cross bonding
- Thread starter dag1167
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One thing I have been taught and realise, in the case of electricity and safety never assume anything, always stop and properly assess the situation!
It may take only a few seconds but can save a world of problems.
As I said before, I don't think it's about the words, and not really about regulations, either - I have been talking about 'BS7671 main bonding' as shgorthand for a direct cable connection back to the MET.
As far as I am concerned, the distinction is between (1) the creation of local equipotential zones (Faraday Cages, if you want), by bonding together all exposed metalwork that could be simultaneously touched, on the one hand and (2) not only doing that, but also connecting the bonded-together metalwork to the MET (and hence the DNO 'earth'), on the other hand.
Kind Regards, John.
I see the problem with 1 as how you might achieve that in some cases
Take an isolated laundry room, so the washer & dryer are connected to earth via there. say, 13A plugs into the sockets, How would you propose connecting metallic water pipes to those earths?
The intent of the regs is to turn the whole house into one Faraday cage, not lots of little ones
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... in the absence of bonding, I presume you mean? That's obviously bad news if one happens to be touching the wrong two things at the time. Mind you, the same could presumably be true with a TN system (and a failed RCD) if there were a partial L-E fault (i.e. not a dead short) such that the OPD did not operate (quickly or at all); on a bad day, it doesn't need anything like 230V to kill.
I'm somewhat reassured by the fact that my TT installation has belts-and braces RCD protection, with upstream time-delayed ones as well as the 'main' RCDs and RCBOs.
Of course, with all-metal plumbing systems, in practice there will almost always be 'bonding' of the pipework via CPCs - probably plenty good enough with a high impedance TT earth. It's when plastic pipes and fittings appear on the scene (none in my house
) that the problems arise.I'm sure it's not your intention, but you are almost selling the idea of having voltage-operated ELCBs (perhaps using a separate, distant, earth electrode) to supplement the other protective devices in a TT installation!
Kind Regards, John.
Ah, you've moved the goalposts somewhat, from pipework to what BS7671 calls 'exposed-conductice-parts'. If some of the metalwork which can be touched is part of the electrical system (e.g. the case of a washer), then one obviously has no choice but to have bonding between pipes and the electrical system (CPCs/MET).
Indeed, and that should be served by the infamous 'main bonding' from MET to where any conductive services enter the premises, theoretically rendering any other bonding superfluous - until you start talking about your conductive walls and floors. If we forget your walls and floors for a moment (since I don't think that many people consider them, at least directly), that's essentially the argument that would say that no bonding of any internal metallic pipes would be necessary if all services entered the property in plastic - but there appear to be dissenters to that view!
Kind Regards, John.
the other metal work I refer to here would include metal bodied appliances, as that is the risk that all this is about (in the main)
Duplicate
In that context, I totally agree with you. Previously, this thread seemed to be talking only about bonding pipework together and/or to the MET.
Kind Regards, John.
There seems to be little point in discussing bonding without realising why and ultimately what is being bonded together, any earthed appliance has to be part of the equation.
Not strictly true if you think of the need to supplementary bond a bathroom. That is creating a smaller equipotential zone with in the larger zone of the house.
I'm beginning to think the analogy with a Faraday Cage is misleading and rather spurious.
Surely, the only way you could compare an electrical installation with a Faraday Cage would be if you were actually inside one of the pipes.
An electrical installation is only akin to a Faraday Cage which also contains various other bits of metal some of which will be connected to it, some of which will be connected to earth, some of which neither, live cables, other cables, various pieces of equipment and parts of the structure of the cage at different potential.
In other words a house is not a Faraday Cage.
Surely, the only way you could compare an electrical installation with a Faraday Cage would be if you were actually inside one of the pipes.
An electrical installation is only akin to a Faraday Cage which also contains various other bits of metal some of which will be connected to it, some of which will be connected to earth, some of which neither, live cables, other cables, various pieces of equipment and parts of the structure of the cage at different potential.
In other words a house is not a Faraday Cage.
I suppose it's all 'belt and braces' really. If main bonding, pipework and CPCs all had zero impedance, then the entire house would be a single equipotential zone and it would be futile/unnecessary to add any additional 'local' bonding within that equipotential zone. Indeed, in practice, I think that's largely true (provided the issue of plastic pipes/fittings is satisfactorily dealt with). As far as a human being susceptible to the effects of electric shock is concerned, it is the local equipotential zone that matters, and I suppose supplementary bonding, when required, serves to make totally sure that there are very low impedance paths between things which can be touched within that local environment, just in case the 'whole house' equipotential zone is not achieving that satisfactorily.
Kind Regards, John.
Yes, I've often thought that when I see it mentioned here (usually by westie). I think westie is really using 'Faraday Cage' to simply refer to an equipotential zone.
As I see it, with a true Faraday Cage, the concept of an 'extraneous-conductive-part' which could introduce a potential (into the cage) would not really exist - because if such things existed, it wouldn't really be a Faraday Cage.
Kind Regards, John.
If you change the bit in bold to potential difference you are about there, the potential difference is what kills.
Don't forget, though, that there are such things as insulators and insulating material which is why each tiny bit of metal does not need bonding!
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