Definitely resistivity is important if we are looking at "copper equivalent".

However, I'm talking about the approx 5 % to 10 % difference in (Z₁+Z₂) over (R₁+R₂) on the "unsafe side" of max Z_s.

SWA is one case where actual Zs measurement would be preferable than using basic resistance measurements, as the measured resistances would have to be adjusted for temperature, and then put back through the PD CLC/TR 50480 formulas, to render a loop impedance.

Thanks again all - that's fairly clear from a regs perspective now although as always there is some room for interpretation in the wording and the science can rapidly provide for some complexity 'dragons'!

But the steel wires of an SWA do not really form a continuous cylinder enclosing the wires  - really there are air gaps, slots  and bunching and the magnetic effect is more like split tube with a distributed 10% gap, rather than welded conduit.

Thinking out loud is it were - most SWA armours strands seems to be 'spiralled' around the bedding - so if there's a continuous gap between any two strands, what we'd then have, in effect, might look like a current transformer, running the length of the cable - the line conductor forming the single turn primary and the armour forming a multi-turn secondary. What impact might that have on an earth fault current ? (either opposing or encouraging it depending on which way the strands are spiralled).

not that significant, at least at 50Hz, 

Again, just pondering. If a fault occurred at a peak in the a.c. cycle, the rate of change of current over time would be very great at that moment - far faster than would be expected with a simple 50Hz wave starting at a zero crossing point. Would that be similar to components of a much higher frequency - if only for a short instant? Probably not an issue if the protective device is a fuse that'll take quite a few cycles to blow anyway, but if it's a fancy circuit breaker that's trying to make the decision to trip in the first few milliseconds in order to discriminate with upstream devices, might things not quite add up as we'd hope?

   - Andy.

Hi Graham,

So if we are installing a SWA direct in the ground supplying a piece of equipment with a protective conductor current higher than 10mA, would we still be ok to run a separate cpc of not less than 10mm alongside the SWA to satisfy the requirements of 543.7.1.203 or would this need to be internal to the SWA or would this need to be 16mm (Table 54.1) as it is not provided with mechanical protection or could we just use the armour if this is sized adequately as a cpc and over 10mm.

Regards

Mark

So if we are installing a SWA direct in the ground supplying a piece of equipment with a protective conductor current higher than 10mA, would we still be ok to run a separate cpc of not less than 10mm alongside the SWA to satisfy the requirements of 543.7.1.203 or would this need to be internal to the SWA or would this need to be 16mm (Table 54.1) as it is not provided with mechanical protection or could we just use the armour if this is sized adequately as a cpc and over 10mm.

I would have thought it unlikely you'd need a duplicate c.p.c. for SWA to satisfy 543.7.1.203 - as the armour will almost certainly be >>10mm² already - the requirement is absolute c.s.a., not copper equivalent. If you did have a protective conductor buried in the ground, then yes it would need to satisfy all relevant requirements including table 54.1 (as per the last part of 543.1.1) - possibly larger if it also connects to extraneous-conductive-parts outside the building (i.e. has to serve as a main bonding conductor too).

   - Andy.

I have often wondered... who writes these regs'??? 

A committee of course (or possibly a committee of committees) - the members of the main committee - JPEL/64 - are listed on page 13 of the current regs (although I gather there are few sub-committees too).

  - Andy.

Thanks, Andy... makes sense! BTW, do you know if these 'committee' people have ever worked on a building site, ,(OR, as domestic electricians'), as sparks'? Thanks!

It does not need to spiral - the magnetic field needs to enclose the conductor, but the conductors need to be parallel (as they are in a conventional transformer.) It is this one turn transformer effect that causes the current dependent voltage drop between the ends of the AWA singles that leads us to criss-cross the armours at link boxes  1/3 and 2/3 of the way along in really high power kit.
In terms of the really fast component to the fault current (*), the load or fault impedance is  not the immediate determine factor if the fault comes on at time 't1' the source cannot 'know' to put more electrons in, until 't2' where the length of cable and the speed of light sets t1-t2.

Initially the current to voltage ratio is limited by the L to C ratio of the cable, its 'characteristic impedance' or 'surge impedance'

After that the current rises in a stepsise fashion, with a step every triple transit, reaching a final state where the current at both ends is set by the load. This is usually assumed to be after a few dozen triple transit times.

(some nice pictures here )

Mike

* here I mean short compared to the wavelength of the highest frequency h component of the waveform - approximately the radian period of  the fastest rise time ( so a  pulse with a 1usec rise time, has a highest frequency component of approx 2.pi MHz. A cable of 100m or so will exhibit triple transit times of order microsecond. A 'short' cable at 1MHz is therefore a few metres to tens of metres.
All cables, unless a DNO, are short compared to 50Hz....

Tomgunn said:

Thanks, Andy... makes sense! BTW, do you know if these 'committee' people have ever worked on a building site, ,(OR, as domestic electricians'), as sparks'? Thanks!

Well, I can't speak for everybody, but watch this from about 2:31 (minutes), which is where the link should take you: https://www.youtube.com/live/70PCl6XYjt4?feature=share&t=151