Earthing and bonding

Hi All,

With the advent of Plastic incoming services and plastic internal services the need for main earth bonding of the Gas meter has become redundant under the BS7671.

However most internal gas piping is either copper or flexible steel with plastic sheath.

The question I'm going to ask is how does the gas meter work, does it have moving parts and does it have continuity across the meter?

The reason for asking this question is because we are not bonding this meter, could there eventually be a build-up of static energy that can't be dissipated or when it does dissipate creates a spark.

I don't know a lot about Gas, but I do know it contains a percentage of water and other minerals, I also do know that to create and explosion you need three elements, which one should not be present in the gas meter (air).

However in practice because of leak's and water there is that possibility of a percentage of air been present, so the three element can be present at a gas meter.

In my case - external incoming services are plastic (water/Gas), the waterpipe is copper to the boiler, boiler pipework is plastic and the internal gas pipe to boiler is a flexible steel with plastic sheath pipe, which we are going to test to see if we have continuity between the gas boiler and the gas meter. If there is continuity from the boiler to the gas meter then it's a simple matter of cross bonding at the boiler with a suitable label and a note on the test certificates will suffice.

(As a additional note the Electrical Contractor has refused to undertake the earth test and to cross bond stating that because the incoming services are plastic they don't need to earth/cross-bond the installation!)

Can someone from the BS7671 team please review this, as I think we might need to earth gas meters, especially moving forward with smart meters etc.

If I've posted this wrongly, please advise as it's the first time posting something for a long time. 

Regards John

  • That is not the purpose of Bonding or Earthing.

    .

    Bonding is to equalise potential difference between various earthed parts.

    Earthing is to cause automatic disconnection in the event of a fault and a metal part becoming live.

    .

    If the gas meter is not an extraneous-conductive-part then it should not be bonded.

    If the gas meter is not an exposed-conductive-part then it should not be earthed.

    .

    So-called cross-bonding beneath the boiler has never been a requirement.

  • Gas pipe is metallic so that nobody burns a hole in it. If the supply is in plastic, there is nothing to bond any more than a picture chain, fireguard, or plug chain.

    I don't have a smart gas meter, but I imagine that they contain a battery. Either that or the gas turns a tiny wee dynamo.

    If you have air in your gas pipes, you have a big problem. What do you think will happen when the burner ignites next time?

  • I support  's view that main protective bonding in BS 7671 is not used to control static, but to help limit touch-voltage in the event of a fault, purely because the extraneous-conductive-parts (gas pipe in this case) is in contact with the Earth.

    In fact, on the other hand, it's worth noting that a proportion of load currents may well flow between metal external gas pipes in many installations (i.e. those having a TN-C-S, or PME, distributor's earthing arrangement), and this can increase to full load current under some conditions of fault in the electricity distribution cables in the street ... which is itself a possible hazard on gas systems.

    Bonding at the boiler is not specifically a requirement of BS 7671 for every installation, but could be a requirement of boiler manufacturer's instructions, or gas installation standards - that is a matter for the registered gas installer rather than the electrician. However, I'm led to believe (although I will say here for completeness I'm not a gas installer) that it's quite probable that the gas pipes at the boiler are electrically connected (via metalwork in the boiler) to the mains protective conductor, and connected back to the installation main earthing terminal that way - which would mean any static in the gas system would be discharged through the boiler protective earthing anyway ?

  • However, I'm led to believe (although I will say here for completeness I'm not a gas installer) that it's quite probable that the gas pipes at the boiler are electrically connected (via metalwork in the boiler) to the mains protective conductor

    Easy enough to check!

    With modern materials, I wouldn't be entirely surprised to find a plastic (or at least some sort of ceramic) boiler.

  • However in practice because of leak's and water there is that possibility of a percentage of air been present, so the three element can be present at a gas meter.

    I would have thought a build-up of static on internal pipework would be pretty unlikely - the pipework will be in contact with building materials which will be far from ideal insulators (unlike say a nylon carpet) - and in many cases, as Graham says, the gas pipework will often be Earthed via the c.p.c. of gas appliances. Static from moving parts within the meter I hope would have been designed out by the meter manufacturers.

    Then igniting gas within pipework is surprisingly difficult. Have you ever watched a gas fitter doing soldered joints? Turn the gas off and apply a blow torch! No purging of gas first. You actually need a quite tight range of fuel-air mix to cause an explosion - above or below that nothing much happens.

       - Andy.

  • What is the purpose of the "cross bonding"? What would the "suitable label" say about your bonding?

  • Cross bonding at boilers is something that plumbers tend to do.  They want to ensure that all pipes are at the same potential, so there are no nasty surprises when working on the boiler.  That includes the gas pipe on gas boilers.

  • Cross bonding

    And I know I needn't point out to most contributors to this Forum, that, according to BS 7671, there is no such thing as 'cross bonding' - 'equipotential bonding' and 'meshed bonding' are the current terms for what people have termed 'cross bonding' in the past (e.g. 'cross bond those two pipes' and 'cross bond between dis-board earth bar and racks, and racks to each adjacent rack' respectively).

  • AJJ "Have you ever watched a gas fitter doing soldered joints? Turn the gas off and apply a blow torch! No purging of gas first."

    Hopefully never. Whilst you are correct that the gas in the pipework would be difficult to ignite, the greater volume of gas in a meter is I believe considered to be a greater risk and gas fitters are required to remove them before using torches on the pipework. And required to maintain electrical continuity whilst removed with a temporary bond (jump lead). Most fitters just install a metal disk in the meter outlet.

    Worth noting that even if the supply is plastic, the gas/water metallic pipework could be run under floors and could be in contact with Terra Firma, so would need bonding anyway.

  • metallic pipework could be run under floors and could be in contact with Terra Firma

    They would then, by definition, be 'extraneous-conductive-parts' and yes would require main protective bonding.