Gas pipe bond (IET vs Corgi)

[RF Lighting]

Hoorah! Someone who actually understands earthing and bonding. Rarer than rarey mcrarington from the planet rare.

 

[Malc]

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 agree. A case in point would be a shower cubical or a tap. These would be isolated or at water potential.

Those pipes are only extraneous-c-ps if they are 'liable to introduce a potential'.

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.

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 .....

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.

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

Yes I realise this although that's only really true with PME. 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.

 

[JohnW2]

Hoorah! Someone who actually understands earthing and bonding. Rarer than rarey mcrarington from the planet rare.

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"!

Kind Regards, John

 

[Malc]

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.

 

[JohnW2]

I agree. A case in point would be a shower cubical or a tap. These would be isolated or at water potential.

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 would tend to assume the worst, a cable fault under the floor which made the pipe live and hence radiator.

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.

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.

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.

Yes I realise this although that's only really true with PME.

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) ....

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.

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.

Furthermore, my installation is also TT, but it illustrates well another reason why main bonding can, in some cases, be important with TT. Although the installation is TT, the incoming water pipe has an extremely low impedance to earth, seemingly because it is bonded to my neighbour's TN-C-S ('PME') installation. I therefore have the reverse of the situation in a TN-C-S installation. Whilst, in the absence of faults, my TT electrode (hence the CPCs of my entire installation) will normally be very close to true earth potential, the incoming water pipes are effectively a TN-C-S 'earth', the potential of which could therefore be appreciably above earth potential. Again, there is therefore a definite need for main bonding, to ensure that everything within my house is 'equipotential'.

Kind Regards, John

 

[JohnW2]

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.

The regulation presumably applies primarily to water pipes, since plastic gas pipework within the house is not allowed.

However, regardless of what sort of pipe it is, bonding it on the consumer's side of an electrically-insulating section achieves absolutely nothing in terms of 'main protective bonding', and, at least in theory, unnecessarily increases hazards. I say 'in theory' since, in practice, internal metal pipework will normally be earthed anyway, whether one likes it or not, by virtue of boilers, immersion heaters etc. - in which case adding additional (albeit unnecessary) 'bonding' will at least not make things any less safe than they already are.

Kind Regards, John

 

[bernardgreen]

seemingly because it is bonded to my neighbour's TN-C-S ('PME') installation.

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.

 

[Malc]

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.

I take your point about different earths on TT.

I agree that visual inspection would be nice but is it actually a requirement? The example model form for a periodic inspection has a tick box for "connection/continuity verified". Is that both or either? You could check a bond with either a meter or giving it a tug and wiggle.

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.

 

[JohnW2]

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.

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.

I'm fairly happy with the present situation since, although as required with TT (and as a safeguard, in case the connection to next door is ever interrupted by plastic pipe), all my circuits have RCD protection, with the main bonding connected all final circuits have sufficiently low Zs for MCB-mediated ADS.

Kind Regards, John

 

[bernardgreen]

because the two premises were actually a single building.

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.

 

[JohnW2]

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.

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.

I agree that visual inspection would be nice but is it actually a requirement?

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'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.

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.

Kind Regards, John

 

[JohnW2]

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.

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.

Kind Regards, John

 

[bernardgreen]

"not permitted" to connect a TN-C-S earth to a TT electrode.

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.

 

[bernardgreen]

"the water the body is standing in" would be isolated from earth.

Doubtful it would be totally isolated. 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 but a current that is too low to trip the RCD ( if fitted ) ? It is considered that 20 mA flowing through a body for a few seconds can do more lasting harm than 100 mA for less than 40 mSecs ( assured trip time of the RCD )

 

[JohnW2]

Doubtful it would be totally isolated.

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.

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....

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.

Let's face it, if the water in a 'seemingly isolated' bath which was standing on the floor had a low enough impedance path to earth to represent a significant hazard (and that only if there were something potentially live near enough to touch whilst standing in the bath), then the hazard would be more-or-less as great when standing with wet feet on the floor beside the bath - and I don't think that even you would try to 'bond' the floor!

Kind Regards, John