But you have to bond them 'even more'. The iee requires that it is bonded. Even with the earth in the supply cable it is deemed to be of too high a resistance. Also as has been pointed out in some of the above threads the earth in the circuit feeding the shaver socket is not neccesarily connected to the exposed metalwork in the bathroom by means of other equipotential and supplementary bonding.
SJW A.C.S. Ltd.
Interesting. I would love to see a proper estimate of the number of people who come to grief(*) through situations involving the lack of kilometres of
4mm2 bonding cable, and how those situations arose, but then I am deeply cynical by nature.(*) Compared with, for example, the number of people who trip over their own feet and fall down flights of stairs.
Of course not. But the regs would have to allow for it.
Dunno. But I'd certainly not like to find out the hard way.
No-one said the regs made sense, or that they solved all common causes of accidents in the home. Some of it is excessive, some is required. It is possible that in some instances such an excessive amount of earthing would be completely OTT, but a blanket rule has to be applied with the 'worst case scenario' being catered for. Not all electricians seem to be able to do things safely when being told what to do as it is, diy'ers would have trouble working out what was wanted and what wasn't. Lazy people just wouldn't bother applying any safety rules at all. Hang on, that's what happens now, none of this is compulsary, it's just a guide. If you buy a car with airbags you may not need them from a safety point of view but they are fitted in most instances 'just in case'. A similar rule is applied to safety measures in electrical installations, they are there 'just in case' an accident occurs and to minimise damage in the event that one does happen.
SJW A.C.S. Ltd.
It`d be interesting to see suggestions on how you earth a concrete floor :-}
10mm bond to the reinforcing ?
Andrew
I've just wired a swimming pool, thats what I did. Miles of 10mm, clamps and other earthing paraphanelia!
SJW A.C.S. Ltd.
I'd love to know how you insulate it *from* earth.
Well, I think I've done some calculations for concrete.
I make the resistance for typical 20cm concrete floor (with some major assumptions) with a wet foot sized imprint about 400 ohms, easily enough to cause trouble for a human touching a 240V live. However, the introduction of floor insulation or a damp proof course would seriously increase this, possibly to safe levels. In any case, it would appear to me that an uncovered ground floor concrete floor is not a safe surface for a bathroom. If I was having to lay one, I would definitely include any internal reinforcement mesh in the supplementary bonding. It should be noted that the conduction path is through ionic transfer in the cement.
Christian.
How? Out of interest.
Steve
The assumptions are too embarassingly huge to reveal my secret formula!
However, I assumed conductivity of 10kohm cm. See what you come up with your own assumptions!
Christian.
I should mention that I think my assumptions err on the high (unsafe) side, possibly. Maybe 200 ohms would be better. It also depends heavily on the concrete mix, particularly the cement.
Christian.
Thanks for making that clear, Bob. As Lurch has said, while it's possible to create scenarios with multiple faults where the supp. bonding causes a sustained potential difference between metalwork and surrounding surfaces
- 'the floor' in your example - the idea is that even in such a case, you've got all of the possible *low resistance* paths for current flow at the same potential, thus no current high enough to kill; leaving only a high resistance path - viz., through the floor - to give a probably unpleasant, but only pretty remotely possibly fatal, tingle. (Around 0.5mA is the lower limit of a current you can feel, which neglecting the body's own impedance means a resistance-to-earth below half a megaohm on a 240V supply gives you a detectable tingle. Muscle spasm is induced at around 10mA, which needs a resistance of some 20 times lower, i.e. about 25kohms; screwing up your heartbeat starts being likely at about 40mA, so 4 times lower again at about 6kohm.) I know of no commonly-used floor construction which will give a resistance to earth in the 10kohm-and-under range... even the 'bath salts and metal drainage pipe' instance reported in this thread was at the 'nasty tingle' rather than 'fatality' level.
Bonding only *some* of the stuff in the bathroom would give you the worst case in the presence of the sort of earth faults we're discussing - leaving a low-resistance path for the current from the live supply to continue on after it's been through someone's body - and that's the reasoning behind the obsessive 'every single bit of metal that might become live OR provide a plausible path to earth' approach. Even the dear old IEE in trying to square this circle gives some explicit guidance over limits to 'might' and 'plausible' in the above: I seem to remember that they're the source of the example of
*not* bonding a metal windowframe in a bathroom, on the grounds that it's not part of structural metalwork (hence doesn't afford a low-resistance path to earth), isn't itself close to anything with a mains supply so isn't going to go live directly, but if it's bonded to other metalwork which *might* reasonably go live, would then become live and so present a risk to a windowcleaner climbing up a metal ladder - for whom even mere muscle spasm could result in falling, splat... :-(Maybe I'm being dense, but I still don't see how the cross-bonding made the fault "more" dangerous. I guess you're saying the following: (a) there was a whole-house fault making all the "earth wires" in the whole installation essentially useless - i.e., floating, capable of attaining a potential more or less that of the incoming live feed and incapable of making any useful fuse-blowing/MCB-tripping current flow; (b) there was then a second fault in some part of the bathroom, which caused something, I'm assuming exposed metalwork, to go live; and (c) that because of the supplementary bonding in the bathroom, there was *more* exposed metalwork in that bathroom now at more or less the potential of the incoming live feed than there would've been without the bonding, hence more places from which you could get a shock. Is that about it?
If so... I'm afraid I still don't see the cross-bonding has made things worse. I think we'd all agree that (a) is the root cause and gives the greatest cause for concern. Given the undetected presence of this first fault, it means any fault-to-earth anywhere - not just in the bathroon - makes every bit of metalwork on all 'earthed' appliances float up to 'live' potential. Ugly, nasty, potentially 'orrible: but even so, the cross-bonding of services (water, gas, and electric 'earth') across the whole house is still trying to make this less of a fatally dangerous situation: *either* the incoming water/gas service provides enough of a path to earth that it makes a fuse blow, alerting the householder to fault (a), *or* the services are well enough isolated from local earth that even though stuff connected to them through pipework ends up at or near 'live' potential, just about all the readily conductive Things in the house have all floated up to the same potential, so the occupants aren't going to get a fatal belt from touching two such surfaces simultaneously. Given that your bathroom was the source of such a fault-to-earth, it's not clear that things were made worse by having
*supplementary* bonding there in the bathroom which made sure that other immediately accessible metal surfaces were really at the *same* potential- rather than, say, the CH-connected radiator or towel rail having a low-resistance-enough path back to earth - oh, say, 1kohm - so that touching that and the actual point of the fault-to-earth would've given someone a fatal belt.
I do appreciate there's a tradeoff - the window-frame example shows the downside of *gratuitous* bonding which merely increases the number of places which could rise to a fault potential (either for ages-and-ages in the presence of a further fault which makes the 'earth' connection ineffective, or just for the 0.4s/5s which the installation design allows between a fault-to-earth occuring and the relevant final-circuit fuse/MCB blowing); and I've mentioned before that there are alternative approaches to preventing fatal shock currents, of which avoiding any 'earth' potentials at all is one (and is the approach often used in creating electrical-test-bench environments, alongside using isolating transformers to remove any reference to local earth from the 'live' feed). But those other approaches aren't generally practical for the nation's bathrooms (though isolating transformers *are* specified for shaving points!), while sensibly-applied cross-bonding rarely increases the risk of fatal shock currents, and in most circumstances usefully reduces such risk.
Stefek
My DVD player does this. If you touch the metal casing you get a very faint tingle. The metal mag-mount aerial I used to have on a mobile phone used to do it too if the phone was plugged into the charger, even if the phone itself was off.
I assume that the casing/0v DC side of these things isn't really 0v. A normal 4-diode rectifier won't give a flat 0v DC but more of a ---- which means if the 0v isn't really 0v relative to your own body's potential you'll feel the pulsing.
Interestingly, since I put a TV card in the computer (in another room) and connected the aerial lead into that (earthed chassis) the DVD player which is hooked up to the TV which is hooked up to the same aerial arrangement no longer gives me that tingle. If I remove the aerial lead from the PC the effect comes back!
Yes - you're then passing a small leakage current from your DVD player to the house earth through the aerial lead and the chassis of your PeeCee. (And if the DVD is on an RCD socket, that leakage is bringing the RCD a little closer to its tripping threshold; though the current is likely to be under a milliamp, so on its own it's a long way from causing an RCD trip.
I'd be surprised if the circuitry responsible for the leakage was as far 'inside' as a bridge rectifier on the low-voltage side, though: more likely suppression components on the live side of a switch-mode PSU. But I'm a software type by profession, not an EE, so this is hobbyist-level guesswork rather than certified circuit designer's opinion ;-)
Stefek
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