because we use the ring-value of R2 to calculate anticipated Zs for circuit length

True, in terms of circuit length, but in terms of maximum allowable Zs, considering ADS & thermal withstand, the assumption is that 100% of the fault current flows through a single conductor - so if max measured Zs is still within limits even with a break in the ring c.p.c. -  then things should still be OK both withstand & ADS wise. Had the circuit been longer, the break in the ring c.p.c. may well have pushed measured Zs above limits - but that not the situation the OP described.

Yes 1mm² c.p.c.s on a 32A OPD are going to be an issue - but that's the same whether the ring c.p.c. is intact or not (or for a spur).

   - Andy,

because we use the ring-value of R2 to calculate anticipated Zs for circuit length

True, in terms of circuit length, but in terms of maximum allowable Zs, considering ADS & thermal withstand, the assumption is that 100% of the fault current flows through a single conductor - so if max measured Zs is still within limits even with a break in the ring c.p.c. -  then things should still be OK both withstand & ADS wise. Had the circuit been longer, the break in the ring c.p.c. may well have pushed measured Zs above limits - but that not the situation the OP described.

Yes 1mm² c.p.c.s on a 32A OPD are going to be an issue - but that's the same whether the ring c.p.c. is intact or not (or for a spur).

   - Andy,

AJJewsbury said:

so if max measured Zs is still within limits even with a break in the ring c.p.c. -  then things should still be OK both withstand & ADS wise

Perhaps best to put this in the plainest terms possible (and in doing so, I fear this may not sit well given some of the previous posts ... apologies in advance if that's the case).

If there is a break in the cpc of an RFC, in BS 7671 terms (otherwise the installation didn't comply with with BS 7671 requirements for protective conductors of RFC, see 543.2.9) there is a fault in the circuit.

BS 7671 only concerns itself with single-fault conditions, and therefore I think weigh of argument is that it's not possible to use BS 7671 alone to justify a condition as being 'satisfactory' where a fault has already occurred.

This is where the individual inspector's faith in their own competence comes in, because they have to make a decision.

Based on the guidance (e.g. BPG4 as previously referenced), we might make the decision that, because there's already a single fault, requirements for  ADS and protective conductors are not met, so C2 is the minimum justifiable code.

On the other hand, we might ignore BS 7671, and choose to make an argument as to why we think it's safe ... but that's down to the person carrying out the periodic verification individually.

I think at this point, John Peckham  would be advising us that whatever we decide, we need to be sure that we can justify that when (to completely rip off John's turn of phrase)  'gripping the rail' ?

gkenyon said:

I think at this point, John Peckham  would be advising us that whatever we decide, we need to be sure that we can justify that when (to completely rip off John's turn of phrase)  'gripping the rail' ?

I am a little surprised that nobody has mentioned Appendix 15.

The plain fact is that either the circuit is a defective ring or it is a radial which branches at the DB. It follows that one way of fixing the circuit would be to reduce the MCB to 20 A (or less). If I did that, I'd also be inclined to isolate the defective cable rather than leaving L and N in situ.