Metal street furniture without an electrical supply: can it be omitted from a simultaneous-contact assessment?

These sort of things are usually considered extraneous conductive parts, and if it was say an indoor car park, with clear boundaries then they would/ should  be bonded to the MET if simultaneous contact between them and something earthed via the mains was credible. It often is not credible for things like brackets for metal drain pipes and odd bollards, and these could safely  be left electrically 'free floating'. 
Now I think what you are looking at is more of an outside situation where escaping the true earth potential is more or less impossible, as even the act of stepping into a puddle  connects the person to terra-firma. (one reason for the gravel at substations, so there are no puddles.)

My suspicion is that reality and the 'pure' advice often diverge here, and in theory it should all be bonded, but in practice it really cannot always be, especially where the lead plus the length of the car extends the 'zone' in some rather odd shapes, or ownership of the bits of metal mean it is not yours to touch.

We have one of the authors of the guidance (Graham) who frequents this forum, and I would not be surprised if he  adds some additional comments shortly.


Mike,

Thanks, Mike, appreciate your thoughts. I’m mainly looking at metal signposts, bollards and post boxes embedded in the ground, with no electrical equipment or connection to an electrical earthing system.

I read Graham Kenyon’s IET article on cross-pavement EV charging. My understanding is that most outdoor extraneous-conductive-parts can be omitted from the simultaneous-contact assessment, with special consideration for items such as HV fences and metalwork connected to electrical equipment.

I’m hoping to clarify whether that applies to our on-street installations supplied from a dedicated feeder pillar. I appreciate the site-specific considerations, but we need clear criteria for when these items within 2.5 m of a charging vehicle can be omitted and when they must be included.

Sam Kindred said:

Can metal signposts, post boxes and bollards embedded in the ground within 2.5 m of a charging vehicle be omitted from the simultaneous-contact assessment, assuming no electrical equipment or connection to another electrical earthing system, even where they may introduce true-Earth potential?

Quite simply, yes.

Reasoning (italicised terms using the definitions in BS 7671):

These might be extraneous-conductive-parts, but they have no exposed-conductive-parts.

The 'simultaneous contact' issue of Regulation 411.3.1.1 applies to simultaneously accessible exposed-conductive-parts rather than extraneous-conductive-parts.

Further, from BS 7671:2008 to present, there is no longer a need to provide 'main' protective equipotential bonding to extraneous-conductive-parts outside buildings (although an indoor car park is an example of being within a building). Having said that,  supplementary local protective bonding, if and where specified, would extend to them from simultaneously accessible exposed-conductive-parts even outside buildings

Thanks, Graham, really appreciate you clearing that up. This has been a bit of a thorn in my side for a while.

What happens for crash barriers and other galvanised metal items, and those with stainless steel bolts and the like. It does appear to be a can of worms.

The well painted postbox and and the plastic sheathed sign posts do fit the no exposed-conductive-parts" rule, along with coated cabinets etc. 

Probably needs that extra bit of careful on-site observation for the unexpected...

I suspect there's still scope for confusion for things like steel lighting columns - are they exposed- or extraneous-conductive-parts? Modern setups use sheathed cables and enclosed equipment internally so the column itself isn't required to carry fault current in the way an exposed-conductive-part would, yet it's solidly bolted to the lighting "head" - normally Class 1 - so would tend to be raised to elevated voltages during faults etc in a similar way to an exposed-conductive-part.  I think by traditional definitions the column itself would be classed as an extraneous-conductive-part, but I can see an argument that it's more like an extension of an exposed-conductive-part. (But the same could be said of many bonded extraneous-conductive-parts - outside taps for example.)

Are we drifting towards two different kinds of extraneous-conductive-parts? One that can import any kind of voltage (Earth or from faults from elsewhere) and another which can import only a local Earth potential (and therefore carries less risk where nearby exposed-conductive-parts have precautions (e.g. open-PEN devices) to limit their variation from true earth (but not from other imported potentials).

  - Andy.

Philip Oakley said:

The well painted postbox and and the plastic sheathed sign posts do fit the no exposed-conductive-parts" rule, along with coated cabinets etc. 

Until you insert a bolt or screw.

Philip Oakley said:

The well painted postbox and and the plastic sheathed sign posts do fit the no exposed-conductive-parts" rule, along with coated cabinets etc. 

or an extraneous-conductive-part?

  - Andy.