The basic question of this thread was query about the philosophy of Section 528.1 (item i and ii ), is it for protecting neighboring circuits from a single insulation fault of a line conductor of a given circuit?

Like I said, I've seen no evidence (or even any suggestion) elsewhere that there's an intention to protect insulation of one LV conductor after the failure of insulation of a conductor of a different LV circuit.

528.1 itself is all about mixing Band I (usually ELV) and Band II (usually LV) systems in the same wiring system - so where you only had LV circuits run together, it wouldn't seem to demand any extra precautions at all. (Although other parts of 528 may apply, depending on the circumstances).

Some others here are closer to the committees' discussions and might be able to enlighten us further - although I suspect that wording goes back quite a time so the original thinking might have been lost in the mists of time by now.

   - Andy.

AJJewsbury said:

Like I said, I've seen no evidence (or even any suggestion) elsewhere that there's an intention to protect insulation of one LV conductor after the failure of insulation of a conductor of a different LV circuit.

AJJewsbury ,yes, as you previously explained your POV and it was well understood. Actually, my latest reply was just to clarify to Mr. gkenyon  my POV that just a single breach in conductor may be risky to neighboring circuits and no need for two breaches to cause fault. AJJewsbury  thank you for your thoughtful contributions to this discussion.

Unknown said:

Actually, my latest reply was just to clarify to Mr. gkenyon  my POV that just a single breach in conductor may be risky to neighboring circuits and no need for two breaches to cause fault

Could you explain how damaged insulation of one circuit becomes connected to the conductor of another circuit, with a single fault in insulation, with the present provisions of a standard?

Unknown said:

will be exposed to voltage between the conductor and its surrounding medium which is  denoted by U0 of 400V rather than 230V.

I don't think you have explained why you think that the remaining phase now has a 'voltage to Earth' (which is what U₀ is defined as) of 400 V. Are you assuming no protection has operated after the first fault, and if so, why?

Could you explain how damaged insulation of one circuit becomes connected to the conductor of another circuit, with a single fault in insulation, with the present provisions of a standard?

I think I can see the OP's point - if insulation on say L1 fails, then the insulation surrounding a L2 or L3 conductor next to it may then be exposed to 400V rather than 230V at that point - so if it's only rated for 230V to its surrounding it might in theory be then expected to fail as a direct consequence of the 1st fault (rather than due to a 2nd independent fault). Probably unlikely in practice (given most ordinary insulation is actually good for several kV - e.g. during surges or indeed routine 500V/1000V insulation testing) but I think I can concede the point in principle. I would even agree that the consequences of such a fault between different circuits might well have more serious consequences than between conductors of the same circuit (as the unprotected conductors of  the smaller circuit may well not co-ordinate with the protective devices of a larger one), but I don't see that BS 7671 demands we worry about it.

   - Andy.

 Mr. gkenyon below is an illustration of what I mean.

 Mr. gkenyon below is an illustration of what I mean.

Unknown said:

Mr. gkenyon below is an illustration of what I mean.

I'm still not seeing the problem.

Regulation 528.1 clearly says that each conductor must be insulated for the highest voltage present if there is no sheath. I believe the wording is clear for indent (i), because it follows immediately with the example of a multicore cable in indent (ii), so the following options to "share containment" for two circuits, one 240/400 three-phase, and one SELV, when using indents (i) and (ii) of 528.1 would be:

• two separate insulated and sheathed cables each rated 300/500 V, one for mains, one for the SELV.
• separate conductors in conduit or trunking rated for at least 400 V
• separate conductors in a cable that would have to be rated at least 400/400 V

Alternatively you could also use any of the options in indents (iii) to (vi).