Earthing with mixed Plastic & Copper pipes.

[xeonzoo]

I am re-plumbing parts of a large house in plastic pipe. However large parts of the existing copper installation will remain. The new installation has isolated long copper sections of pipework and radiators from the main earth bond. In addition to re-re-reading the regs and studying Paul Cook’s article on plastic pipes I’d like to run my conclusions past this forum.

1) Metal Water and Gas mains bonded to main earthing point at their entry to property in 10mm.
2) All non special areas with plastic pipes exclusively supplying them (even if a short metal tail is used to the radiator for aesthetic purposes) are NOT earthed.
3) Special areas (Bathrooms) all metal paperwork cross/supplemental bonded together. None of the rooms has any Class I or II equipment in them (lights, extractors etc are all SELV) and as such there is no equipment with which to supplemental bond. (In 4mm)
4) The existing runs of copper pipe have been bonded together to ensure the earthing continuity where plastic pipe has been inserted. (In 10mm)
5) The boiler, immersion heater, CH pump, shower pump etc are located in the same cupboard and have all been supplementally bonded together. (Not required I know but all that Class I in one small space and with water about during maintenance I felt it was a good idea.) (In 4mm)

I’d welcome your views on this approach. I should point out that the installation will be electrically inspected once the rewiring work (another story) has been finished– but at this stage I’d like to keep things safe and avoid rework if the inspector disagrees. The installation is TNC-S single phase.

 

[Damocles]

1) assuming services coming in are metal, then yes. If they are all plastic then very little to bond to. if they change to metal shortly within house then bond there. Though as far as I can see if there is a 1m length of plastic pipe entering the property it does not meet the definition of an extraneous conductive part and hence should not be bonded. So who expects the OSG not to be self-contradictory?

2.you mean, don't add any earthing,yes.

3. Arggg. Again, you are supposed to bond earthy conductors entering the area. This means plastic pipeing (not a conductor) entering the area need not be bonded even if it turns to metal inside. The caveat is that there must be a long enough run of plastic to ensure it is insulated. 1m should be more than enough. Others may disagree, but in principle a 1m copper pipe on the end of a long plastic section is no different to a 1m towell rail screwed to the wall.

3b. Myself, I do not use metal paperwork. Too heavy.

4. which runs of copper, where? see answer 3 for inside bathrooms, 2 for everywhere else.

5. I think you are correct, not required. However should not do any harm. i do not know if there might be any electrolytic corrosion implications of these extra earth straps everywhere, and god do they look ugly if people insist on adding them half way down your kitchen wall.

 

[xeonzoo]

Thanks for the detailed reply.

 

[Damocles]

I recommend reading other old posts on this thorny subject. Somewhere are some links to IEE stuff on bonding and posts by various manufacturers.

Whether you bond or do not bond is entirely up to you, except where it affects safety and therefore has become a regulation. The point to note is that earthing may actually make a situation less safe. This is because it takes two connections for a current to flow, and one of them is the earth.

Earthing is used because in general it is desirable that a fault will cause a short circuit (to earth) and therefore blow a fuse/breaker. Better than something staying live indefinitely. The trouble comes if someone gets in between the earth and the fault.

Anything which is isolated from earth is safe as it can not conduct a current through you. So towel rails are safe, taps on the end of plastic pipes are safe. Risks arise where there is doubt about whether something conducts. This is solved by connecting together all possibly earthy things. This means conductors entering the room. Nothing is assumed about what happens to the pipes or circuit earth wires once they go outside the bathroom. Only that they might carry electricity, but if all possible paths for electricity are connected together inside the room, it is impossible to connect yourself across any two of them at different voltages.

Safest would be to literally bond every conductor entering the room, including the live. Very safe, but pretty pointless as nothing would work. So you still have to put up with the risk that somehow you might contact a live (say smashed bulb) in one hand, and earthed pipe in the other. Thus it is desirable to minimise the number of earthed pipes/taps. Plastic plumbing would be safer.

Pipework is historically earthed by accident, being as it is made of metal and comes out of the ground. The idea of bonding at the point where the supply enters the house is really identical to the idea of bonding conductors entering a bathroom. That all conductors within the premises are at the same voltage. However, this is considered a lesser risk than in a bathroom, because being wet all over makes the risks from shock much greater. It has a secondary role of improving earthing overall where the house supplies are metal.

The most dangerous part is where metalwork is separated by small plastic sections. Metal pipe with plastic joints is a real problem in bathrooms, because you can not guarantee either that they are satisfactorily insulated or conducting.

It is very hard to get a straight answer on exactly when bonding makes things more dangerous. Health warning, do not take my opinion as the last word on this subject. Quite a few people here might disagree.

 

[brown-nought]

I agree with all the comments here. The real problem is that the jury is still out on exactly what best practice is with regards to mixed installations. If you have 100% metal or 100% plastic the guidance is relatively clear. As the designer (which is what you are) you have to make choices, but choices within the regulations. There is no black and white answer here as can be seen by the previous debates in this forum.

I think it’s worth a little background/amble to help our discussion :-

1) Metal pipe work entering a building forms a conductive route/part. You must not use metal pipe work as a source of earthing an installation but often it will act as a path to earth. A houses pipe work may be conductively linked to neighbouring installations. Both these points allow pipe work the opportunity to be at a different potential (P.d.) to the main earthing point of your installation. Without any bonding you could get a shock by touching say your kitchen tap/sink and your washing machine at the same time. By bonding all incoming conductive parts to the main earthing point you eliminate any potential difference between them. This is the reason why the regs require metal supply pipes to be bonded. [Clear a safety issue]
Plastic pipes entering a building may not offer a conductive route (I say may because you cannot necessarily guarantee how long the plastic section is unless you are familiar with its installation.)
There is no need to bond incoming plastic pipes; however you are required to bond the pipes once they become metal. I agree with Damocles on the potential contradiction in the OSG. Apart from short plastic sections the subsequent metal pipe bond does nothing to equalise external P.d’s (after all there cannot be one). It has more to do with reason (2).
2) There are two forms of electric shock we are trying to prevent. Direct and In-direct shock. Direct shock occurs when you form part of the circuit. For example you touch a bare conductor and form a circuit to earth. In-direct shock occurs when you cone in contact with a conductive surface that should not be live but due to a fault has become live/risen in P.d. For example, you washing machine case has become live and you touch it. We try to prevent direct shock risks by placing bare conductive parts out of reach and behind non conductive materials, for example the bus bar in a consumer unit should have a plastic guard over the bar. H.V power lines are placed high in the air etc. In direct contact protection is provided by EEBADS – Earth Equipotent Bonding and Automatic Disconnection of Supply. Put simply, we connect external conductive parts of an appliance etc to earth. Our washing machine fault would cause a short circuit which would result in a blown fuse. We rely on the fuse/ circuit breaker providing the automatic disconnection. In order to minimise the time the fault is present we need a high fault or short circuit current to blow the fuse. If this cannot be achieved or is doubtful then devices like RCD’s can and must be used. (Another topic I think)
If our installation has metal conductive pipe work it has a path to earth (back to the main bond). In areas of special risk (bathrooms being the main one) it is important that all conductive fault routes are tied closely together. If an electric shower developed a fault. The current would flow back to the earthing point (and beyond) down the earth conductor of its supply cable. If at the same time you were touching a towel rail which was conductively linked to earth it would also offer an alternative route. The resistance in the pipe work and the shower earth wire could create a P.d. between the two routes with you in between. You would get a shock. This would be even worse if the shower earth was lost. For this type of reason all conductive parts/routes need to be supplementally bonded together, thus equalising any P.d between them, even if under a fault condition they had a P.d. to the earthing point. This is the basis of the supplemental bonding requirements.

Now plastic pipe over 1m in length has such a high resistance that is makes a poor conductor. The current will be too low to produce a shock (don’t try it though!!) and certainly too low to blow a fuse. Therefore metal pipe supplied by plastic pipe over 1m in length will not offer a conductive route in itself back to earth and therefore cannot form part of a circuit. If you earth the metal pipe however you would introduce a route to earth and thus a potential P.d. This is clearly less safe than having no risk of a P.d due to isolation by insulation. Insulation pipe work does not need supplemental bonding.

The problem arises however when there is a risk that the metal pipe work could become live via another fault route. Two potential routes include: (a) the metal pipe work connects to a Class I device (a boiler being a good example) and under fault conditions is raised to faulty devices P.d with earth. (b) the metal pipe work becomes live due to contact with a broken cable – say a screw in a ceiling through a cable touching a pipe. In the latter case this would be a classic indirect shock risk as there would be no circuit or current flow to cause automatic disconnection until you complete it.

I’m sure you can now see the balancing act as a designer you have to carry out – it sounds to me Xeon that you are aware (if not totally sure of best practice) of this balancing act. Only you (or experts/professionals on which you call) can see your particular installation and assess the risks.

I and Damocles and for that matter everyone else on this forum can express a personal opinion but in the end that’s all they are however well or badly informed they may be.

Going back to your original list of things to do

1) Bond your metal service pipes to the main earthing point in 10mm
2) If you have a bathroom supplied only by plastic pipes, even if short metal sections are used in the room you do not need to supplemental bond it. However check on the location of the metal pipe work that it is not at risk of becoming live from another source – if there is a risk it should be bonded, bridged or better still relocated/ changed for plastic.
3) If a conductive route enters the special zone it should be supplementally bonded only to the Class I or II equipment adjacent and not to the nonconductive pipes.
4) Bridging to electrically isolated metal pipe isn’t covered in the regs and is really a design decision. If the pipe work is away from all potential fault sources it could be left isolated. If however it passes close to potential fault sources (say mains halogen lamps in the ceiling below) it may be safer to bond it to earth rather than risk the indirect shock risk.
5) Cross bonding the boiler plumbing sounds a good idea if you have mixed conductive/non conductive pipes and pumps etc – there is no requirement however.

If I were you I would re-assess your design decisions, run them past the inspector – he may disagree with me totally. He will want to see the main bonds are correct and accessible to inspection and that bathrooms has suitable supplemental bonding.

My basic philosophy is that non conductive isolation is better provided it does not create/increase a risk that a non earthed conductive surface could become live under an extraneous fault condition.

 

[Damocles]

The worst situation of all is where you are put in a position of having to decide the best thing to do. Then, someone else comes along with their own ideas and tells you it's all wrong. Don't you just love freedom of choice?

Generally people accept over-bonding. It becomes an issue if you reckon that extra bonding might actually make things less safe.

Actually, I have no idea how important all this is. The risk may be evident, but sometimes evident risks are less dangerous then ones people do not notice. How many cases are there each year where bathrom bonding comes into play?