if one of the conductors(say line  L1)  in the group had insulation failure, the other conductors (of lines L2 and L3) will be exposed to voltage between the conductor and its surrounding medium which is  denoted by U0 of 400V rather than 230V.

True, but the same is equally true for a single 3-phase circuit - generally if the insulation is damaged then all bets are off (and let section 434 deal with any consequences). I can't see a physical reason for grouped cables being any different if they're all part of the same 230/400V system.

I suspect the intend of 521.6 is to deal with mixed voltage systems - e.g. 12V run alongside 230/400V - especially for unsheathed singles in a shared conduit or inside the same multicore cable - in which case you'd want the 12V circuit to have 230/400V insulation, rather than 12V.

So I reckon that when evaluating "the highest voltage present" you can still treat between-conductors and between-conductor-and-earth separately.

   - Andy.

AJJewsbury said:

(and let section 434 deal with any consequences)

The  idea that I suppose there is difference between conductors of same circuit and conductors of different circuit is that section 434 states in its introduction " This section only considers the case of a fault between conductors belonging to the same circuit". So it's a bit confusing here.
 

This section only considers the case of a fault between conductors belonging to the same circuit

Indeed. One of the many "gaps" in BS 7671 (and probably IEC 60364 too). Little protection from small overloads of long duration, the underlying theory of shock protection that's probably not safe for around 5% of the population, 5s disconnection times for larger circuits which is far far in excess of any safe duration for shock protection, nothing to prevent short thin c.p.c.s carrying excess current from faults on larger circuits where exposed-conductive-parts are bridged together by extraneous-conductive-parts. Nothing to prevent end users plugging in large loads in a way to unbalance a ring final circuit and so overload the short leg. The list goes on... Mostly mitigated by the low probability of these things happening in practice and usually tolerable damage, rather than any mathematical or logical certainty.

Likewise no precautions are demanded for faults between live parts of one circuit and c.p.c.s of another - which would be reasonably plausible in situations like consumer units where conductors of different circuits cross over each other.

Don't expect any wiring regs to be entirely thorough, complete, consistent or logical.  They might have been once, but have spent over 100 years being quite literally "amended by committee" (or probably more accurately in recent years a hierarchy of several diverse committees), so despite the undoubted best efforts of all those involved, perfection is yet to be achieved.

   - Andy.

,,,, a hierarchy of several diverse committees), so despite the undoubted best efforts of all those involved, perfection is yet to be achieved.

That is a rather sad but very accurate description of the problem - In many ways regs are  almost an example of something that can either be incomplete or incorrect, but never both correct and complete at the same time as there are always awkward corner cases.

Mike.

well noted thank you AJJewsbury 

mapj1 I agree with you

Thafer Ali said:

This section only considers the case of a fault between conductors belonging to the same circuit".

Or, between conductors and Earth.

This is not so problematic in general ... BS 7671 (and IEC 60364) only considers single fault conditions wherein a fault between two lines requires there to be two breaches of basic insulation.

The requirements for appropriate mechanical protection (or, 522.8.10, an earthed armour etc.) help to redress this.

AJJewsbury said:

One of the many "gaps" in BS 7671 (and probably IEC 60364 too).

Given the above, is this really a "gap"?