I never quite understood why when wiring an out building it is not advised to use the main property earthing . In most cases the only option is an earth spike that is a dubious alternative?

Looking at the point Andy made "not being able to create a reliable equipotential zone" ...

We no longer use this concept, and certainly from 17th Ed onwards BS 7671 recognises that there may well be no extraneous-conductive-parts. Hence, that particular "justification" is perhaps no longer relevant?

Looking at the point Andy made "not being able to create a reliable equipotential zone" ...

We no longer use this concept, and certainly from 17th Ed onwards BS 7671 recognises that there may well be no extraneous-conductive-parts. Hence, that particular "justification" is perhaps no longer relevant?

I thought the word "equipotential" still survives in BS 7671 (titles of regs 411.3.1 and 411.3.1.2 for starters). I'd certainly agree that there's no need for extraneous-conductive-parts to be present & bonded in order to achieve an equipotential zone - in many ways an all-insulating building with no metallic services is ideal - although there is still an equipotential zone between items of class I equipment via the c.p.c.s (hence the requirement for all exposed-conductive-parts within reach of each other to be connected to the same earthing system).

Where it gets messy is where there are parts that can introduce a potential but aren't practical to bond - e.g. damp concrete floors (you'd need a buried metallic grid under the entire floor area).

Granted the term "equipotential" is falling out of favour - not least because the situation isn't really one of equal potential - very often it's not entirely equal, and sometimes (e.g. during the clearance times for Earth faults) not anything like equal by perhaps hundreds of volts. Never the less the attempt is there to try to keep the voltage on accessible things as similar as we reasonably can in the circumstances. Bonding is still very much a requirement (where extraneous-conductive-parts are present) even if the method of shock protection is now just referred to as 'Automatic disconnection of supply'.

Your local DNO's rule on use of their PME terminal are usually a good read too - the likes of damp floors usually feature significantly.

   - Andy.

AJJewsbury said:

Where it gets messy is where there are parts that can introduce a potential but aren't practical to bond - e.g. damp concrete floors (you'd need a buried metallic grid under the entire floor area).

BS 7671 only really covers that in one place now - Section 705 - although it's common practice in certain cases, e.g. for a shower block for a sports club or similar supplied from PME, as it's well-known that (probably very innocent and not hazardous) tingles might result.

I agree with the other points you make, although to be honest, if DNOs are worried about damp floors, how can we not worry about situations such as EV charging?