[Megaohm7]

I have no problem whatsoever with this suggestion. It is exactly the same solution that we came too, in our last confrontation/debate on this issue!!!

Hi Larnacaman. Could you please explain what the difference is between a TN-C-S and a PME system? The reason I'm asking is that I've just completed my c&g 2391-10 course and I have diagrams in my notes of fault paths for the different earthing arrangements. The one I have for a TN-C-S system also Includes extra earth points to earth along the route back to the transformer. I do respect peoples opinions of course and none of us are perfect but I am a bit baffled by what you said. Mega.

 

[Sidewinder]

Megaohm7,

As far as you are concerned in the UK mate nothing TN-C-S = PME ESQCR says so and that is statute law.

 

[Larnacaman]

As i understand it, and this was partly explained to me, actually by Steptoe and others here, as well as a DNO friend of mine, that a PME system should be neutral roded at every joint on that cables run from the supply TX. On overhead distribution systems roded at regular intervals. (sorry, i'm not sure of the actual spacing distances) Also remember that almost all overhead supplies, are in fact initially supplied via an underground cable...

Now as far as i can make out, DNOs TNC-S hasn't been roded as per PME requirements, most of these supplies are work in progress, converting older TN-S systems to PME. So joints have been roded, but not all, as to meet PME requirements. For the DNO to guarantee a 0.35ohm Ze all or most of those joints need to be roded. Which is maybe why the DNO in this case quoted the OP, that upto 0.80 ohms is OK. I'm not saying this is an ideal situation, and that maybe other factors are also present in this particular situation.... As far as you would be concerned, if the head neutral also contains an earth tail, it's a PME system. This was basically the point i was trying to get accross above...

It's OK to say that all TNC-S is PME in the UK, but that statement doesn't always give the DNO room to complete conversion of previously other designated systems. It doesn't magically just happen, it takes time and a good deal of money to convert to PME, so conversions are completed over varying time periods and is continuously ongoing throughout the whole country, ...but it gets there in the end ...lol!!!

 

[Chr!s]

T Terre

N Neutral and Protective conductor

C Combined in distribution circuit

S Separate in the installation

PME Protective multiple earthing - Requires two or more earth electrodes ESQCR

PNB One electrode connected to the pen

So both tncs just variations.

Draft for Public Comment - BS 7671/AMD 1 - IET Electrical

Megaohm7,As far as you are concerned in the UK mate nothing TN-C-S = PME ESQCR says so and that is statute law

I dont believe ESQCR precludes other variations of tncs, i think you will find PNB complys with Part II section 8 ESQCR