Hi all I need to install supplementary bonding in both kitchen and bathroom, as there isn't any at all at the moment! Two queries arise please...
I have the luxury of having no ceiling at the moment in the room below the bathroom, which means all the piping is readily accessible. I could very easily install earth clamps connecting all the pipework, very neatly and invisibly. However, is that allowed? Do you need to be able to see the clamps etc in an installation? Where exactly do I need to attach the clamps; as close as possible to taps/radiator etc?
I need to take an earth cable right through the house to the kitchen sink. Do I need to have a separate, continuous cable going from the earthing point at the fusebox all the way to the kitchen (which means going through the bathroom en route) or is it sufficient to piggy-back the kitchen earth cable on to a clamp on a bathroom tap, say?
Indeed, there isn't: and providing a 'better' (lower resistance) earth connection for just *some* of the metalwork in the kitchen can make some possible shocks *worse*, not better. Nor is there any need for the bathroom bonding to go back to the main earth terminal. It's *supplementary* bonding, designed so that all the exposed metalwork which either might become live or provided some sort of path to earth is all held at the same electrical potential, *even**if* a fault develops in any of the other earthing arrangements. It's a *local* (to the bathroom) same-potential scene you're trying to create, *not* one which is necessarily referenced to the house earth. In practice it will be referenced to the house earth, because much - probably all - of the metalwork you're cross-bonding is already connected to earth, either through an "earth wire" if it's an electrical fitting, or because it's connected to metal water pipes which have a main bonding conductor from where the water main enters the house to the main earthing terminal.
You don't need to be able to see them, however they do need to be accessable.
You'll need to run a 10mm earth from your meter position to all of the incoming supplies, gas, water, etc.. directly, no joining or piggybacking. You''ll also need a 4mm earth around the pipes, radiator and all other metallic surfaces in the bathroom. And assuming your water supply is under the kitchen sink, you'll also want to run a link across the hot and cold, (and sink if it's metallic), in the kitchen. There are some nice colouurful pictures on the web somewhere but I can't seem to find them at this time!!
You should earth bond from the incoming electrical mains supply cable, preferably into the Henley block or head block that is used to provide the main earth to the consumer unit, and then on to the mains supply pipes of both the water and the gas systems. This bonding should be provided using
10mm csa' (cross sectional area) Green/Yellow sheathed stranded conductors. This bonding gives you a main earth bond to one end of the pipework on both systems. Taking the shortest route is better because it saves on cable and is safest because it provides the shortest path back to the main earth point on the electrical supply.
The rest of the bonding to the pipework around the house is easiest done where the pipes come together at the closest points where they come to the taps or to one side of any metal radiators you have. You should also bond together all the pipework around the hot and cold water storage tanks. Making the connections at the closest points also helps to save cable and gives fault currents the shortest path back to the main earth point at the electrical supply. All the pipes which connect to the central heating boiler, including the gas connection, should also be bonded together at the closest point where they meet at the boiler. The boiler itself should provide an earthing lug if the casing and chassis have exposed metal parts, which most do.
All the supplementary bonding should be done using 4mm csa' Green/Yellow sheathed cable to provide a low resistance path, and should be connected to the pipework with proper earth straps, not just wrapped around the pipes, and remembering to clean the pipes with wire wool or fine sand paper at the point where the strap is to be positioned before putting the clamp around it and tightening until the strap can not be turned or removed from its position by accidental mechanical damage. The pipe cleaning helps the earthing point make a good contact with the metal and it isn't trying to work through any tarnish or gunge that might have built up on the pipes and will increase any resistance in the pipework.
If any plastic pipework or joint fittings are used, then the earth bonding should by-pass it and be connected to the metalwork where any valves or tap connections have been made to all appliances, this includes all points on the water and gas distribution systems.
Once you have a main earth point to one end of the main gas and water supplies from the mains electrical supply, the supplementary bonding between any gaps or breaks in the rest of pipework act in the same way as looping everything together so that any fault current can take the shortest, lowest impedance path back to main earth and cause the fitted safety devices to act in a correct manner by quickly breaking the current flow to all parts of the system that are affected by the fault.
Hope this helps answer some of your questions David. :-))
If plastic pipework (of 1m length or more) is used, then the taps/metal bath/radiators etc. should NOT be supplementary bonded. Doing so reduces electrical safety. If the internal pipework of the house is substantially metal, but with a plastic supply (i.e. MDPE), then the internal pipework should be main bonded.
Most I've seen seem to be fitted to the pipes rather than the taps themselves. If you've got soldered joints, it seems to me they could go anywhere reasonably close to the taps, since any high resistance is likely to be between the pipe and tap, given the fibre washer employed and usually some jointing compound.
I'm interested in this as I'm about to re-decorate the bathroom which has no bonding at all, but I really don't want exposed clamps and cables to the toilet cistern and bidet which have exposed pipes albeit at the back. Also, how do you bond a cast iron bath which I don't think has any provision for this? It's fully enclosed, so looks aren't a problem.
All my pipework is copper with solder joints where possible.
The implication of the IEE's advice is that if there is at least 1 metre of plastic in the supply immediately before the tap it is OK not to bond. However, it isn't really explicit about the length of insulating break required, so I'd be a little circumspect in doing it just for 30cm of insulated flexible pipe without further official advice. Perhaps a bit of the concealed fixed pipework could be converted to plastic?
Obviously, if there's any chance of the metal bath being earthed by other methods, such as nearby metal pipework that may touch it, or structural metalwork, then things change and bonding would be required anyway.
The earth clamps need to be as close as possible to the taps, in order to protect against the possibility that a section of plastic pipe might be inserted somewhere in the pipe run. Obviously it will be better if their location is hidden, yet still reasonably accessible.
To bond a cast iron bath, you should be thinking in terms of drilling holes in the feet or legs, assuming these are solidly fixed to the bath. A suitable nut and bolt will then allow an earth to be connected.
If separately mounted handles are fixed to the bath, these should also be provided with an earth wire.
Hmm, my Sime combi is certainly metal! but I can see no sign at all of an earthing lug. There is a 3-core flex entering the boiler, but having taken the boiler's cover off, I can see it dissappearing into its bowels and I'd have to half-dismantle the boiler to trace where the earth lead is wired to (possibly into the pump?) - surely that shouldn't be necessary?
Think this is probably where I was getting confused (Stefek et al!) abiut taking an earth bonding cable (10 mm2) all the way back to the fusebox from the bathroom (or indeed the kitchen, if that's where the water main comes in) ... from what you are saying above, isn't that exactly what I'd need to do if there is any sign of plastic plumbing being used (although that isn't the case here, in fact)?
Absolutely, thanks to you and the others for the advice! David
How do you prove that you have a correctly x bonded bathroom if you have an electrical inspection on your property and all the clamps and earth cables are hidden under the floor?
Copper has stood the test of time - plastic hasn't yet. And given the way many plastic things deteriorate with age, I'd be amazed if pipe doesn't do the same.
To quote from "Commentary on the IEE Wiring Regulations" (Paul Cook) "There is particular requirement that the supplementary bonding itself be carried out within the bathroom, simply that the parts themselves be supplementary bonded. Supplementary bonding maybe installed underneath floorboards, above ceilings or in an adjacent airing cupboard, as long as it is accessible for inspection"
"Use can also be made of copper pipes as the supplementary bonding conductor if the joints are electrically continuous"
There is no requirements to festoon yours tanks and boilers with supplementary bonding. Supplementary bonding only applies to "locations of increased shock risk, such as those in Part 6" (471-08-01)
However, if a generally plastic-plumbed situation, there is no need to earth bond short sections of copper used as final runs to taps, radiators etc.
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