Main bonding, supplementary bonding

[Pretty Mouth]

A technical question:

Regulations require us to bond extraneous parts back to the MET with an appropriately sized bonding conductor. I don't dispute that, that's what the regs say so that's what I do.

In situations such as outbuildings with extraneous parts, where the distributor's earth is utilised, it can mean a long run of perhaps 10mm² copper back to the met.

What is the technical reason that supplementary bonding can't be used to achieve the same thing, ie. equal potential between exposed and extraneous parts in the outbuilding?

 

[LastManOnline]

A technical question:

Regulations require us to bond extraneous parts back to the MET with an appropriately sized bonding conductor. I don't dispute that, that's what the regs say so that's what I do.

In situations such as outbuildings with extraneous parts, where the distributor's earth is utilised, it can mean a long run of perhaps 10mm² copper back to the met.

What is the technical reason that supplementary bonding can't be used to achieve the same thing, ie. equal potential between exposed and extraneous parts in the outbuilding?

Supplementary bonding would suffice as protection between exposed and extraneous parts during a line to earth fault. But it may not suffice under an open PEN fault where the extraneous part (water pipe? ) is now carrying a shed load of current from the neighbours downstream. Where would that go if there was no main bond back to the MET.
In practice though the extraneous part is almost certainly already main bonded at the house and that being the case I would think that supplementary bonding should suffice in the outbuilding

 

[telectrix]

it also relates to the fault current in the bonding conductor back to the MET. esp. with PME a small bonding conductor may suffer thermal damage and then be unfit for continued use.

 

[pc1966]

The underlying issue is the huge and sustained fault currents under PME faults, basically you might be the neutral return for whatever imbalance there is on half a dozen properties. This is far worse than a local fault where the OCPD would clear it in 0.4/5s depending on the design.

That is why it is sometimes suggested to TT out-buildings rather than run 10mm (or SWA armour equivalent) to them. It is a trade off, keeping with TN-C-S allows OCPD based disconnection which is reliable but means large bonding conductors to any extraneous parts, where as going TT puts your life in the RCD's hands.

 

[LastManOnline]

The underlying issue is the huge and sustained fault currents under PME faults, basically you might be the neutral return for whatever imbalance there is on half a dozen properties.

If the extraneous part in an outbuilding (waterpipe) is coming directly from the house and is determined to already be main bonded, is there any reason why the installation (house and outbuilding) could not be treated as a multi occupancy building where there would be one main bond and the individual flats would require just a 2.5mm.?

 

[pc1966]

If the extraneous part in an outbuilding (waterpipe) is coming directly from the house and is determined to already be main bonded, is there any reason why the installation (house and outbuilding) could not be treated as a multi occupancy building where there would be one main bond and the individual flats would require just a 2.5mm.?

Ideally it would be. The issue is if someone changes part of it for plastic in the future but you still have 100m of buried copper out to the building that is now to "true Earth".

That is the underlying problem with not going with the regs - they are written to try and deal with all problems that might arise, some may be unlikely. But it comes down to the person not applying them to be able to stand up in court, etc, and justify the decision should there be a fault in the future. So usually you follow them as you have that posterior-covering aspect.

 

[LastManOnline]

Ideally it would be. The issue is if someone changes part of it for plastic in the future but you still have 100m of buried copper out to the building that is now to "true Earth".

That is the underlying problem with not going with the regs - they are written to try and deal with all problems that might arise, some may be unlikely. But it comes down to the person not applying them to be able to stand up in court, etc, and justify the decision should there be a fault in the future. So usually you follow them as you have that posterior-covering aspect.

That is certainly a case to consider.And perhaps that is the answer to the OP, s question. Is there anything technically wrong with using supplementary bonding in the outbuilding?. Technically no, providing the extraneous part is already main bonded at the house.
But The regs however stipulate otherwise for the reasons you point out. Future possible changes to the extraneous part.
I, m personally not convinced by those arguements though I naturally comply with the reg.

 

[Pretty Mouth]

Thanks for all the comments guys, very well explained.

Just thinking out loud: I wonder if this could be the reason for the minimum 6mm² bonding conductor, even for TT installations? Bonded extraneous parts being connected to the PME terminal at the neighbour's house, under an open PME fault, could we see that current flow through the bonding system at the TT property?

 

[pc1966]

Just thinking out loud: I wonder if this could be the reason for the minimum 6mm² bonding conductor, even for TT installations?

It is a possible reason for it. Another possible reason is simply to have a mechanically robust cable and termination even if some vibration and corrosion is present?

 

[LastManOnline]

Thanks for all the comments guys, very well explained.

Just thinking out loud: I wonder if this could be the reason for the minimum 6mm² bonding conductor, even for TT installations? Bonded extraneous parts being connected to the PME terminal at the neighbour's house, under an open PME fault, could we see that current flow through the bonding system at the TT property?

Yes. You are correct pretty mouth. When I started my apprenticeship it was in a 100% TT environment. NOW we have a 100% TNC-S environment. The larger main bonding is directly connected to the open PEN faults which we never had under the older TT arrangement.

 

[LastManOnline]

It is a possible reason for it. Another possible reason is simply to have a mechanically robust cable and termination even if some vibration and corrosion is present?

Yes. The concept of mechanical protection is a feature of most bonding and some earthing. In your regs you will see many examples where the bonding cable size is increased or decreased depending on whether mechanical protection is provided.Some of these regs contain a degree of subjectivity and can vary considerably from country to country.
Where there is no subjectivity (and neither can there be) is in regards to main bonding in TNC-S systems. They must be sized correctly for the reasons pointed out in previous posts