Please have a quick look at 411.3.1.2 and note the words "within a building".
If you're standing on concrete on earth, and you touch a post that is sunk into the same concrete/earth, the post isn't extraneous.
I really don't think extraneous conductive parts should be part of your thinking at all here.
The closest clue to this in the regs is maybe in section 714 which explicitly states that metal structures (inc. fences) near an outside lighting installation don't need bonding.
What you might have is simultaneously accessible exposed conductive parts which in certain very specific circumstances (like a bathroom) may require supplementary bonding. But AFAIK only if any of this stuff forms part of a special location and the relevant section 7xx requires that additional protection is in place. (In which case 415.2 would apply, and you then indeed need to satisfy the formula in 415.2.2)
At least, that's my take on it from what you've said.
Hello timhoward
Hummmmmmm this is getting quite deep now, and interesting, we may never get to the bottom of it but lets give it a go
Had trawl through the book and came up with this.
411.3.1.2 does reference '' Within a Building'' it however doesn't reference outside of the building. 411.3.1.1 Protective earthing, references Simultaneously accessible exposed conductive parts shall be connected to the same earthing system individually, in groups or collectively.
Simultaneously accessible parts. Conductors or conductive parts which can be touched simultaneously by a person
or, in locations specifically intended for them, by livestock.
NOTE: Simultaneously accessible parts may be: live parts, exposed-conductive-parts, extraneous-conductive-parts, protective
conductors or earth electrodes.
415.2.1 Supplementary protective equipotential bonding shall include all simultaneously accessible
exposed conductive parts of fixed equipment and extraneous conductive parts including where practicable the main metallic reinforcement of constructional reinforced concrete. The supplementary protective equipotential bonding system shall be connected to the protective conductors of all equipment including those of socket-outlets.
417.3.1 Simultaneously accessible parts at different potentials shall not be within arm’s reach. A bare live part other than an overhead line shall not be within arm’s reach or within 2.50 m of the following:
An exposed-conductive-part
An extraneous-conductive-part
A bare live part of any other circuit.
418.2.2
Protective bonding conductors shall interconnect every simultaneously accessible exposed conductive-part and extraneous-conductive-part.
714 is a special location covering, Outdoor lighting installations, highway power supplies and street furniture?. Could the reasoning for metal structures (inc. fences) near an outside lighting installation don't need bonding be due to 7614.411.3.4 sating that lighting that is accessible to the public shall have additional protection by an RCD having the characteristics specified in 415.1.1.