Power - Earth-RCD's- ISO/TX's and workshops.

First off, Thanks Chris for splitting this, as the thread had taken on a life of it's own.

Interesting links Terry and Rob, I hadn't seen this before (the DIY forum), as usually the DIY type sites don't go into much detail on this, and usually spread the the myth that ooh! you can't export TNC-S

I have actually done these types of jobs, and had to design and cost out the different ways of achieving it.
I have actual experience of doing this, it can be a bit of a balancing act, as each system has it's pro's and cons

Most of my information originally came from the IET Guidance Note 8, which is all about Earthing and bonding and goes into far more detail than the snippets I posted here.
I actually just posted off the top of my head without looking at any books, just by experience so I hope there are no mistakes in it.

The Wiring matters article is now quite out of date (16th edition), and out of step with current regs, but the basic principals are still valid, and it is still a good and easy to understand guide.

On a more serious note, anybody contemplating doing anything contained in this thread, then please get an electrician in to oversee it, as a forum limits the amount of advice that can be given, and the consequences of doing something wrong can be fatal.

Whenever I have read threads on other forums about running power out to a shed I always end up being non the wiser. I get the strong impression that you can't do it yourself these days even if you are a qualified 17th edition electrician.
What I concluded rightly or wrongly, is that you need to be registered with the Competent Person Register; it would be great to get some clarification on this.
What also seems strange to me is that within the competent fraternity, there is debate about whether you are allowed to "export" an earth to your shed or not. I suppose export in this context means the physical act of running or extending an earth from the house, I mean why wouldn't you?
I did wonder at one point if this CPS was just some more red tape madness, but having followed a recent thread on here illustrating some poor building work, I'm starting to see why the Government might think it necessary.

Eddie

Hi Valvekits,

I will try to answer your questions for you.

Since Part P was introduced several types of electrical work became notifiable, running a supply out to an outbuilding was one of them.

See here for a more in depth look, note there are different versions of the document.

http://www.planningportal.gov.uk/buildi ... p/approved

Basically if you are not a Part P registered Electrician (whether qualified or not) then you have to go through building control before you are allowed to do notifiable work (see documents above).

If you are a Part P registered electrician you can do the work and then self test and self notify (again see documents above).

Part P is about to be ammended in a couple of months, there are quite a few changes afoot.
Looking at the '2013 draft document' (also in the link above), it looks as though running a supply to an outbuilding is not on the list, however adding a new circuit is notifiable., so not much change there then.

'Exporting' is not really the best word to use, as you are 'extending' the earthed equipotential zone.

There can be problems exporting the Earth (see some of the previous posts and links earlier in this thread), it all depends on the earthing system at source, and if there are metal pipes in the outbuilding or not, or if the outbuilding is made of metal or not, there are other things you need to know, as it is not as simple as just running the earth cable out, there are other serious considerations to take into account too, the choices are not 'black and white' as each situation is different, it also depends on the intended use of the outbuilding to some extent.
Costs to comply also come into the equation too (see earlier posts).

There are also technical pro's and con's to consider. (see some of the links earlier in this thread)

Things like power to metal greenhouses also come under different regs too (horticultural regs).

What else causes confusion is that some DNO's (Electricity supply companies) at one time prohibited exporting TNC-S, and so some electricians think this is still the case, even some DNO's can't give a straight answer if asked, and it depends on who you speak to at any given time, at any given DNO.

It is quite a complex subject, and not particularly easy to advise someone what to do on an internet forum, without seeing the situation on the ground so to speak.

I hope I have answered some of your questions. Phew!

Okay R to B some heavy reading there but I think I got it. You can't create a hypothetical shed scenario because although there appears to be great similarities, each case can be slightly different.

I think I understand this now and basically there are three avenues that I can see:

I chose to do it myself, I must contact building control before work commences.
Building control may chose to inspect themselves or get a third party to check the work, but they will be looking to recover their inspection costs so it might not save me any money.

I get my 17 edition electrician who isn't Part P registered but able to test to BS 7671 to do it, building control still need to be notified before work commences.
When he completes a BS7671 installation certificate, Building control will issue a completion certificate if they are satisfied, but may well request a third party approved inspector to verify the work. So it still might not save me any money.

I get a registered Part P installer to do it, they self test and provide me and building control with a compliance certificate. It won't save me any money but it could save me a lot of hassle.

It isn't possible to circumnavigate all this by making the cable to the shed plug in to an outside waterproof socket on my house (Not fixed installation) as this socket installation would have to comply with Part P anyway.
If I choose avenue 1 or 2 and don't involve building control at the outset, I can't get the work approved retrospectively even by an approved inspector because BS7671 cable routing requirements might not of been met and they have no way of checking.

Eddie

Hi VK,

Jeffrey is of course correct, it is generally cheaper to use a part P sparky, who will notify it on your behalf, as BC charges vary enormously depending on the local autority.

I hate Part P with a passion, it is just a money making scam as far as I can see, and yes I am a part P registered spark and for which I have to pay about 400 and odd quid each year, to renew just to stay legal, IOW Iam competent just so long as I have paid my cheque

During the new amendments, at the consultation phase, the government listed scrapping it altogether as one of the options, which they should have done IMO, but the Scams (Part P providers) lobbied heavily against this (vested interests ?), and produced some dubious data to prove electrical accidents in the home had decreased since it's introduction.
Then there was the fact that if the government scrapped it, that would imply they were wrong to introduce it in the first place!, and we cannot have that now.

It was also flagged up during the consultation that the scams (Part P providers) had a vested interest, ie. the person/trader they are supposed to be monitoring is also paying their wages.

I would much rather have had a Gas Safe type of registration, one governing body for all, Part P could and should have been great, but as always the implementation of it was a farce, although with the new version some parts have been improved, whilst other parts have been watered down.

BTW it is not policed at all , it is just insurance companies and some mortgage lenders have began to insist on certain requirements.

Hi Jeffrey,

What I meant was if we have to have some form of registration for electricals, then it should have been modeled on the gas safe scheme, which at least is concerned about safety.

Where as the electrical scams are more about money (suprise suprise).
It started off with the best of intentions, but IMO sadly lost it's way.

Personally, provided you have either qualifications and/or competancy, calibrated test equipment, PL insurance and can fill in the paperwork correctly, then I think you should be able to sign off electrics without being a member of a scam (and paying for the privilege), which is similar to the model used in Scotland.

The other thing of course, registration leaves a paper trail

In the context of Workshop Earths, Safety Earth, RF Earth and Radio use read about these!
http://www.theregister.co.uk/2013/02/05 ... eneration/

Comments
http://forums.theregister.co.uk/forum/1 ... #c_1718543

Hi Michael,
The pages on there are not really relevant to the UK, or Europe for that matter, the US electrical system is totally different, they tend to use split or Bi-phase supplies, 110V on one phase, and 220V using two phases for heavier loads.

Unless of course I am missing something ?

So far I have only commented on protective earths (PE) on the supply side, I haven't touched on functional earths (FE) yet, which tends to use a cream coloured cable. I can of course though, should the interest arise ?

I was going to post some drawings of the various schemas for different earthing systems in the UK, time permitting.

It may be just me, but I find this subject rather interesting

The point is that the EXISTING mains networking products if tested properly would be illegal.

These new stupid products WILL be sold in UK, because Ofcom only cares about raising revenue, not protecting spectrum, and as is pointed out by you and others, the assumed earth / phase system doesn't exist outside US, and increasingly these proposed products would conflict with newer RCD standards in US.

CE is of course totally broken due to (a) Fraud and (b) self certification and (c) Almost no proactive in market policing effectively none in Ireland). At least in UK the Tradaing Standards people frequently take action when informed.

No, you didn't miss anything, you are right on the money.

Interference issues aside, which I don't know enough about (does anyone truly understand the 'black arts' ?), I cannot see how this would affect the RCD's primary function or operation in the protective purpose they are used for.

I could see how the RCD/RCBO could affect the data maybe, but not the other way around.

One other point the US uses RCD's or more correctly GFI (ground fault interrupters ) as low as 6mA trip thresholds, in the UK domestic side we mostly use 30mA devices, of course there are other higher rated ones for other purposes.

Maybe I am still missing something ?

Good evening Gentlemen,

I would also like to thank/apologize to Chris for having to moving this thread from his own workshop story.

As I seemed to started this interesting thread by simply asking a question about Ethernet over Mains devices, I thought I should complete my power installation description with a drawing of what exactly was done 13 or 14 years ago, as there may of been some confusion with my explanation.
This was all fitted by an electrican and tested at the time.(I would not of attempted to do this job now or then!)
I have been talking to a friend of mine who is part p registered and he says that it would still be done this way now and should pass all saftey requirements.

Here is a basic drawing showing my installation.

From memory, the cable used from the house to workshop was 16mm2 armoured cable with 10mm2 wire for the earth connection to the stake. The stake is a 10 - 12 ish mm copper rod over a metre long. There are no pipes or anything in my workshop to require bonding.

Hope this clears up any confusion that may of arisen from my poor description.

Best regards
Jon

Good evening Gentlemen,

I would also like to thank/apologize to Chris for having to moving this thread from his own workshop story.

Jon

Jon no apologies necessary, this is the nature of conversations/threads, they spawn other interesting avenues. I thought it best to split as together they diluted each other and, would be difficult to follow. Anyway the team hardly ever have to moderate you're all so well behaved and we don't get to play with the tools much so it's good practice splitting.

As the last line of our rules states

Above all have fun, going off topic is allowed within reason as therein lies a voyage of discovery.

Chris

Thanks for posting Jon,

One thing to remember here is that the current regs are not retrospective, just so long as the installation complied at the time, there is no need to update anything if you don't want to.

I will post a few comments in a minute, and some drawings for you to look at.

Hi,
Thanks for the replies again.
Yes at the time of installation all was within current regs(not sure what it would of been then).
The house was rewired in the mid 80's so again was in spec then. I have no plans to change anything for the moment, but if I did I would change the consumer unit for a split load type as per current regs.

With reference to the 100mA trip, If I have a fault in the workshop of say 40 or 50 mA, then surely the trip at the house end will not operate because I have not reached it's trip point or am I missing something??
I have blown the trip in the workshop many times (by doing something stupid like we all do or is it just me?) but the 100mA trip has never gone.
Yes it is tested regularly!

Jon

Just so we are all singing from the same hymn sheet, I have posted some Earthing system diagrams, I will try and explain a bit more this about in a minute.

Jon, I do have a couple of comments on your drawing too, which I will post later.

There are commonly five basic earthing systems in use, although there are other sub types, five are the most common.
The T stands for Terra or Earth, the N for Neutral, C for combined, and the S for seperate, this should help explain to an extent why the systems are named the way they are.

In all cases except IT systems, the supply star point which is also the Neutral is earthed at the substation, the IT system is the exception in that it usually has a High impedance in the path between the star point and earth.

Of these five, only three are used for public electricity supplies, at least in the UK and most of Europe, which are of the type of most interest to us in this thread, but for the sake of completeness I will mention the other two which are a bit specialised.

Ze = External Earth loop impedance at the intake.

IT: The first Type which is not used in public supplies is the IT type, this usually found in hospital supplies and other life support systems, it is fault tolerant, which means it won't trip on a fault but will usually indicate an alarm of some sort or other, it must be monitored and under supervision at all times.

TNC: The Second type which is also not used in public supplies, and is illegal to use anywhere in the UK, is the TNC, this uses the Neutral conductor or PEN (protective Earthed Neutral) as the sole means of earthing, although similar to the TNC-S system, there are important differences.

On to the Public supplies then:
Ze = External Earth loop impedance at the mains intake.

TNC-S: the TNC-S system also uses a PEN conductor similar to the TNC system, but this is only on the DNO side of things, and is not allowed to be used after the service heads incomer, it is seperate from then on.
There are two types of TNC-S, one is PME or protective multiple earthing, which is earthed to multiple ground points along the supply route (unlike TNC).

The other is PNB or protective Neutral bonding. , PNB is usually used on private supply transformers and associated switch gear.
Most of Europe except the UK also require an earth rod at the intake (Michael W)
The Max Ze allowed is 0.35 ohms

TNS: the TNS system usually uses the Lead sheath of the supply cable as the Earthing conductor, this connected back to the substations star point.
The Max Ze allowed is 0.8 ohms

TT: In the TT system the supplier does not supply any Earth at all, the consumer must have an earth rod or similar fitted, and unless this is very low impedance at TN values (not usually possible in domestic) then the installation must use an RCD to provide adequate disconnection times in case of an earth fault
The Max Ze allowed is 200 ohms*
* this is the max that BS7671 states, as anything higher may be unstable.

If anyone is still interested I can post up some further information on the domestic incomers ?

Thanks for that R to B,
I often wondered what all the different types of earthing systems stood for. I considered being an electrian when I was younger, after reading this Im glad I chose TV repairs, they seem less compicated.

Jon

Thanks for that R to B,
I often wondered what all the different types of earthing systems stood for. I considered being an electrian when I was younger, after reading this Im glad I chose TV repairs, they seem less compicated.

Jon

No bother at all Jon , I actually did it the other way round, in that I was a TV engineer first, and became an electrician later.

They are both quite complex in their own right, just in different ways, I would say electrical is slightly easier once you get used to it, but both are vast subjects.

Anyway here are pictures of the general layout of common domestic service heads for the three systems.

Jon I will post a couple of comments on your drawing after this post, just so it doesn't get confusing.

Jon, I have only really two points to make on your drawing.

Point one,
Is the earth cable I have circled in yellow, now it may be a drawing error, or it could be that the earthing system was originally TT and has been converted to TNC-S (PME) at some time, this is not that uncommon (even on O/H supplies), it would also explain the seperated earthing of the armoured cable at the shed end.

Bearing in mind this is only from looking at the drawing provided.

Point two,
Is the supply to the shed should have an overcurrent protection device (either fuse or MCB) to protect the supply cable (circled in blue) inserted here, and this should be sized to suit the csa of the conductors in the armoured cable.
Before or after the RCD does not really matter electrically, but usually the 100mA time delayed RCD's are suited to fit directly on the tails with an MCB or similar after.
RCD's do not offer overcurrent protection, only earth fault.

Most DNO's don't allow the tails to be extended by more than three metres, without having a switched fuse or being fused down within the three metres.

But this again is just going by the drawing you provided, obviously I cannot see the installation first hand, and it could be just a case of it being omitted from the drawing.
Don't worry I am not the Part Pee police

Hi R to B,

Point one,
Is the earth cable I have circled in yellow, now it may be a drawing error, or it could be that the earthing system was originally TT and has been converted to TNC-S (PME) at some time, this is not that uncommon (even on O/H supplies), it would also explain the seperated earthing of the armoured cable at the shed end.

Yes the earth seems to originate from the main incomming fuse like my drawing and in your picture TN-C-S, but there is also a stake as in TT so I assume a combo as above.

Point two,
Is the supply to the shed should have an overcurrent protection device (either fuse or MCB) to protect the supply cable (circled in blue) inserted here, and this should be sized to suit the csa of the conductors in the armoured cable.
Before or after the RCD does not really matter electrically, but usually the 100mA time delayed RCD's are suited to fit directly on the tails with an MCB or similar after.
RCD's do not offer overcurrent protection, only earth fault.

Most DNO's don't allow the tails to be extended by more than three metres, without having a switched fuse or being fused down within the three metres.

The length of cable in the blue circle is no more than 0.5 of a metre if that. No MCB of fuse is used here. Should I add one or is it only required if the cable is more than 3 metres? Is this a new reg or would it of been required late 90's?

Jon

Yes the earth seems to originate from the main incomming fuse like my drawing and in your picture TN-C-S, but there is also a stake as in TT so I assume a combo as above.

Obviously without physically testing it I cannot assume it is one type or another.
If it has been converted to TNC-S at some point, then really the main bonding conductors in the house should be 10mm^
If the earth rod is still connected then this is not a problem either, it can just be left as is.
As I say it's anyones guess on here.

The length of cable in the blue circle is no more than 0.5 of a metre if that. No MCB of fuse is used here. Should I add one or is it only required if the cable is more than 3 metres? Is this a new reg or would it of been required late 90's?

This is not really anything to do with part P, or BS7671 (wiring regs), either then or now.
This is a DNO reg, and was probably in force then, it is to protect their supply equipment, as well as your sub main (see below)

Also what it means is that the supply cable to your shed is only protected by the 80A incoming fuse, and really it should/must be fused down, but by how much depends on the size (csa) of your supply cable to the shed, it needs to be calculated, you would really need to consult with an electrician to do this properly. although if you know the csa of the supply cable and the approximate loading at the shed we can calculate it on here.

It is not just the 0.5 metre to the RCD, as the RCD does not provide overcurrent protection, it is the whole supply cable to the shed.

BTW, I am just pointing this out, what people do in private is entirely up to them.