A collection of electrical earthing questions

[dave]

I have some questions regarding domestic electrical earthing - and would appreciate some enlightnment.

The house is a 50's semi. Earthing is provided by a conductor bonded to the cold water supply where it enters the house. There is an RCD fitted in the CU. Both rings and lighting circuits have mcb's.

Q1: Is this an illegal installation because of the earthing?

Q2: Is this a TTS system?

Several years ago we had a quote from the electricity company to rewire the house (which we should have got around to but didn't). In that quote they say (under comments), "PME Earthing". I'm not sure if that meant they intended to provide PME earthing as part of the quote - or that (for some reason) they thought we already had it. I think the former. Anyway...

Q3: Given that (afai can see) the only conductors into the house a single phase and neutral cable, how/where would they have obtained that earth from?

I'd like to install an earthing rod. If I do,

Q4: Is this still TTS?

Q5: If I do install the earthing rod, can/should the bond to the cold water supply remain *as well*. Would it do any harm to leave it in place (electrically) anyway?

Q6: Is it possible, and is there any virtue in, asking the electricity company to provided PME earthing to the property? (How they would get that to the house I have no idea - maybe is expensive).

Q7: If PME is installed, would the installation then be TN-S? Thanks

[Andy Wade]

Yes, you seem mighty confused...

may shed some light.

Are you sure you're not confusing the "means of earthing" with the (required) main bonding connection to the water pipe? Is there any other connection to the main earth terminal? Is your supply from an underground cable, or overhead wires? A photo of the supply intake, meter and consumer unit would help if you have a digital camera and some Web space.

If the water pipe is the only means of earthing it would certainly be illegal to install now. Right now it requires urgent investigation and possibly remedial work.

TTS? There's TN-S (cable sheath earth), TN-C-S (PME) and TT (own earth rod). These are explained in the above-cited FAQ article.

Perhaps. On a 50s property PME will only be possible if the service cable has been replaced, or an overhead network upgraded.

From the neutral - that's what PME is - or from a local earth electrode (TT system). TT is a non-preferred option now and you should use it only if the supplier cannot give you PME, or TN-S. What does the cable look like, by the way?

Before you do that, let's be sure it's necessary.

It would still be TT, yes, provided there was no supplier's earth connection. (The bond to the water pipe doesn't affect this, and is still necessary.)

Yes, it's a requirement - main equipotential bonding. There should be one to any gas pipe too, and any other incoming metallic services.

Yes, use PME if available. If necessary, you will need to bring the earth bonding, both main and supplementary, up to modern standards before the supplier will connect a PME earth.

No, it would be TN-C-S.

If you're going to DIY this you need to learn an awful lot more. Could I suggest using a qualified electrician? Start by getting a PIR (periodic inspection report) done, as there may well be other nasties lurking in your old wiring.

[dave]

Why?

Hang on. You say I am "mighty confused" based on the assumtion that *you think* I *may* be confusing the means of earthing with an equipotential bond! I think maybe remedial English comprehension and elementary logic class would benifit you.

No! Clear enough I hope.

Overhead.

Intake! Mmm. Ducted fan I think it is. :)

Eh! It just a plain honest to god Wylex 6 way consumer unit (split) with an RCD and mcb's! Why do you need a picture of that?

Well of course it would be to install NOW. That's why I clearly stated the age of the property. Do you think I ripped out the PME and installed a equipotential bond to the water supply in place of it? My actual question was, is it illegal NOW. It may well be unsafe - that's clearly a separate point. (remember the RCD though).

That's right. The past 30 years it's been teetering on the edge of black-hole collapse. Yes I know that just because it's been a long time doesn't mean it safe etc etc. Anyway, that's what I'm "looking at" now.

Ah ok well that is useful.

[useful info snipped] thanks for that.

Mmm while I thank you for the purely informative parts of your reply I object to your presumptuous and patronizing statements. While earthing arrangements can certainly be involved, getting a collection of letters to tie-up with the configuration is no giant intellectual feat. I just didn't have the detailed info of the coding and schemes in use today. I am more than happy to debate electrical theory with you (if I have the time anyway).

Suggest it but totally unnecessary. I have been involved in electronics for 30 years and used to install high power multi phase system (steel rolling mills/ motor controllers - you name it). I **openly admit** though to being wayyyyyyyy out of date with the latest domestic TTT etc coding regulations and best practice. It is remise of me not to have done this before but I am quite capable of quality workmanship. btw (and I could be wrong here) but I think years ago, connecting to the cold water main was commonplace for an "earthing system". And in fact before plastic pipes came along it wasn't a bad option. I'm not advocating doing it now.

A closing thought is that I may not be allowed to do it anyway - given the latest legislation.

[Andy Burns]

Another decent article with helpful explanations and diagrams

[Grunff]

What a charming attitude you have! Andy Wade takes the time to respond to your post, giving you some pretty helpful tips, and all you can do is reply in a rather childish, defensive manner.

[Christian McArdle]

From the neutral. That is what PME or TN-C-S actually means. The combined earth/neutral conductor goes into the supplier's cutout and is split there before it gets to your installation.

If there is a third conductor, such as the cable sheath providing an earth, it is not PME, but TN-S type earthing.

If you are getting the house rewired, they will be unable to change the earthing system (if any) supplied by your electricity supplier. The only possibility is that they can use a TN-C-S earth terminal provided by the supplier that is currently unused because the original TT system wasn't suitable for direct connection (perhaps due to main equipotential bonding conductors being too small).

Christian.

[David Hansen]

On Tue, 04 Apr 2006 13:59:43 GMT someone who may be dave wrote this:-

I think that given that reply few people will be keen to give you any more advice. The advice which you were given is good advice, all of it including the bit about you seeming mighty confused.

The TT and so on terminology goes back at least 25 years, probably more.

One last thought. Proper earthing is important and can be made worse by those that don't know what they are doing. I suggest reading the articles you have been pointed to, followed by one of the various guides that are available.

[chris French]

I don't think it's that uncommon Andy, it older properties not recently rewired. Our last house - 1938 semi, some wiring done in the 1960's ish probably, but still old rubber stuff on the lighting circuit.

Our only earth was via the lead water main.

[Andy Burns]

and you're practically *boasting* about that?

[meow2222]

no. The right q is is it safe: check the water pipe is metal and goes into the ground. If its been changed to plastic, you have an immediate problem.

TT

Keeping it and conecting them would be an improvement on a water pipe earth TT system. Redoing it to 16th edn spec would be best.

Water pipe earthing is widely criticised because the metal pipe, which makes a very good earth, could be changed to plastic. And that makes a none to impressive earth.

PME is much lower earth impedance, and allows you to have no RCD on lighting, which is a safety advantage to some extent. Lower E impedance means touchable metalwork stays at lower voltage during a momentary fault. The lower impedance also means to clear earth faults youre not relying on the rcd, which to some extent you are with TT. So yes PME is better, fwiw.

PME has its own risks, multiple neutral faults on the supply side can make all your earthed metalwork live, but this is a lot less likely to occur than with TT, which can also go the same way in the case of multiple faults, with higher chance of so doing. Having said that, TT is perfectly adequate.

TN-C-S

NT

[Andy Wade]

Yes, I've never doubted that that used to be quite common practice. OTOH confusion has often arisen between the earthing conductor and the main bonding connection to the water service in threads such as this in the past, hence I felt that the possibility should be considered and eliminated (or otherwise) at an early stage in the discussion.

[Andy Wade]

No, I only said you _seemed_ (to me, at that point) confused about many aspects of the subject. The other source of possible confusion, between earthing conductors and bonding conductors, has been common in threads like this, so it helps if it's resolved early in the discussion, that's all.

[...]

I know of no reason which makes it illegal now. The building regulations, Part P of which which now cover the electrical installations of dwellings, are not retrospective.

So long as the water service pipe remains metal and provided that the RCD is regularly tested to ensure it works, then it's probably not electrically unsafe (although any competent electrician doing a PIR would flag it as 'code 1' - "requires urgent attention" - if only to cover their own arse).

It also sounds as if you have a single whole-house RCD (30 mA?) and that, nowadays, is not regarded as a safe arrangement since a single earth fault will result in loss of supply to all circuits, including lighting, which may be dangerous (trip/fall hazards) and will almost certainly be inconvenient.

[See reg. 314-01-01 of BS 7671 which says: "Every installation shall be divided into circuits as necessary to: (i) avoid danger and minimise inconvenience in the event of a fault, and (ii) facilitate safe operation, inspection, testing and maintenance."]

For upgrading you have two earthing options, either install a proper earth electrode (usually one or more rods), keeping it as a TT system, or get the supply and installation upgraded for PME. At the same time you could consider replacing or upgrading the consumer unit to provide a split-load arrangement complying with 314-01-01. For TT this should usually have a 100 mA RCD for all circuits except those socket circuits "reasonably expected to supply portable equipment for use outdoors," which must go via a 30 mA RCD [reg. 471-16-01]. For a TN system only the latter, 30 mA, RCD is required.

Part P doesn't forbid DIY wiring work of any kind. If the work concerned is notifiable then a building notice should be submitted to the local authority, who are supposed to arrange for inspection, testing and certification of the completed work. For non-notifiable work you are responsible for doing (or arranging) your own inspection, testing and certification. Installing or upgrading equipotential bonding is not notifiable work, but altering the earthing arrangements and consumer unit replacement are notifiable.

[jim_in_sussex]

Fie!

You are receiving excellent expert advice.

My first impression from your post was that you have an elderly TNCS/PME supply, so may be AW is a little alarmist, however it is also clear you have some homework to do to familiarise yourself with the theory and practicalities of domestic power wiring.

snip....

. I am more than happy to debate

snip...

. I have been involved in electronics for 30

There is considerable difference between electronics and power supply practice.

Ohms law stays the same, but after that everything is at opposite poles: eg

generally in electronics: currents are low; conductor size is seldom critical; frequencies may be high; impedances contain R,L & C components; compenents get hot but conductors don't; heat is dissipated with fans; circuit fault protection is ignored.

in power supply work (at least for domestic and small offices) : currents are high; corrrect conductor sizing is vital; frequency is low and fixed (50Hz); impedance is pure resistance; components (fittings) stay cool but conductors get hot; heat is dissipated to ambient surroundings (and thus much attention is paid to routing conductors); circuit protection, especially from unforseen external causes, is important.

There is also a significant difference in bonding practice: in electronics common practice is to bring all grounds to a common point and avoid mesh circuits because of eddy currents: in power supply work everything in sight is bonded at every opportunity creating a huge mesh network.

it might help to peruse:

+ other items on Kevin Boone's website. [highly recommended if you are know your elec theory but are new to power cable selection and protection]. IEE On-Site Guide

The Electricians Guide John Whitfield

HTH .