Do I need to do anything about single pole Isolator owned by DNO

I think this was about diverted neutral currents and potential broken PEN conductors. Very valid but isolation won't help in that case ... only removal of the main earthing conductor, but at that point it may well be a live conductor according to EAWR and you shouldn't really be working on or near it, so call out the DNO (Reg 114.1 only says the Neutral of the supply shall be deemed to be reliably connected to Earth, not that the Neutral is anything other than a live conductor, and not that the protective conductor is always considered safe to touch).

Ah yes. So the lights have gone dim and you decide to isolate at the nice shiny metal CU ... then you get a belt off it!

Personally I'd argue on that basis we ought to permit split concetric to be routed like SWA in TN-x' settings, the regs disagree....

Although in BS 7671 land the N might not be quite so reliably connected to Earth as a PEN - it could well be fed through a DP switch, a N switching RCBO or even a plug and socket - all of which are somewhat less reliable than a simple joint and even when working correctly might not offer any reassurance that N makes first and breaks last. Which then leaves the broken-PEN like problem of the N occasionally being at line voltage due to connected loads (perhaps a concern for insulation withstand for lightly covered N strands as well as from a shock perspective).

   - Andy.

Chris Pearson said:

Ah yes. So the lights have gone dim and you decide to isolate at the nice shiny metal CU ... then you get a belt off it!

But you were implying a little earlier that wouldn't happen due to bonding?

I just had another quick look on YouTube for the IET video, but cannot see it.

It prompted me to buy a single pole contact voltage indicator, I used it a few weeks ago when I did a call out for a gas contractor replacing the old metal gas mains pipework with plastic, they disconnected the old steel gas pipe and the meter became live as did everything else that should have been earthed in the house, because the old gas main was the earth electrode for the TT installation.

Do not trust either a single or double pole switch to truly isolate an electrical installation.

A double pole switch is useful when testing, I have just come back from trying unsuccessfully to diagnose a tripping 30 mA RCD in a TT installation.

We are now onto plan B and I have moved the circuit that the customer thinks is causing the problem onto a RCBO to see what, if anything, now trips one of the RCDs.