Keir Stitt said:

And they were right to be perplexed because there's rarely a compelling argument to TT where supplementary electrodes for the TN[-C]-S won't suffice.

There are some caveats here ... and these are exemplified because of the increased reported accident stats with MEN (Australia/NZ version of PME) vs UK ... in Australia, but not New Zealand ...but in general, I agree

Keir Stitt said:

Especially now we've got plenty of options for PEN fault protection if it really is a concern.

Yes, new standard IET 01:2024 now available, which should help to address issues with unwanted operation etc.

Keir Stitt said:

Generally I'm of the school that having everything at the same potential via main and supplementary bonding which I can be sure of the integrity often by visual inspection trumps separation where to validate safety someone's got to prove the absence of conductivity and hope nothing happens between inspections.

This is, I agree, the best option if you have it ... the problem is, there are limits to the concept of 'equiopotential boding' unless you make sure it happens ... BUT even then, there's a difference making it happen at 50 Hz, vs GB frequencies ... and then when you look at the size of a site, distance amplifies resistance and inductance ...

... there is something that we call 'IGZ' ('ground window' in the US) that is a technique for dealing with that .. but believe me, it's all real. There are definitely very real limits to what we used to call the 'equipotential zone'.

The problem is that of course the earth is not flat, neither geographically, nor electrically,  So when I have my feet on the mud in one point and hold a wire, and my colleague holds the other end, and does the same, a current may well flow up one of us, along the wire and down the other, - there is a small DC for no better reason than the earth is a magnet, and we are rotating,  near built up areas there is a lot of AC from substations earthing arrangements and currents induced by nearby pylons, and everywhere there are radio frequencies  and impulsive currents from solar storms and lighting strikes around the world, and looking at the news, perhaps coming soon, though that has been said all my life, very large currents,  if there are any nuclear bombs being let off in the atmosphere...

The saving grace is that over distances tens of metres these voltages are sub-volt, except the ones near substation earth electrodes, and generally pose no issues, but on large rambling campus installations or even big farms they become worth considering properly. Then, knowing that all the voltage offsets are safely piling up in the gap across SWA armour in a box or an insulated joint in the gas main you can point to, gives you the ability to measure and to plan to limit currents in a way that accommodates the effect rather than being caught out by it.

If you don't,you risk introducing near field hazard zones around bits of metal that act as earthing of opportunity that do not have a keep out zone, that really should.

As things get bigger or large chunks of metal get planted, the argument for TT increases. Interestingly I was working alongside a Kiwi sparks at an international scout event this summer, and over the odd beer we discussed earthing among many other things, and how they do not see the neutral as a 'live conductor' in the way we do but more as an earthy thing. The no of rods we drove in (one per genset)  amused him somewhat.

Its not a simple topic, and all the methods exist for good reasons and have their place.

Mike

I’d be interested to see the official guidance that you mentioned.

My memory isn't what it used to be, but I'll try to dig it out...

I think in this case they didn’t like the aesthetic  of a box under the charger. 

Does the charger have a metallic enclosure then? The might suggest another problem ... how are they ensuring the double/reinforced insulation of the live conductors up to the RCD (or equivalent)?

   - Andy.

ha! there is some semi official guidance that gets it bady wrong - see here

https://www.voltimum.co.uk/articles/armoured-cable-supplies-outbuildings and note that the gland outside the box is accessible.

These glands could do it https://www.cmp-products.com/Installation%20Instructions/CMP%20ZEN.pdf  if you do not strip the cladding too far back

Miike

note that the gland outside the box is accessible.

I think it's meant to be covered with the shroud/boot (the close up shows dotted lines each side, presumably trying to imply a cut-away to show how things are arranged inside the boot).

  - Andy.

AJJewsbury said:

I think it's meant to be covered with the shroud/boot (the close up shows dotted lines each side, presumably trying to imply a cut-away to show how things are arranged inside the boot).

The shroud/boot can be removed 'without destruction' and is therefore not suitable for basic protection (Regulation 416.1). This is where I was going with 'cold shrink sleeving' (although appropriate heat shrink might perform similarly, it's just that when you're working on many of the sites I've worked on heat shrink needs a 'hot works' permit).

In my experience, the shroud/boot is only protection against water from certain directions (i.e. to meet IPx5).

Agree, the boot can easily be removed, A correctly sized plastic bodied gland is preferable.

M

A correctly sized plastic bodied gland is preferable.

I don't disagree at all ... (as per my earlier posts) but in terms of what the regs actually require...

The shroud/boot can be removed 'without destruction' and is therefore not suitable for basic protection (Regulation 416.1).

But where does it say that earthed parts of a different earthing system need to be covered with basic insulation? If we go down that route the sheath covering the armour isn't rated for insulation at all, so the problem then gets a lot bigger than just the gland. I thought we only needed to prevent things being simultaneously accessible - if it's covered it's not accessible. A properly fitted shroud shouldn't be easily removable, but I might suggest a cable tie around the cable to prevent it slipping to make sure. If you're wanting to make it foolproof then you'd need a way of stopping people running an extension lead from the other zone...

  - Andy.

all true - the problem is that the CPC from the wrong zone is not just a CPC, but a part introducing a potential. But if it needs basic insulation, maybe argued.

Now there are not enough accidents to worry about really but in principle there is a risk - its why we do put insulation on the  neutral which is also at the 'wrong' earthy potential.

M.

AJJewsbury said:

But where does it say that earthed parts of a different earthing system need to be covered with basic insulation?

It doesn't. However, BS 7671 requires that simulatenously accessible exposed-conductive-parts are connected to the same earthing system.

A permanent barrier (or enclosure) ... perhaps insulation ... could be used to prevent simultaneous contact, depending on the circumstances.

However, a consideration is also that we need to protect electricians going about their work. A brass gland won't do this, unless insulation (or some other form or barrier or enclosure) is used.