In the process of having our house rewired (1970's wiring done by spider man!) and the discussion of bonding came up. As we have none at all, it has already been put in, but the query I have is, should one be unfortunate enough to touch a bonded item (sink/tap etc) and a live source, surely the resulting electrocution would be far more severe than an un-bonded house?
I know theoretically you shouldn't be able to reach any electrical source while touching bonded "bits" but that's never going to happen, so is bonding MORE of a health risk than getting a little tingle from an un-bonded house?
After all, you're more likely to get a shock from exposed electrical appliances than you are from a wire somehow connecting with the copper pipes etc?
Just a point for discussion rather than a "thats the law" kind of reply. Ta.
The shock might or might not be more severe (depending on, for example, whether your water service pipe is plastic or metal). Talking about the severity of electrocution though is a little pointless: dead is dead.
That's irrelevant. Earthing and equipotential bonding are measures to prevent electric shock from 'indirect contact' - i.e. contact with accesible parts which have become live as the result of a fault. Protection against shock from 'direct contact' relies on other measures - principally insulation! The wiring regulations (BS 7671) recognise the use of residual current devices (RCD, RCCB, RCBO) as a _supplementary_ measure against direct contact. That's why RCD protection is required for socket-outlets likely to be used to supply portable equipment outdoors.
Protection against shock from indirect contact by 'non-conducting location' is recognised in BS 7671, but not for domestic situations where it would be completely impractical to implement.
The EEBADS measure (earthed equipotential bonding and automatic disconnection of supply) is there to ensure that the 'touch voltage' between accessible metal items, be they electrical or not, is constrained to a safe combination of voltage and time. Bonding ensures that the same earth reference point is used for both the earthed electrical items ('exposed-conductive-parts') and non-electrical metalwork ('extraneous-conductive-parts'). Without bonding, the touch voltage could be higher, especially with PME supplies.
It's not the law yet, but a public electricity supplier can refuse to connect you if your wiring does not comply with the 'British standard requirements - i.e. BS 7671. So thou shall't bond.
For further (fairly technical) background on why the wiring regulations are as they are I recommend Paul Cook's Commentary - see
Thanks for your time Andy. My concern arose after getting a 120v shock from our microwave oven whilst leaning against the stainless drainer to remove some blinds.
The socket the Microwave was plugged into had a fractured Neutral (not connected) so, the 120v from the microwave case screw could have been far greater than a "tingle" if the sink had been bonded?
And, If it were bonded, would such a neutral fault/shock have tripped the RCD? as on this occasion it didn't.
That's an interesting diagnosis. The case shouldn't be live under any circumstances (well, not these days anyway), and unless you have a high impedance supply and the neutral in the socket connected itself to Earth I'm not sure how this happened... and why 120V?
Possibly in this case because:
It seems to me as if as well as a neutral fault you also have suspect earthing.
On a TN- supply if any part of an earthed object becomes live (as you are implying the case of your microwave did) then enough current should normally flow to blow the circuit fuse or trip the breaker. This is why a whole-installation RCD is unneccessary for these supplies.
With a TT supply (local earth rod) the impedance of the earth path is usually too high for this to happen, and so whole-installation RCDs are required. We won't get into the types here, but the fact is that they trip with minute amounts of current and your RCD *should* have disconnected the supply the moment the case of your microwave became live; it should not have had to wait for you to touch it.
You don't have a double-insulated microwave, do you? If so, it is highly suspect and needs checking out.
The problem that bonding is designed to address is that although you would think that your (say) cold water pipe shouldn't have anything to do with your electrics, it is in fact probably quite well attached to the electrical system through the earth. The bonding regulations are there in order to ensure that any metal parts which could be "earthy" are connected together. In that instance, if the case of your microwave became live, then rather than the pipework being at (near) earth potential, it would rise (due to the bonding) to the same potential as the case. This protects you during the finite period it takes for a fuse / MCB to blow, and further protects you should circumstances conspire to prevent them blowing.
Should you then happen to touch both live case and bonded metalwork, rather than some current flowing, no current should be able to flow because both are at the same potential. Sounds silly, but it works.
Supplementary bonding is only strictly required in a bathroom because it is here where your body's resistance is lowest. Whether or not to install it in a kitchen is a moot point; the OSG (guidance from the IEE) suggests it isn't usually necessary, but the electrician's "bible" (John Whitfield's book) insists that it is.
Where supplementary bonding *isn't* required is when the metalwork in question is effectively isolated from the earth. This could be, for example, a metal door frame or a metal radiator supplied by plastic pipes: beyond a certain length of pipe (somewhere around 1m) the resistance of the water is high enough to be able to consider that the radiator isn't "earthy" at all.
My feeling is that supplementary bonding wouldn't have made any difference in the above case.
The sink will almost certainly be pretty solidly connected to your wiring earth because of the requirement to bond to incoming water main. Even if that bonding wasn't in place there's a fairly good chance that the sink will be connected to earth by the pipework anyway. The only way that the sink could be (relatively) isolated is if it had all plastic plumbing, in that case you'd be better off it it *wasn't* bonded in the above scenario.
The real cause of you getting a shock would appear to be that the earthing on the microwave had failed. If its case had been correctly connected to the wiring earth you wouldn't have got a shock regardless of the state of the neutral connection. I don't really understand how your 'fractured neutral' caused the microwave to be live.
The basic premise of the OP is correct. It is safer to have isolated metalwork in the bathroom than to have earth bonded. However, if there is a chance of earth bonding by whatever means, then it is much safer to ensure that all the metalwork agrees what that earth is by doing the supplementary bonding.
The result of this is that if the parts are already isolated by being connected via plastic pipework and not having an electrical connection and not touching structural metalwork in the building, then supplementary bonding should be omitted for that item (usually a bath or radiator) to maintain that isolation. If, however, there is a chance that the metalwork might be earthed in any way (via pipes or whatever), then the supplementary bonding should be applied.
Best: Complete isolation. OK: Supplementary bonding Unacceptable: Extraneously earthed, but not supplementary bonded
No, nor me, unless it was touching the earth.... ? As for the 120v (measured potential between sink and Microwave case, I just assumed it was a half wave (?) of the 240v
Either way, both have been replaced and the bonding hs been run to the taps/sink.
The only time I have experienced an electrical shock in a kitchen was when a rat had eaten through the cables in the cellar of a house I was renting. Due to the bizarre series of noshings on the part of the rat, it had managed to allow the line to come into contact with an earth bonding wire, without tripping the RCD. I went down in the mornign to make tea, as I touched the cold tap I had an "interesting" experience. Probably made worse by concrete floor and no shoes.
Yes, supplementary bonding is for bathrooms. The kitchen sink shouldn't have been bonded, although if fed through main bonded metal pipework, the effect of supplementary bonding would have been minimal.
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