Gas and water bonding

I need to install electrical bonding to my gas and water supply.
Unfortunately the gas meter, electric meter and rising main are about as far apart as possible in my house. The water is under the stairs in the middle of the house, gas in the garage at the front and electricity at the very back of the house, which is a long thin extended semi. I have a TT earth and can't see any point in connecting the water and gas to the main earth, surely providing an earth rod for each would do exactly the same job and it'd be a lot easier. As I understand it, bonding the services (in a TT system)is to ensure that if any part becomes live there is a low resistance route to earth so the rcd trips, so an earth rod under the stairs would do the same job as the main rod (earth is earth surely, if there's a significant potential between two points 15M apart then the worlds energy problems are solved).
I can't find any regs which say this is OK unfortunately, does anyone know if I'd get away with it if I had to have an electrical inspection. If not, is 10mm earth wire OK, are there limits on the length you can use, and how it must be installed. Can I bury it? if so, does it need to be in conduit (what sort?), how deep etc.
Any suggestions gratefully recieved, don't really fancy running the earth around the outside of the house but even less keen on trying to hide it inside. Could go up and through the loft but that'll be a long bit of wire.
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On 21 Apr 2005 06:31:48 -0700, snipped-for-privacy@engenuity.net (Kevin Chambers) wrote:

sorry they both need bonding to main earth terminal (Separatly at that) dont think its alright to bury it how about thin plastic conduit around the outside? 10mm earth cable will be fine regards bob
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snipped-for-privacy@engenuity.net (Kevin Chambers) wrote:

[...]
Nothing to do with tripping the RCD really, though that may be a side-effect. The point of bonding is to ensure that if there is a fault to an "extraneous conductive part" (bit of metal) that *all* similar parts are at the same potential. In other words, if you should be unfortunate enough to make your metal bathroom taps live, the radiator will also be live and so if you should touch them both at the same time no current will flow. It's counterintuitive, but seems to work.
Your idea of separately earthing wouldn't work as even an extremely well installed earth rod will have a resistance to earth of several tens of Ohms and thus under fault conditions there will be a PD between services bonded to different rods. The resistance to earth depends on a lot of factors, not least of which is how dry the soil is, and I suspect that the soil under your under-stairs cupboard will be about as dry as it could get.
As for significant potential between points 15M apart, well it depends on the amount of current flowing into the earth rod. Note that farm installations have *stricter* RCD and earthing requirements than domestic because of the possible PD across the length of (say) a cow :-)
Hwyl!
M.
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Martin Angove wrote:

Think about why birds can perch on high-voltage lines. The line voltage, measured with respect to earth, doesn't matter. What matters is that both of its feet are at the same potential, so no current flows through the birdie itself.
The aim of equipotential bonding is exactly the same: to equalise the potential (voltage) of all metal objects within reach, so that no current can flow between them... via you. It doesn't matter what the potential of your whole bathroom is, relative to planet Earth, so long as no current is flowing through you.

If it still seems counterintuitive, it's time to get some new intuitions about electricity. (Available in various shades of grey.)
However, this all rests on the assumption that you and your bathroom floor are well insulated, so that no current can flow through you to "real earth" - the dirt under your house. For most types of house construction, with suspended wooden floors, that is an excellent assumption so equipotential bonding is the way to go.
Now here's my problem: we have just moved into a traditionally constructed cottage with a solid screeded bathroom floor. At present there is no floor covering except for the damp bath-mat. The floor is "dry" in the normal structural sense, but it obviously has a much lower resistance to earth than a suspended floor.
The supply is TT and the earth spike is at the front of the house; but the bathroom is at the rear. That means the bathroom floor may not be at quite the same potential as anything that is bonded to the mains earth.
What's the best *practical* way to bond the pipes and radiator in this situation? I know it's recommended to lay a metal screen under the floor and bond to that too. Unfortunately the Old Dead Guys who laid the floor didn't think of that, and in practical terms that's simply not on.
I am strongly tempted to put in a second good earth spike, right outside the bathroom window, and bond the bathroom metalwork to that earth incomer on its way to the CU. That would bring the metalwork as close as practically possible to the potential of the floor.
Longer-term, the earth leakage will be greatly reduced because we will be installing a sealed floor-covering which will also provide very good insulation... but I'd like to get the bonding right as well.
--
Ian White

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It's ok, I understand it perfectly well with a physics background and all that, but I do find that "the great unwashed" (i.e. those without physics backgrounds) are confused by the thought of making everything "live" in order to *prevent* electrical shocks.

Define "low". Plucking figures out of thin air, if you consider 10mA to be a safe current (note that "safety" RCDs operate at 30mA) then at 240V you're going to need a resistance to earth of 24kOhms or less to beat this target. Is this likely, even with a concrete slab? Why should wood, which may well hold a reasonable amount of moisture, insulate any better than cured concrete?
I honestly don't know if this is likely to be a problem with your setup, and await others' thoughts on the matter. One thought which occurs to me though is that when I worked in a factory dealing in static sensitive electronics the floor had to be regularly painted with "conductive" paint to allow static to dissipate. Since the rating of the "conductive" mats and wrist bands and so on used was of the order of 1MOhm or more this implies to be that the factory's concrete slab must have had a very high effective impedance in order for the paint to be necessary.
Is there a possibility of upgrading the installation to a TN-C-S system? If so then the system earth is tied to the neutral at the incoming, and impedances fall through the floor, as it were. Most houses in my village are still TT though the distribution network was upgraded to TN-C-S some years ago. Some houses still have voltage operated circuit breakers as the main switch where the house electrics may not actually have an earth rod at all; the earth being used simply as a reference for the breaker.
Hwyl!
M.
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(Kevin Chambers) wrote:

I see your point and the important thing is whether it'll pass an inspection so I guess I'll be ordering 100M of earth cable, is 6mm OK at any length? I thought I needed 10 anyway but 6 would be easier.
I'm still confused over the logic though, surely part of the reason why TT systems require rcd's on all circuits is because the earth rod cannot be relied upon to give a low enough impedence to earth to prevent a dangerous shock (or trip a circuit breaker).
Multiple earths on TT system worst case. Touch two services, one earthed at rod A, the other live but earthed at rod B and passing <100mA, assuming all earth rods meet the requirements there must also be another impedence between live and earth or the RCD would have tripped?
Non TT system worst case. Touch two services, one fully connected to suppliers earth, the other live and poorly connected to earth, i.e. compression fittings, pushfit, dodgy joint etc, could be passing anything up to the current of the highest non-rcd circuit mcb (could be 32A?) before anything trips.
Is the multi earth TT scenario any worse than the non-TT one
Unless there is a requirement to ensure continuity to metal fittings of all services (which there may be, I'm no expert, but in that case the rules for bathrooms would surely apply everywhere) then the regulations don't attempt to protect against having different earths on different parts anyway (outside the bathroom). If all fittings should have continuity then why do I have to bond at the meter, the boiler is right over my leccy meter, suplementary bonding everything there would be no problem.
I can't remember any warnings on pushfit connections saying they need bonding if used with copper pipe, if they do I bet practically no-one does.
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snipped-for-privacy@engenuity.net (Kevin Chambers) wrote:
[snip my previous reply]

As Andy says, 6mm is generally ok for TT systems, but installing 10mm would be sensible in case you are able at a future date to upgrade to a TN-S or (more likely) TN-C-S system.

Well yes, that is partly true...

Not quite sure what you are trying to describe, I'm afraid. Don't forget to include source impedances, resistance of human body (greatly reduced if barefoot and wet as in a bathroom) and so on...
With TT systems there should be *two* RCDs. It is only the 30mA/40mS RCD which is supposed to protect you from a dangerous electric shock, and that is only (usually) connected to socket outlets. 30mA for 40mS is the level that someone has decided is safe for most people (children and those with heart conditions beware). The 100mA RCD should also be time delayed in order to avoid nuisance trips of the lights when a fault occurs on the sockets, so not only does it pass more current, it passes it for longer thus leading to a greater chance of injury or death. Its main purpose is to avoid damage to the installation, not to the occupants.

Your point being that if your kitchen tap somehow becomes live and you touch your plugged-in metal kettle (earthed through the circuit), you are likely to receive quite a shock? Yes, that's true, but one has to consider likelihoods. Some people insist on equipotential bonding the kitchen sink and taps, but there is only a BS7671 requirement to EP bond in the bathroom, or other location containing a bath or shower.
[...]

The argument over when and when not to bond continues. In a bathroom you *do* need to bridge plastic sections, unless they are over a certain length (around 1m) when items on the "far side" can be considered to be sufficiently isolated from earth for it actually to be dangerous to bond them. Elsewhere, who knows?
Gotta stop and let someone else take over. Coffee is calling...
Hwyl!
M.
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(Kevin Chambers) wrote:

snip more
I've given up on the multiple earth idea but...

Argument against multiple earths centred on the possibility of different services being at different potentials, (all my services are on plastic incoming so can't become live from anyone elses property). I was trying to compare what I saw as worst possible cases for multi earth TT compared to any other non-TT system in the highly likely event that continuity wasn't maintained throughout the houses internal pipes.
Worst case for TT system is a fault which only just doesn't trip the 100mA rcd? A low resistance fault, even to an earth rod should trip the RCD so you need just enough resistance to pass less than 100 mA.
Can't see that this is clearly any worse than a non TT system which is only protected by the fuses. And thus can't see why a multi earth TT system is any less safe than a non TT system. Regs don't require maintaining continuity in services, so you must surely assume that there isn't continuity.

Only needs to be time delayed if in series, and they are so bloody expensive that I've "got" one for each set of circuits.

Absolutely, that's partly what I'm thinking.

What I've come round to thinking is that bonding services at the entrance to the house is either a) totally pointless or b) protecting against a set of circumstances which are very unlikely and where there are other, equally likely, circumstances which would be made worse by bonding.
This is in the circumstance where incoming services are plastic and knowing that there is no requirement to maintain continuity internally (and it's therefore highly likely that at some point continuity will be broken)
Bonding services is so that if any pipe somehow became live it would be earthed so you don't get a shock and die? As soon as you introduce pushfit to the world (but before then too), any particular piece of pipe which may become live is very likely to be insulated from earth. At this point having anything properly bonded to earth actually makes matters worse if you happened to touch two different bits of pipe. The fact that bathroom bonding regs are different in my opinion recognises the fact that, generally, the danger from services becoming live poses an acceptable risk. Unless some part of you is well earthed when you touch a live pipe you probably won't die. Unless you can guarantee that all metal parts are earthed (no attempt to do so, phew..) surely it would be best to reduce the chances of being well earthed.

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snipped-for-privacy@engenuity.net (Kevin Chambers) wrote:
[large snip]

No. This is a common misconception. The point isn't to make sure that metal services etc. are "earthed", but rather to ensure that they are electrically connected.
Under normal circumstances you would not expect any reasonably likely electrical fault to be able to make such "extraneous conductive parts" (i.e. conductive things which are *not* part of an electrical appliance) live. Again, under normal circumstances any "exposed conductive parts" (i.e. conductive things which *are* part of an electrical applicance) which become live through a fault in the appliance should cause the circuit protective device (ideally the fuse/MCB, but in a TT system quite likely the RCD) to operate.
However, where an extraneous conductive part does become live, or where earthing is faulty or where there may be an appreciable delay before the circuit protective device operates, there is potential for an unfortunate householder to simultaneously touch both the "live" part *and* some other extraneous/exposed conductive part which is "earthy". With no bonding in place, there will exist a Potential Difference (PD, Voltage) between these items and current will flow through said householder at a rate governed by various resistances.
Bonding prevents this by ensuring that all such metalwork is electrically connected together and thus is at substantially the same potential wrt earth (the limit is 50V). The householder is now touching two items with little or no PD between them and thus little or no current will flow. Connect a bulb between two lives (on the same phase for the pedants) and it won't light up; it needs to be connected between a live and a neutral.

*If* a particular section of pipework is completely and utterly isolated from earth, then it is "floating" to use the technical term, and it doesn't matter whether you touch it while holding the incomming phase wire in your teeth (usual disclaimers apply). There is no PD between that phase and anywhere and so there is nowhere for the current to flow. Such a section shouldn't be bonded at all, even in a bathroom.
Individual pushfit joints cannot be described as good conductors, but neither can they be described as the perfect insulator as the gap between copper and copper (if copper pipework is used) is actually very small, and the water in said pipework will be slightly conductive. General consensus seems to be that you need about 1m (3ft) of plastic pipework before you can consider a metal section to be effectively isolated.
If I understand your argument correctly, what you are worried about could be illustrated by a metal radiator on an otherwise plastic heating installation. If this radiator somehow (how?) becomes live, then yes, you are going to have problems if you touch it and something which is earthed. It is far more likely however that something else will become live and you touch the radiator. If the radiator *is* isolated then it matters not at all that the water stopcock is bonded; the radiator is floating (no pun intended) so there is no problem. If the radiator is *not* properly isolated then there is every chance that it will be "earthy" somehow. Without bonding, a PD will exist. With bonding it shouldn't.

The ideal situation would be one where everything conductive was isolated both from everything else electrical, and from earth. In practice this is impossible to achieve. The only reason earth is a problem is that earth is a part of the electrical distribution system; one end of every transformer is connected to an earth electrode. In a TN-C-S system this is even more obvious as the "circuit protective conductor" (otherwise known as the earth) is connected directly to the neutral where the supply enters the building. The neutral is itself earthed at multiple points along its route back to the transformer by the use of (for example) earth rods near the distribution poles and, of course, it is earthed at the transformer itself.
There are many different facets to the problem of earthing and bonding, and I've only covered a few. It is impossible to make any electrical installation 100% safe while still being economical to install and maintain, and able to perform useful work. The question is, where do you draw the line? It is generally thought that the system we have evolved over the years in this country is practically the best in the world (particularly by those who have worked in the States). As you will discover should you every be foolish enough to search for the "Part P" arguments on this group (especially those from 2003/2004), Britain has an incredibly low rate of death directly attributable to installation wiring, even installation wiring which hasn't been performed to BS7671.
Run out of things to say now...
Hwyl!
M.
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Kevin Chambers wrote:

6 will meet the regulations [547-02-01], although it might be challenged by an inspector who is not too familiar with TT installations. There's also the issue that your (presumably) overhead supply may be upgraded to offer PME in the future and to take advantage of the PME facility you'd have to upgrade the bonding again. So using 10 avoids any controversy and is a bit more future-proof. The length simply isn't an issue - the resistance of 100m of 6mm^2 wire is about 0.3 ohm, which is around two orders of magnitude less than the ball-park resistance of your earth electrode.

The latter of those. A fault may cause a high touch voltage, but must be cleared within 0.4 s for socket circuits and 5 s for fixed equipment circuits.

If that's the case the touch voltage will be less than 50 volts - deemed safe in dry conditions. If the OSG recommendation of keeping the earth electrode resistance <200 ohms is followed then (with a 100 mA RCD) it'll be less than 20 volts.
there must also be another impedence between live and

I'm not sure what you're getting at there.

But there you're postulating an un-bonded extraneous-conductive-part (see below) becoming live of its own accord. How is that going to happen?

Not from the circumstances you've described, but consider that an incoming metal service pipe may become 'live' due to a fault in someone else's installation, or in the street. Your RCD won't clear this, so, if you don't have a single equipotential zone (EPZ), a dangerous touch voltage could exist for a long duration.
You may think that's a extreme example - and it is - but the bottom line is that this is not negotiable: within one building you must have a single EPZ if you want to comply with BS 7671.

There's an important distinction between exposed-conductive-parts and extraneous-conductive-parts. The former term means the metalwork of electrical equipment (excluding Class 2 - 'double-insulated' - items). These parts could become live under fault conditions and must be reliably earthed via circuit protective conductors.
Extraneous-conductive-parts, OTOH, are metal items which are not part of any electrical equipment, but which could "introduce a potential" - IOW they're conductively connected to earth (usually) or some other voltage. These parts don't have to be earthed individually. The philosophy is that if any/all metal pipes etc. entering the building are bonded to the main earth terminal at their point of entry then it's highly improbable that a dangerous touch voltage could exist between any extraneous-c-ps and other extraneous- or exposed-c-ps. (In the shower/bathroom local supplementary bonding ties together all extraneous- and exposed-c-ps to reduce the risk further.)

There's no requirement for that.

Because that's not bonding at the point of entry to the building. It could leave other extraneous-c-ps still connected to the outside world and so violates the EPZ concept.

At the risk of repetition: you don't have to "earth all your plumbing." You bond at points of entry to create an EPZ, backed up with local supplementary bonding in high risk locations.
HTH
--
Andy

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Kevin Chambers wrote:

No that would fail - see Martin Angrove's reply.

10 is OK. The min. size for main bonding for TT-earthed installations is 6mm^2.

You can directly bury green/yellow insulated earth wire, but then the minimum size becomes 16mm^2.
--
Andy

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Grrr... it is Angove; "an Gof" in Kernwek (Cornish) or "the Smith". Most of the Angroves are Canadians who got it wrong one year :-)
Hwyl!
M.
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Martin Angove wrote:

My profuse apologies - and nice to have you back here BTW.
--
Andy

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