Earth rod needed for shed supply or pme allowed ????

[Nigel]

It’s funny but I was talking to the guys from the DNO on a new build site a few weeks ago, all new supply’s to the homes are tncs.
I asked how far apart do you apply the earth electrodes for the pme connection, he said there’s a rod at the transformer end and a rod at the end of the cable , that’s it.
Now one guy was connecting up the street lighting and one guy applying a joint for 2 supplies to some properties but both said the same thing.

This is my point. We cannot know what is going on on the DNO side so it is irrelevant to the design process. All we need is the earthing arrangment supplied to us and BS7671 guides us from there.

 

[Nigel]

I suspect most houses are supplied as TNCS not PME as per your definition, but the DNO still calls its supplies to a property as PME.

I think any joint put in by the DNO will have an earth tap installed as standard so probably easier for the DNO to call it PME as a TN-C-S could become a PME at any time.

 

[Midwest]

I think any joint put in by the DNO will have an earth tap installed as standard so probably easier for the DNO to call it PME as a TN-C-S could become a PME at any time.

I'm no expert here. I've relayed a story before, when I had an issue with a TNS supply. A DNO engineer turned up. I asked if I could have it converted to a TNC-S supply, he said whats that in English, do you mean PME? He said, it would take 6 weeks, do it yourself, and demonstrated how to do so (I didn't take his advice).

So does TN-C-S in reality exist, all is it just a load of PME bull?

 

[Nigel]

I'm no expert here. I've relayed a story before, when I had an issue with a TNS supply. A DNO engineer turned up. I asked if I could have it converted to a TNC-S supply, he said whats that in English, do you mean PME? He said, it would take 6 weeks, do it yourself, and demonstrated how to do so (I didn't take his advice).

So does TN-C-S in reality exist, all is it just a load of PME bull?

I think it probably does exist on supplies with no joints and that have had no work done. I bet the DNO have no idea what supplies are and what supplies are not so just say all TN-C-S supplies are PME.

 

[westward10]

I know. Sorry.

I think @darkwood has completly misunderstood PME, extending EBZs and metallic buildings but there is only so much you can discuss in text so I will move on.

Why do you keep quoting EBZ that went out with the Spice Girls.

 

[Midwest]

Why do you keep quoting EBZ that went out with the Spice Girls.

I thought they were 'In' again?

 

[Nigel]

Why do you keep quoting EBZ that went out with the Spice Girls.

Do you know what an EBZ is?

 

[Darkwood]

I have TNCS at home, it is connected to a converted TT, this is now classed as a TNCS, a TNCS can be supplied in a concentric cable or conversion of a TT or TNS system where all they have done is strapped both the E/N together where the earth existed or made the N in a TT the N/E, the ability to do this is done on a case by case to ensure it is a safe option as oppose to running new supplies.

PME (Protective multiple Earthing) is exactly what it says on the tin, the concentric cable is earthed a regular intervals all of which can be inspected and tested but that is the DNO's side and duty.

PME/TNCS system stop at the cutout of the property where you are provided a common terminal or in some cases they split to 2 terminals to take a N and E from, within this thread and the title people discuss extending PME, this is wrong, that is a very different system and would mean you extended the combined N/E with a concentric cable and depending on length you may have to strap it down along it length, this has to be agreed with the DNO and cannot be done as an option otherwise. The E/N facility that the DNO supply at there cutout is where PME or TNCS stops and you tap you N and E to as separate cables, once you have separated them your installation is based on a TNS design thus no circuits past the DNO's facility can be called PME or TNCS hence my continuous repeated point that you cannot word it as extending the PME.

 

[Darkwood]

Why do you keep quoting EBZ that went out with the Spice Girls.

I also was only using the Term because essex introduced it into the conversation and been old hat I continued its use, it still exists but under a new guise that is all, the actually application and practice hasn't changed but EBZ was deemed confusing if I recall so they changed it by name only.

 

[Nigel]

I have TNCS at home, it is connected to a converted TT, this is TNCS, a TNCS can be supplied in a concentric cable or conversion of a TT or TNS system where all they have done is strapped both the E/N together where the earth existed or made the N in a TT the N/E, the ability to do this is done on a case by case to ensure it is a safe option as oppose to running new supplies.

PME (Protective multiple Earthing) is exactly what it says on the tin, the concentric cable is earthed a regular intervals all of which can be inspected and tested but that is the DNO's side and duty.

PME/TNCS system stop at the cutout of the property where you are provided a common terminal to take a N and E from, within this thread and the title people discuss extending PME, this is wrong, that is a very different system and would mean you extended the combined N/E with a concentric cable and depending on length you may have to strap it down along it length, this has to be agreed with the DNO and cannot be done as an option otherwise. The E/N facility that the DNO supply at there cutout is where PME or TNCS stops and you tap you N and E to as separate cables, once you have separated them your installation is based on a TNS design thus no circuits past the DNO's facility can be called PME or TNCS hence my continuous repeated point that you cannot word it as extending the PME.

Everyone knows what is meant when they say 'extending the PME'. Nobody means physically extending it and taking over the DNO's job.

It is like correcting someone for saying 'sub-main' or 'ring main'. We all know what is meant and no real need to correct it.

 

[westward10]

Do you know what an EBZ is?

No I am an idiot.

 

[Darkwood]

Everyone knows what is meant when they say 'extending the PME'. Nobody means physically extending it and taking over the DNO's job.

It is like correcting someone for saying 'sub-main' or 'ring main'. We all know what is meant and no real need to correct it.

I disagree, the forum is full of people from different stages from fresh faced apprentices to seasoned sparks, read the last several posts, it is clear members still have trouble understanding the systems and the differences 'by their own admission', I disagree also that this is somehow a similar comparison to ring and sub 'main' ... they were accepted and taught terminology it is only that they have been reworded that changed anything, PME and the term 'extending PME' with regards to a consumers side is wrong and has always been wrong, the fact it is a completely different system is why it shouldn't be casually used like it is, I have seen N/E linked together at a Distribution boards before, I expect they weren't sparkies who installed it but when incorrect terminology is banded around it wouldn't surprise me if they were...like you said, everyone says it but we know what we mean..Yes we do but not everyone knows is my point.

Either way, I'll break bread here and lets agree to disagree

 

[Strima]

No I am an idiot.

Well done for admitting it, that's the first step on the road to recovery...

 

[ruston]

No I am an idiot.

Class answer.

 

[Nigel]

No I am an idiot.

Then your comment about it being removed is irrelevant.

 

[MDJ]

I suspect most houses are supplied as TNCS not PME as per your definition, but the DNO still calls its supplies to a property as PME.

Hmm, if you say so

 

[Midwest]

Hmm, if you say so

Nope, just surmising what essex was saying; whats your thoughts then?

 

[Marvo]

A pure TN C S supply network and a PME'd TN C S network behave very differently under fault conditions. I know the DNO's can be a law unto themselves but surely they've got a national standard that dictates the multiple earthing point requirements a supply network must achieve to qualify as PME. Just calling a system PME when it's not can be dangerous for those designing end user installations on that system; you really don't want to locally rod a supply that's not PME'd whereas there's advantages to a local rod if it's a correctly multiple earthed TN C S supply.

 

[Darkwood]

That's the problem Marvo, can take months to get an answer sometimes from DNO as we are not privy to that kind of info normally, the only guide is a PME label on the cutout so if it ain't there then I don't treat it as one.

 

[westward10]

As you have stated previously you do not export the PME as this stops at the service head, you are exporting for want of a better word TN-C-S. The earthing system is judged at the service intake point and not what is occurring previous to it.

 

[Gavin John Hyde]

The earthing arrangement is pme , in process of joining cps scheme tried ringing for advice and they said they can’t give me any advice as not member yet

I will let you in on a secret with the niceic and elecsa if you know the regstration number from a member, its on most websites and similar then you enter it on the voice prompt menu on the phone, then they put you through. never asked for anything more!

 

[westward10]

I never in 40 years of my working life asked for advice from any scheme.

 

[Nigel]

I never in 40 years of my working life asked for advice from any scheme.

Wow! Really?! My hero.

 

[MDJ]

As you have stated previously you do not export the PME as this stops at the service head, you are exporting for want of a better word TN-C-S. The earthing system is judged at the service intake point and not what is occurring previous to it.

read marvos reply above, he explains it properly.

 

[westward10]

read marvos reply above, he explains it properly.

I wasn't trying to explain that.

 

[MDJ]

I wasn't trying to explain that.

It must be your moustach putting me off my posting lol, apologies then

 

[Midwest]

I never in 40 years of my working life asked for advice from any scheme.

Have you an admission to make; have you been a member of one of the schemes?

 

[Michael Sands]

To be honest who needs to ask a schemer we have all we need here and more
24/7

 

[Midwest]

Whilst I value the knowledge & experience members on this forum have, if you have a question thats pertinent to a design or install on a working job, I don't see that you can rely on an response from a forum member. If it all goes horribly Pete Tong, I wouldn't like to be using 'the advice I received from Mr Miggins, on the electrical forum was' in my defence. Hence when I did seek guidance from my schemes technical, it was always in an email format, copy secured away.

I would use and have used this forum for advice, but always knowing the answer almost, and just seeking confirmation or agreement, like you would from any other working colleague.

I have read quite often the response, 'why ask some faceless individual on a forum, ask your scheme'.

 

[Michael Sands]

As you said you need confirmation that your thoughts are on par with the trusted members on here you know who to heed and who to just ignore as some like a windup

 

[Toneyz]

I never in 40 years of my working life asked for advice from any scheme.

Only had dealings with NICEIC via the previous so-called electrical contractors I worked for, spoke to their technical helpline on a few occasions and I found them extremely knowledgable

 

[Strima]

Only had dealings with NICEIC via the previous so-called electrical contractors I worked for, spoke to their technical helpline on a few occasions and I found them extremely knowledgable

Spoke to the NIC tech helpline on Friday for a sanity check and he knew the reg number straight off the top of his head before I'd even finished speaking.

 

[oracle]

You can use two RCD's, one type S at the CCU protecting the supply to the shed and normal RCD protecting the shed final circuits. This will give you discrimination between the two

 

[Strima]

You can use two RCD's, one type S at the CCU protecting the supply to the shed and normal RCD protecting the shed final circuits. This will give you discrimination between the two

What if the RCD in the CU is providing additional protection for other circuits?

 

[oracle]

Obviously I would use RCBO to supply the shed. I don't use split boards, I use breakers and RCBO's as suitable for each circuit.

 

[Nigel]

You can use two RCD's, one type S at the CCU protecting the supply to the shed and normal RCD protecting the shed final circuits. This will give you discrimination between the two

Why would you need a Type S? RCD?

 

[oracle]

BS7671 Regulations 314.1(iv)and 531.2.4 require installations to be subdivided into circuits to prevent accumulated protective conductor currents causing unnecessary tripping of RCD’s. Where there are RCD’s in series for Additional Protection, then Selectivity must be provided to comply with Regulation 536.3.
This can be provided by the use of Type S RCD’s which will provide a time delay to allow for a lower rated RCD down stream of the main RCD to operate first under fault conditions.

 

[Nigel]

BS7671 Regulations 314.1(iv)and 531.2.4 require installations to be subdivided into circuits to prevent accumulated protective conductor currents causing unnecessary tripping of RCD’s. Where there are RCD’s in series for Additional Protection, then Selectivity must be provided to comply with Regulation 536.3.
This can be provided by the use of Type S RCD’s which will provide a time delay to allow for a lower rated RCD down stream of the main RCD to operate first under fault conditions.

That does not answer my question.

 

[westward10]

Isn't that the 18th, too lazy to look.

 

[Dillb]

BS7671 Regulations 314.1(iv)and 531.2.4 require installations to be subdivided into circuits to prevent accumulated protective conductor currents causing unnecessary tripping of RCD’s. Where there are RCD’s in series for Additional Protection, then Selectivity must be provided to comply with Regulation 536.3.
This can be provided by the use of Type S RCD’s which will provide a time delay to allow for a lower rated RCD down stream of the main RCD to operate first under fault conditions.

That does not answer my question.

@oracle didnt quote you

 

[Nigel]

@oracle didnt quote you

Agreed. Thank you.

 

[oracle]

Reason I didn't quote you is I wasn't addressing it to you, simply posing an axiom. If you don't know about Type S RCD’s, I suggest you research them, along with the other types as well. BSEN 61008 and 61009 are not the only RCD’s out there.

 

[Nigel]

@oracle didnt quote you

Agreed. Thank you.

Reason I didn't quote you is I wasn't addressing it to you, simply posing an axiom. If you don't know about Type S RCD’s, I suggest you research them, along with the other types as well. BSEN 61008 and 61009 are not the only RCD’s out there.

I know all about Type S RCDs.

Again, why would the distribution circuit feeding the CU in the shed need a Type S RCD protection. You are the one saying to use it but cannot back it up.

 

[petermouse]

On this subject I recently spoke to an electrician who was repairing a street supply cable under the pavement. He agreed that out buildings should be TT’d. Because when a supply cable is repaired the tendency is to make it Tncs connecting the earth to the neutral to get better Ze readings. So a property with an apparent tns at the supply end may well be a tncs. Also on the subject of on a completely wooden/plastic construction no extraneous metal parts, then allowing the installer to extend the local earth to it. No mention is made of the danger of using class 1 equipment in this environment. I feel the more I read the less I actually know.

 

[Wilko]

... I recently spoke to an electrician who was repairing a street supply cable under the pavement. He agreed that out buildings should be TT’d. Because when a supply cable is repaired the tendency is to make it Tncs ...

Hi - supplies that present at the installation origin as TNCS (with or without PME) can be extended to outbuildings. The means of earthing at the origin can be reused at the outbuildings, it just needs to be properly designed. So my 20p worth is Mr StreetSupply is wrong .

 

[FatAlan]

I have a couple of out buildings total run about 50m which were fed by suspended 4mm T&E from RCD protected CU at house. Each shed CU was/is rodded. In the process of swapping to buried 6mm SWA on seperate MCB protected circuit with RCBOs at shed. Will use PME but also keep rods. This brings up old discussion that DNO engineer when converting a TT to PME advised that the rod be disconnected as it can cause nuisance tripping. I did leave it in place with no problem.

 

[oracle]

Agreed. Thank you.


I know all about Type S RCDs.

Again, why would the distribution circuit feeding the CU in the shed need a Type S RCD protection. You are the one saying to use it but cannot back it up.

Told you already, I was posting an axiom also told you before you appear to be a Troll. I will not respond to anything you post in future, so please don't tell me that I haven't answered your questions. I won't!

 

[Strima]

This brings up old discussion that DNO engineer when converting a TT to PME advised that the rod be disconnected as it can cause nuisance tripping.

That's where I've been going wrong, just remove the earth and your problems go away...

 

[Midwest]

Wasn't the 18th draft proposing rodding (pardon me) all PME or TNC-S supplies?

 

[Strima]

Wasn't the 18th draft proposing rodding (pardon me) all PME or TNC-S supplies?

It did but never got written in, it will probably be in the 1st amendment.