Extraneous conductive parts

but wouldn’t anyone doing that plumbing be obliged to consider this risk at that time?

Unfortunately not - UK plumbers traditonally haven't been trained to know much about electricity - even the 'safety electrical connection - do not remove' label appears to confuse them on ocasions. In certain circumstances where there are strict procedures (like some particularly stringent work places) you might be able to get it written into site procedures that metallic plumbing shouldn't be replaced by plastic without consideration of the bonding requirements, but as a general rule - certainly for ordinary domestics - we can't rely on plumbing maintaining its continuity.

Why would the copper pipes need to be main bonded one to another close to the bathroom (in an accessible place for testing, like in an airing cupboard)?

Normally you wouldn't - main bonding would usually only be applied around the perimeter of an installation to deal with anything that might introduce a potential into the installation. Bonding for a bathroom would normally be supplementary bonding (again bonding everything that would introduce a potential into the bathroom). The only situation I can think of where you'd need to main bond internal pipework for a bathroom is where you're trying to omit supplementary bonding but the condition for any extraneous-conductive-parts entering the bathroom being main bonded hasn't been satisfied. That condition is there to stop faults from elsewhere in the installation entering the bathroom via the metallic pipework - say from an immersion heater in an adjacent bedroom cupboard (as was the fashion a few decades ago). Often (but perhaps not always) it would be easier and cheaper just to supplementary bond within the bathroom rather than a run main bonding conductors up to the bathroom from the MET.


   - Andy.

but wouldn’t anyone doing that plumbing be obliged to consider this risk at that time?

Unfortunately not - UK plumbers traditonally haven't been trained to know much about electricity - even the 'safety electrical connection - do not remove' label appears to confuse them on ocasions. In certain circumstances where there are strict procedures (like some particularly stringent work places) you might be able to get it written into site procedures that metallic plumbing shouldn't be replaced by plastic without consideration of the bonding requirements, but as a general rule - certainly for ordinary domestics - we can't rely on plumbing maintaining its continuity.

Why would the copper pipes need to be main bonded one to another close to the bathroom (in an accessible place for testing, like in an airing cupboard)?

Normally you wouldn't - main bonding would usually only be applied around the perimeter of an installation to deal with anything that might introduce a potential into the installation. Bonding for a bathroom would normally be supplementary bonding (again bonding everything that would introduce a potential into the bathroom). The only situation I can think of where you'd need to main bond internal pipework for a bathroom is where you're trying to omit supplementary bonding but the condition for any extraneous-conductive-parts entering the bathroom being main bonded hasn't been satisfied. That condition is there to stop faults from elsewhere in the installation entering the bathroom via the metallic pipework - say from an immersion heater in an adjacent bedroom cupboard (as was the fashion a few decades ago). Often (but perhaps not always) it would be easier and cheaper just to supplementary bond within the bathroom rather than a run main bonding conductors up to the bathroom from the MET.


   - Andy.