What about an earth rod in case of of PEN failure?

A single rod trying to hold down PEN voltage for a decent sized installation to my mind comes under the same category as a chocolate fireguard. It might well work with very load load installation (e.g. a street light of a few tens of watts), but trying sink even a few tens of amps and keep things below say 50V means you'd need single figure electrode resistances, when most of the time we're luck if it comes in below a hundred Ohms. Sure extra electrodes do no harm, and if you happen to have quite a few installations with them upstream of the PEN break, the risks can certainly be reduced, but it's no panacea.

Likewise clarity on Reg 717.411.1

I'm not seeing that reg is my copy...(2022 edition, incl A2) - maybe the committee is ahead of you on that one.

   - Andy.

Les 2024 said:

What about an earth rod in case of of PEN failure? Or can this be omitted because points (i) and (ii) have been met? 

You would need such a low resistance in general (typically 3 ohms or less for domestic, sub 1-ohm in installations with more demand) that this option is impracticable, unless, for example, you've got lots of structural steel in the ground to act as that electrode. Having said that, there are sites with lots of structural steel ... but that would be in the installation to which the transportable unit is connected, not the unit itself.

However, I advise you look at BS 7671:2018+A2:2022, as Regulation 717.411.4 was extensively re-written, and there are other circumstances in which mobile and transportable units may be connected to PME without supervision:

717.411.4 TN system
A PME earthing facility shall not be used as the means of earthing for mobile and transportable units falling within the scope of this section except where one of the following applies:

(i) the installation is continuously under the supervision of a skilled person (electrically) or instructed person (electrically), competent in such work and the suitability and effectiveness of the means of earthing has been confirmed before the connection is made, or

(ii) the unit is located within, or outdoors on an upper storey or roof of, a building or structure containing the electrical installation which supplies the unit.

Les 2024 said:

Lets say I test this TNC-S supply and I am happy with the readings, is that now good to hand over?

You would need to check not only the earth fault loop impedance, but perhaps also that there are no indications of diverted neutral current that might occur during an open-PEN event. Guidance for this is provided in Appendix D to the 9th Edition of IET Guidance Note 3: Inspection and testing.

Also, a caution there is no formal "handover", technically, because the installation is to be "continuously under the supervision of a skilled person (electrically) or instructed person (electrically), competent in such work"

Les 2024 said:

Likewise clarity on Reg 717.411.1 'For a supply in accordance with 717.313(ii), automatic disconnection of supply shall be provided by means of an RCD......not exceeding 30mA". A

Regulation 717.411.1 was deleted 2 years ago, in BS 7671:2018+A2:2022. It is no longer a requirement.

Les 2024 said:

I am aware that the 3rd amendment is bringing in changes around mobile units, however I am curious to know how things stand with the current regs quoted. Really appreciate any competent responses

Really? The DPC for AMD3 is out now, and there's nothing for Section 717 ! However, as stated, there were changes to the 2018 requirements in the latest (brown) version published in 2022 (BS 7671:2018+A2:2022).

Thank-you both gkenyon and Andy for your instructive answers! And apologies for not referencing the correct edition of the regs. There is not much written on this topic in general or on this forum, so this type of discussion is certainly an asset to Electricians working with mobile units. 

As you indicate, there is a wold of difference between a lone burger van with a tatty looking bit of arctic flex trailing across a car park to a caravan socket on the side of a building or to a portable genset, depending on where it parks on different days and a full blown hundred amps plus  3 phase supply to a theatrical rig that never travels without its own gaffer, grips and runners who work as a well-oiled team who set it up and take it down at each venue.

Arguably the latter is far more 'under supervision' than the former in terms of the being someone on the team to report 'tingles' or damaged cables to who knows how to use a meter and so on, and even if there isn't they have a sparks on speed dial.

The problem is that the 'burger van man' probably has the more dangerous case, despite being a much simpler looking installation - for that one the 30mA RCD makes sense. For the big one a programmable earth fault relay ;-) and whatever the regs suggest, neither is all that likely to bring a transformer with them.

Mike

apologies I originally composed this a  few days ago , and somehow failed to post it, and the discussion has moved a bit since.

mapj1 said:

The problem is that the 'burger van man' probably has the more dangerous case, despite being a much simpler looking installation - for that one the 30mA RCD makes sense.

Additional protection by 30 mA RCD is required for all single and three-phase socket-outlets rated up to and including 32 A, that can be used by ordinary persons (electrically), so it would almost always (have) to be there (for new socket-outlet provision).
(Whilst BS 7671 is not retrospective, risk assessments for workplaces have to be reviewed when standards change, so this use-case ought to be caught for existing installs without RCD).

Of course, the question of PME vs not PME, and the capability to use a 13 A plug to 16 A single-phase socket converter (from the caravan shop), so such a vehicle or trailer can be plugged in within the installation through the office window is also a possibility that the electrician can't prevent .... but when such a vehicle is used for work, the dutyholder carries the can.