The above discussion almost but not quite addresses my own question. What is the minimum CSA I need if a sub-main of length < 1m is to be protected by my existing cut-out fuse - I assume to BS88 - and confirmed by the DNO to be rated at 80A?

Looking at the gG tables above, the 5s disconnection time for this fuse is achieved with a current of 430A. Presumably that is worst case, various manufacturer's data gives other results e.g. mersen give 300A.

The graphical method, looking at where the adiabatic curves cross the fuse characteristics, then shows for this "time to blow" I need somewhat under 10 sq mm conductors. Is that correct?

Also, can someone please explain where the figure for Zs of 0.27 came from in the original calcs above? The tables I have found e.g. these https://www.dungannonelectrical.co.uk/dun1-shop/pdf/loop-impedance.pdf show for 5 seconds a value of 0.425 ohms for the original 100A fuse and 0.525 ohms for my 80A case leading to rather different results. And why is a figure of 1 second then chosen (seemingly arbitrarily) to put in the adiabatic equation?

Hoping someone can enlighten me, I am more at home with electronics!

Neither main meter tails nor a final circuit. Original Q was "What is the minimum CSA I need if a sub-main of length < 1m is to be protected by my existing cut-out fuse". It is to be teed off the load side busbars of item 5 below. From the above discussion it would seem that 3 x 4.0 sq mm is necessary but sufficient.

1. 80A DNO fuse
2. Meter
3. 600mm stone wall
4. REC isolator
5. 4-module  enclosure with SPD and multifunction meter
6. Main CU (via changeover switch to/from ESSS)

Chris Coffin said:

The graphical method, looking at where the adiabatic curves cross the fuse characteristics, then shows for this "time to blow" I need somewhat under 10 sq mm conductors. Is that correct?

Yes BUT depends on the actual prospective fault current at the origin. You may be able to lower the rating by using a non-adiabatic plot for the particular cable or wiring system arrangement.

Chris Coffin said:

Looking at the gG tables above, the 5s disconnection time for this fuse is achieved with a current of 430A. Presumably that is worst case, various manufacturer's data gives other results e.g. mersen give 300A.

Yes, it's based on the requirements of the standard, or "worst case seen from certain manufacturers" - meaning in the real world, things may get "over-engineered".

AJJewsbury said:

(whether in a new installation or as an addition to an existing one)

That's also an interesting discussion ... and I would say not always true of existing installations.

To cite an extreme example, if one was asked to provide an additional socket-outlet in premises with lighting protection system to BS 6651, and there were no SPDs, it would not be practicable to re-assess the whole installation against BS EN 62305 and provide SPDs, in order to simply fit a socket-outlet (regardless of whether the premises was of a type that would now require SPDs under 443.4.1). Luckily, we are talking about Construction Work, as defined by CDM Regulations, so the Client, or someone appointed by them as Principal Designer, would (for most of those types of premises) be involved in the discussion about what should and should not be done, and any relevant risk assessments, and the LPS/SPD issue would be one of ongoing review and planning for future works at an appropriate time.

Neither main meter tails nor a final circuit. Original Q was "What is the minimum CSA I need if a sub-main of length < 1m is to be protected by my existing cut-out fuse".

Whatever you want to call it, BS 7671 434.3 would apply wherever you have a conductor that's not protected by any of the installation's protective devices but relies instead on the distributor's devices.

   - Andy.

Implying what?

Chris Coffin said:

Implying what?

I would suspect that AJJewsbury  is considering specifically indent (iv) of Regulation 434.3.

The implications being, if you are relying on the DNO cutout:

1. You have to comply with the distributor's requirements (which may require a minimum csa) ... otherwise they may not agree "that such a device affords protection to the part of the installation between the origin and the main distribution point of the installation where further protection against fault current is provided"; and
2. The wiring must be selected and erected to minimize the risks of faults, fire and danger to persons.

All too difficult. Am now considering  supplying the HP from the existing main distribution board - which might overload my Victron Multiplus inverter in a power cut - or to avoid that, its AC 2 Out terminals which get disconnected on loss of mains. Which will mean in either case that ordinary 4.0 T&E can be used.

Graham's suspicion is indeed correct. The implication that if the DNO's generally ask for 25mm² but might sometimes tolerate 16mm², then 4mm² might be pushing things a bit... It's only up to the first protective device of course - after that your 4mm² will likely be fine. Could you not take it from an outgoing way in the 4-module enclosure (perhaps with a 'width upgrade') - in effect making it a 'non essential' DB?

   - Andy.

No, it is full (2 modules of SPD and 2 modules of Estron export/import meter. No space to expand it.

Every time I think I have got to the answer someone pulls another consideration out of the woodwork citing some new arcane aspect of the regs.

Equally I am not sure it is helpful for the OSG to say 3 x 4.0 is (just) OK on an 80 A fuse if it is subject to DNO approval anyway.

Using my professional judgement I would say the likelihood of <1 m of cable surface-mounted >1.2 m from the floor in a little-trafficked larder area getting damaged to the point of blowing the 80A fuse is very small, and the likelihood of a short circuit overheating the cable and not blowing the fuse even less. In any case it does not matter if the cable suffers thermal damage because the repair would be to replace the entire short length.

So I am grateful for everyone's contributions but equally I am happy it has led me to an entirely different solution to the underlying problem!

Equally I am not sure it is helpful for the OSG to say 3 x 4.0 is (just) OK on an 80 A fuse if it is subject to DNO approval anyway.

To be fair what the OSG says would be right in 99.9% of cases (well 100% of cases for circuits within the installation itself) - it's just because you're depending on a device that's outside of the BS 7671 installation and under someone else's control (which I suspect the OSG deals with explicitly elsewhere - possibly under the heading of meter tails, although as already discussed that's probably not a helpful description in this unusual case). The DNOs always want the right to change things - the rating or indeed type of fuse (or maybe even some kind of circuit breaker in the future, as is done in many other parts of the world), or reconfigure their network (e.g. feed your bit of road from a different substation as loads increase) which may well alter Ze - and they have a responsibility to ensure (AFARP) that their changes don't undermine safety. That's a lot easier to do if they know that everyone has (say) <3m of 16 or 25mm² - one set of worst-case calculations back at the office would cover everyone - life would get a lot harder for them if they had to account for odd bits of smaller conductors hidden all over the place. The DNOs have already changed the type of fuses they use - from BS 1361 to BS 88-3 - and their characteristics aren't entirely identical (even some of the ratings differ) - in a few edge cases what just scraped though with a BS 1361 fuse could be just a fail with a BS 88-3 one. It's in the DNO's interest to design-out the awkward cases before they happen.

Every time I think I have got to the answer someone pulls another consideration out of the woodwork citing some new arcane aspect of the regs.

It's not that new - the current form of words seems to go back at least as far as the beginning of the 17th Ed (2008) and the general gist probably goes back even further.

  - Andy.

Equally I am not sure it is helpful for the OSG to say 3 x 4.0 is (just) OK on an 80 A fuse if it is subject to DNO approval anyway.

To be fair what the OSG says would be right in 99.9% of cases (well 100% of cases for circuits within the installation itself) - it's just because you're depending on a device that's outside of the BS 7671 installation and under someone else's control (which I suspect the OSG deals with explicitly elsewhere - possibly under the heading of meter tails, although as already discussed that's probably not a helpful description in this unusual case). The DNOs always want the right to change things - the rating or indeed type of fuse (or maybe even some kind of circuit breaker in the future, as is done in many other parts of the world), or reconfigure their network (e.g. feed your bit of road from a different substation as loads increase) which may well alter Ze - and they have a responsibility to ensure (AFARP) that their changes don't undermine safety. That's a lot easier to do if they know that everyone has (say) <3m of 16 or 25mm² - one set of worst-case calculations back at the office would cover everyone - life would get a lot harder for them if they had to account for odd bits of smaller conductors hidden all over the place. The DNOs have already changed the type of fuses they use - from BS 1361 to BS 88-3 - and their characteristics aren't entirely identical (even some of the ratings differ) - in a few edge cases what just scraped though with a BS 1361 fuse could be just a fail with a BS 88-3 one. It's in the DNO's interest to design-out the awkward cases before they happen.

Every time I think I have got to the answer someone pulls another consideration out of the woodwork citing some new arcane aspect of the regs.

It's not that new - the current form of words seems to go back at least as far as the beginning of the 17th Ed (2008) and the general gist probably goes back even further.

  - Andy.

Thanks. Perhaps instead of writing "citing ... new..." I should have written "newly citing..."

But as I no longer draft contract terms for a living perhaps I am excused for once.