Alan can you explain where else a PD might come from, if not Ground.
One option is a cable possibly from somewhere, with a metal sheathing, similar to a BT cable.
That is covered in the regs that it MAY need bonding with permission.
Also
Do you think that a copper vent pipe poking through an external wall is An Extraneous conductive part.
Do you think a stainless steel cooker hood vent protruding through a restaurant roof is an Extraneous conductive part.
Lets be sensible for a moment!
I can give you scenarios easily enough then you decide where the difference comes from. For the moment I'll go along with the no bonding concept for the moment.
1. Plumbing
Common fault I used to go to plumbing wise was the replacement of immersion heaters in cylinders and single point of use water heaters. Often cause of failure not necessarily just actual open circuit but where the water had corroded through the sheath of the heater.
Immersion heaters are often installed with PTFE tape or lashing of gunge. The heater itself is connected to earth via its own connection but the effectivness of this is often reduced either by the fitting method or corrosion around the area caused by water corrosion over time.
These heaters may go years and never used but the corrosion still happens on the sheath normally around centre or towards ends.
Now if they fail anywher from around centre portion to neutral end the heater will continue working, ok noisily and customers would often say that. Earth conection efficiency has been reduced so the current will now flow in part via the flexible cable earth and the hopefully bonded pipework. With your no bonding concept all will try to flow down the flex but the pipework, since no path to earth, simply rises in potential to a dangerous level. PME system no RCD so how long can toy MCB, of fuse if that old, survive increased current, due to effectivly shortened element. Indefinitly?
2. Elecrtic central heating boilers - exactly same as above
3.
I have some cables running across a heating flow and return. What would you do to protect them? They are first floor joists that they running in by the way. Thanks.
Under floorboards this could go on for decades undetected cable sheath being heated and cooled many times daily. Sheath not designed for that situation so sooner or later fails leaving uninsulated wire, (went to many a jobs in 60's with VIR cables and all that was left under floorboards was lengths of naked wire! Same will happen eventually to PVC if heated and cooled too many times.) near pipe, live touches unbonded pipe, nowhere for current to flow so RCD, if fitted not activated MCB detects nothing so does nothing. All the while the pipe is at mains potential all ready for someone to touch their kettle and turn on the tap touch any bit of metal and an earthed object.
More examples of isolated metalwork at a different potential to electrical earth wanted?
Comment I posted earlier, and got a credit for. Why do we need earthing? If there was no earthing the world would be a safer place. Well apart from the fact the distribution companies cannot guarantee they could have an undetected earth fault on their network.
If one could predict what would cause a dangerous fault then somebody would design into the offending object a means of preventing it. That is not possible 100% of the time so a fail safe backup was devised - Equipotential Zone - but this can only work if all internal metalwork is bonded together hence power companies requiring it. Yes even Equipotential Zones don't make for a 100% safe enviroment but is best on offer at the moment apart from making regulations to prevent anything, non electrical being, constructed in non conductive materials. Not going to happen.
BT sheath has no requirement for bonding since it is not normally possible to physically touch the sheath plus the ends are normally sealed with heat-shrink. Important thing is it can only be touched internally if BT property is disturbed in any case externally if tou touch it where the sheath has degraded needs you to be on the same earth as the cable is burried in, or you are sufficiently insulated.
