Difference between cross bonding and supplementary bonding?

[robgil]

Anybody know in layman's terms?

 

[MdB]

http://wiki.diyfaq.org.uk/index.php?title=Earthing_and_Bonding

This states that they are the same thing (which makes sense to me).

 

[robgil]

ah , so it is the difference between cross and main bonding that i was having trouble understanding , should be easier now , thank you.

 

[Penners]

MdB - is there any point in equipotential bonding if the bathroom water supply runs in plastic pipe? Do the regs have anything to say about this?

 

[robgil]

MdB - is there any point in equipotential bonding if the bathroom water supply runs in plastic pipe? Do the regs have anything to say about this?

This is exactly what i am wondering also Penners , the way i see it is if everything in the bathroom is bonded together then isn't this more dangerous if the earth bonding does not run the the earth in the CU?

 

[Flyfisher]

I don't think equipotential bonding can make things more dangerous, but the lack of a main earth bond would certainly be a cause for concern.

Voltage is a 'potential difference' between two conductors. If there is no potential difference, no current can flow between the conductors. Equipotential bonding ensures that all conductors are at an equal potential, thus there cannot be any voltage between any of the conductors, thus no current can flow between them.

If, for some reason, a tap becomes live then without equipotential bonding the (e.g.) towel rail can be a different voltage to the tap. Place one hand on the tap and one on the towel rail and you could receive a very nasty shock (through the chest, which is about the worst case) - especially if the towel rail happens to have a 'good' connection to earth (i.e. a low resistance path). If the tap and towel rail are bonded then both will be at the same voltage so no current (shock) will flow between them, or through your chest if you place one hand on each.

BUT.

Because there is no main bonding (plastic water pipes etc.), there still exists a shock hazard. By touching any of the bonded (and now live) conductors, current will flow through you to ground in just the same way as it would if you touched the live wire itself. The good news is that your connection to ground is not likely to be very good (especially if wearing shoes and standing on an upper storey wooden floor), so the resistance will be fairly high and therefore the current flow will be fairly small. Also, any shock current is unlikely to be passing through your chest and will be typically down one arm and a leg. If you're lucky, it will be too small to do any harm. Also, if the CU is fitted with an RCD then it will detect that current is being diverted to ground somewhere (in this case through you!) and will trip quickly enough to save your life.

So, I think equipotential bonding would still make for a safer installation even if there were no main bonding.

However, I doubt it, alone, would comply with the wiring regulations and I would guess that if you have plastic water pipes then a big earth wire back to a known good earth would be also required (but I'm not up to date with the latest regs and, besides, I'm not a qualified electrician so this is all conjecture!).

 

[MdB]

Yes, the regs do cover all of this.

The link above and Flyfishers comments sounds about right to me with the exception of the following:

However, I doubt it, alone, would comply with the wiring regulations and I would guess that if you have plastic water pipes then a big earth wire back to a known good earth would be also required (but I'm not up to date with the latest regs and, besides, I'm not a qualified electrician so this is all conjecture!).

Equipotential bonding is acceptable to the regs (when done to the regs) and it isn't necessary to bond everything metal in the bathroom if connected by plastic pipes. For example, if you have plastic pipes everywhere but a metal bath, metal taps and an electrically powered metal towel rail then it is only necessary to connect the towel rail to the earth wires of circuits entering the bathroom. If the towel rail became live for some reason, then touching the taps would be perfectly safe as there is no route to ground for the electricity. The thickness of earth wire required for earth bonding is given in the regs and depends on the type of earthing system you have.

Again, I'm not a complete expert in this and you need an expert in the 17th edition to confirm all the above.

 

[robgil]

i found this , if i am reading this right in a bathroom you need a main bond , equipotential bonding and an rcd?

http://www.diynot.com/forums/viewtopic.php?t=224735

 

[Flyfisher]

Interesting that equipotential bonding is not always required. I take your point, MdB, about a 'floating' bit of metalwork not providing a path to ground but isn't the very fact that it is 'floating' mean that it's likely to be at a different potential to another bit of metalwork that is live because of a fault? E.g. the 'floating' metal will be touching the walls, which might be damp etc etc, thus providing some path to ground - albeit not particularly good - and therefore giving rise to a potential difference and a shock hazard. But with equipotential bonding there cannot be any potential between them, so doesn't that make things safer?

Anyway, the thing is, Robgil, that these electrical works you're doing will require certification by a suitably approved person. Working from theory is all very well but I would be concerned that the installation might be electrically safe but still fail, in some way, to meet the detailed requirements of the wiring regulations. Only a suitably qualified electrician will be able to confirm everything is right.

I've done some electrical works here but I've checked first with an electrican, who I've used for other works, that what I'm doing is in accordance with the regs and then I have him come out to certify my work.

 

[robgil]

so the best thing may be to contact an electrician and see what i need to do then get h/she back out to certify it? sounds about right , electrics is one of those things that you have to get right first time round.

 

[Flyfisher]

That's the approach I've taken.

I also believe that Building Control can inspect and certify electrical work as well, but I've no experience of that route.

 

[robgil]

i am not quite sure but i think you have to pay a fee for that dont you? on my budget 'nothing' i really have to consider every cost.

 

[Penners]

Yes - last time I heard of a figure, I seem to remember that it was about £160. But don't rely on that. And I don't know how it compares with a typical electrician's certification charges.

FF and MdB. Many thanks for the explanation about the minutiae of equipotential bonding with plastic pipes. If you had a bathroom with an acrylic bath with metal taps, a porcelain washbasin with metal taps, and a loo - all supplied in plastic pipe - a radiator supplied in plastic pipe, an electric shower, zone-compliant lighting and a shaver socket - all with their own earth conductors - would you install equipotential bonding?

 

[MdB]

I would put the shower in particular on an RCD (or RCBO if that is easier) and preferably the lights / shaver socket. I am fairly confident that it is unnecessary to earth all your taps etc. I am less sure if it is necessary to suplimentary bond the earths from the shower, lights and shaving sockets. I don't think so but my guide to the 17th edition book isn't very clear on this.

 

[Flyfisher]

Re: certification: Yes, I'm sure BC would charge, but so will an electrician. I don't know which would be cheapest; it'll probably depend on the extent of the works to be checked/inspected.

Re: bonding: I don't know what the regulations actually require but in Penners' example I suspect the unbonded taps represent very little, if any, risk providing all the other electrical devices are properly earth-bonded and supplied via RCDs.

However, the good thing about all the regulations is that if the system complies, and has been certified as compliant by a qualified person, then it is deemed to be safe. That may not be any consolation to a house guest who manages to contort themself into a strange shape and use long thin metallic implements to simultaneously probe the innards of your electric shower and shaver socket, frying themself in the process, but it will be of great help during the inevitable inquest and far better than having to argue your case from first principles of electrical theory. :wink:

 

[AMc]

Re: certification: Yes, I'm sure BC would charge, but so will an electrician. I don't know which would be cheapest; it'll probably depend on the extent of the works to be checked/inspected.

One advantage of an electrician over an building inspector is that they are more likely to advise you and help you fix anything non-compliant - rather than tell you it isn't right and leave. You would need them 'on your side', perhaps you could do the setting up and pay them to make the final live connections, check and certify the lot?
I'm not sure if your local BCO have anyone who is qualified to certify electrics - when part P came in there was a lot of hoohar about the skills needed to certify electrics were more complex than many other things they deal with as a matter of course.

 

[robgil]

i might give one of my sparky mates a call , do the work to their instructions then get a local spark to certify it all.
and this is all for a temporary bathroom that will need moving into the proper bathroom in a few weeks.

 

[Flyfisher]

Beware of 'temporary' works . . . lest they become permanent fixtures! :lol:

 

[robgil]

, ms robgil seems to think its going to be very temporary , that is until she is sitting in a hot bath , then it will be 'take as long as you need rob'.
it will be nice to have the pressure off for a while though , baby due in two weeks isnt helping matters either.

 

[Penners]

MdB - yes the main consumer unit switch is a master RCD, and all circuits are on MCB's.

FF - I take your point. Better to hold your trousers up with belt, braces and baler twine.