Bonding Extraneous Conductive Parts

[johnduffell]

How does that work if for instance the fault current comes into the location via the pipework?

I think what you're meaning is a situation where there's some kind of conductive item, completely within the equipotential zone (i.e. not extraneous) and not an exposed conductive part. This means that it is unrelated to the electrical installation.

Since all live conductors must be double insulated or protected by earthed metalwork, there should be no touchable live conductors in a single fault situation (for more than the relevant disconnection time) This is why metal backboxes would be earthed as someone may thread a copper pipe down the stud wall or similar.

Of course in a double fault situation, *any* metal in the area is a hazard as it increases the chance of touching something that's live. However double faults are not considered and this is sometimes known as the "bonding teaspoons" mindset and is not necessary under bs7671.

Hope that essay makes some kind of sense!