Hoorah! Someone who actually understands earthing and bonding. Rarer than rarey mcrarington from the planet rare.
I agree. A case in point would be a shower cubical or a tap. These would be isolated or at water potential.It only has to connect exposed metalwork (and exposed-c-p of electrical items) if they are extraneous-conductive-parts. To unnecessarily connect metal work to earth (in the name of bonmding), increases, rather than decreases, hazards,
I would tend to assume the worst, a cable fault under the floor which made the pipe live and hence radiator. Plastic pipe in the CH means the boiler bonding does not protect. If the radiator had plastic pipe above the floor then I'd say there was no risk. This is mainly because it is a bathroom and I'd accept that I'm probably over cautious. For a similar case not in a bathroom of metal pipe to below floor level, assuming there weren't any mains cables over the metal pipe work, I would assume no risk and no need for bonding.Those pipes are only extraneous-c-ps if they are 'liable to introduce a potential'.
The reverse really. Rather than prove from bond back to MET, prove from wire disconnected from MET to bond point to exposed section of gas pipe. That tests bond point even if it is not accessible.This is one reason why the bonding to the gas pipe needs to be accessible for inspection. AS for testing it, about the only thing one can really do is to disconnect the bonding cable from the pipe (which obviously requires that connection to be 'accessible') and measure the resistance of that cable back to the Main Earthing Terminal (MET) at/near the CU ... which I think is probably what you are going on to say .....
There is generally a requirement to bond the incoming gas pipe to the MET of the installation, regardless of other ways in which it may be connected to 'earth'.
Main bonding is not really anything to do with 'earth', per se. The whole idea is to ensure that all touchable metal (e.g. pipes) in the building are at the same potential as the protective conductors of the electrical installation - i.e. as connected to the MET at/near the CU. That potential may or may not be the potential of true earth. It is because extraneous-c-ps like incoming service pipes may try to introduce true earth potential that one needs the main bonding - to, if necessary, raise the potential of those incoming pipes to the same (above earth) potential as the MET.
Kind Regards, John
Thank you, kind Sir! I'm no authority, but I do believe that I understand the concept of (and reason for) main bonding a lot better than does the idiot who wrote the regulation which requires it to be connected to a service pipe on the consumer's side of an insulating section or insert"!Hoorah! Someone who actually understands earthing and bonding. Rarer than rarey mcrarington from the planet rare.
You keep on talking about "water potential", but I'm not really sure what you mean by it. At least in the case of metal pipework, the conductivity of the pipe is so much greater than that of the water, that one can forget the water.I agree. A case in point would be a shower cubical or a tap. These would be isolated or at water potential.
Bonding does not really attempt to address such possibilities. The incredibly rare scenario you are postulating is no more likely with a pipe that with, say, anything screwed to a wall with metal screws - and it clearly would be ridiculous (and not required) to attempt to bond ever screw etc.I would tend to assume the worst, a cable fault under the floor which made the pipe live and hence radiator.
Fair enouigh, you could also do it that way However, a satisfactory test result does not guarantee that the connection is satisfactory - the connection could be 'loose', or corroding, hence likely to become a problem, even if electrically OK at the time of testing - hence the need for the point of bonding to be accessible for inspection.The reverse really. Rather than prove from bond back to MET, prove from wire disconnected from MET to bond point to exposed section of gas pipe. That tests bond point even if it is not accessible.
It's certainly at it's potentially worst with TN-C-S (PME) but, even with TN-S and TT, the potential of the installation's 'earth' can be different from potential of extraneous-c-ps (e.g. incoming metal service pipes) ....Yes I realise this although that's only really true with PME.
For a start (and this is true of all TT installations), in the case of an L-E fault in one's installation, the potential of one's TT 'earth' can (and does) rise to a very high level above true earth - quite possibly approaching the full mains voltage. If there were unbonded extraneous-c-ps (e.g. water/gas pipes) entering the house which were at around earth potential, there would be a potentially lethal potential difference between those pipes and the exposed-conductive parts of all electrical items in teh house (e.g. metal light switches/sockets/whatever, cases of kitchen appliances etc. etc. For that reason, one could argue that the need for main bonding is therefore probably greater with TT than with other earthing systems.My new house isn't PME as the impedance is too high. It has earth spike instead (TT). Arguably they will all be the same "earth" potential anyway, even without bonding.
The regulation presumably applies primarily to water pipes, since plastic gas pipework within the house is not allowed.I think the reasoning behind "consumer's side of an insulating section or insert" is that all gas pipes inside the house will be copper. This copper pipe comes through a wall and may get an earth from the wall itself, i.e. this could be below dpc so very damp, or the wall could have rising damp.
Then there should be an isolating section section of non conducting pipe between your TT earthed pipework and the neighbour's pipe which is effectively connected to the supply Neutral. As the DNO required between my new PME supply and the adjacent existing TT after the property was divided into two separate properties.seemingly because it is bonded to my neighbour's TN-C-S ('PME') installation.
The reason your DNO requested that was presumably because the two premises were actually a single building. In my case, the neighbouring house in question is totally separate and about 35-40 metres from mine, so there really isn't a similar issue.Then there should be an isolating section section of non conducting pipe between your TT earthed pipework and the neighbour's pipe which is effectively connected to the supply Neutral. As the DNO required between my new PME supply and the adjacent existing TT after the property was divided into two separate properties.
As I recall it was because they did not want the Neutral of my supply being connected in any way to the "earth" of a TT system. There was the additional consideration that if the TT was ever converted to a PME then the Neutrals of two different street cables would be connected together via the pipe work linking the two properties.because the two premises were actually a single building.
Unless earthed by a bonded waste fitting, "the water the body is standing in" would be isolated from earth. That's one of the reasons why unnecessarily earthing (maybe in the name of 'bonding') a metal bath (or fittings) can appreciably increase hazards.By water potential I mean what voltage the water is at. Worst case is obviously wet person in shower/bath reaching for towel on towel rail. All metalwork wants to be visibly obviously isolated or at same potential as the water the body is standing in.
As I've said before there is a requirement for all "joints" in an electrical installation to be accessible for inspection, maintenance and testing, unless the joint is of an exempted type (soldered, brazed, crimped or in a non-screw-terminal 'maintenance-free' junction box) - and a I would think that applies to the 'joint' between a bonding cable and pipe as much as it does to anything else.I agree that visual inspection would be nice but is it actually a requirement?
In common sense terms, I agree. However, in regulatory terms you would be violating one regulation in order to satisfy another - so you would have to decide how sensible that is.I'm thinking that maybe I should take the bond wire to the outside box so it can be inspected even though that is outside the house so technically wrong. Practically if the copper pipe starts there it makes no difference if the bond is there or inside.
That's a bit like the mythical belief that it is "not permitted" to connect a TN-C-S earth to a TT electrode. Apart from anything else, a TT electrode is no different from a service pipe entering your house - and, of course, in a good few countries it is a requirement to have a local earth electrode with a T N-C-S supply.As I recall it was because they did not want the Neutral of my supply being connected in any way to the "earth" of a TT system.
Connecting a PME "earth" to an ultra low impedance TT earth is harmless and even beneficial in normal network conditions. It greats a hazard if the Neutral in the local network goes open circuit. In that abnormal situation the cable connecting PME MET to the TT electrode may have to carry the Neutral current from all properties downstream of the break in the network Neutral back to the substation."not permitted" to connect a TN-C-S earth to a TT electrode.
"the water the body is standing in" would be isolated from earth.
If 'totally isolated' means literally an infinite impedance, you're obviously right. However, in the absence of an earthed waste fitting (very unlikely unless there is a metal waste pipe or deliberate {unnecessary} 'bonding') the impedance of the path to earth of a bath sitting on the floor is going to be far too high to represent any hazard.Doubtful it would be totally isolated.
Exactly - and, as above, in the case of a 'seemingly isolated' bath, that impedance is virtually always going to be far too high to present a significant hazard, let alone a potentially lethal one.The question is how much current can flow through the person's body due to the lack of total isolation of the bath and the water in it. What is the impedance between bath and ground ? Is this impedance low enough for an eventually fatal current to flow through the body....
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