mapj1 said:

We should not rely too heavily on any 'agreed' definition, as generally it is only agreed within a small clique who wish to use the meta-language to keep the magic to themselves.

So, I'd agree with that in terms of academia.

To that end, BS 7671 determines the symbols to be used in the standard (which generally aligns with BSI and IEC basic principles). This makes the definitions very public, and removes the 'magic' (again, that is a function of the BSI and IEC principles).

The guidance relating to BS 7671 necessarily aligns with BS 7671 for the same reason.

Why education deviates from industry practice is a key question, though. It's not just this branch of the IET's portfolio either.

So if I is current, and U is potential, does that mean that we should specify a supply as 100 imps at 230 uolts?

imps and uaolts - prefeclty tickety-boo, in the land of eletrickery the Professor Unwin way perhaps...

https://www.youtube.com/watch?v=iNEWovL90sM

one wondery tinkaro he might have accomplieshemento a  texto the OSG..

Might even have been clearer for some.

Simon Barker said:

So if I is current, and U is potential, does that mean that we should specify a supply as 100 imps at 230 uolts?

The symbol used is formulas are independent of the symbol used for the units of measurement (SI and SI-derived units).

So, for example in our 'constant acceleration' formulas, both u (velocity at time=0) and v (velocity at time=t) have units of ms^-1.

mapj1 said:

in the land of eletrickery the Professor Unwin way perhaps...

gkenyon said:

If anyone's wondering why U is used for potential difference or emf, it's because V is volume.

Thank you, Graham, that makes sense.

gkenyon said:

So, for example in our 'constant acceleration' formulas, both u (velocity at time=0) and v (velocity at time=t) have units of ms^-1.

Ah yes. As in v = u + at where a is acceleration and not amps; and t is just plain old time (in seconds). If t can stand for anything else, please let us know.

Any road, it's all on pages 46 and 47.

But only the European sparky standards  version.

The bigger one is here..
Physics Alphabet 

you will be pleased to see no doubt, that 't'  for time, as used in that most technical sentence

" surely it is 't' time ? ' is one of the few we all more or less agree on.

Let's skirt around the military and their H hour, D-Day stuff for now...

Chemists of course use many of  the letters we wanted to use for physical quantities for elements, and have to use other ones for the times and concentrations, while biologists tend to do something else entirely - consider the language of  gene sequences perhaps, - which is my earlier  point about cliques.

You may think we have a standardized language, and to some degree it is, but iff (*) you are in the know. Ideally you need to visualise current flow, not worry if current flux is being described as 'J' or 'D'

It is often useful to be aware of the other languages as sometimes they are more concise.

M.

*  iff = if and only if - in the shorthand of the mathematicians...

jbrameld said:

Does anyone support the theory that use of a separate CPC is no longer allowed, and

That wouldn’t be a bad thing if it were true!

I would suggest that the separate cpc with swa has been part of the furniture for many years. Done automatically by both consultants and contractors alike. 
My question would be why? I don’t believe there are too many circumstances where simple selection from table 54.7 would be breached and if it was, reference to the adiabatic would be the get-out. 
Agree that there may be times when a separate conductor might be unavoidable but for the vast majority of cases, it can be done without.

Coming back to the question at the top.  It all rather depends what you want to happen during a fault, and the touch voltage you are prepared to be exposed to at the far end while the fault clears.

In terms of operation of prompt ADS, there are indeed very few cases when the armour alone is not adequate. (ignoring some folks inability to connect to it in a solid and reliable manner)

But if fault currents are high, or there is a desire for a lower than normal touch voltage then firstly the armour in parallel with an internal CPC is more elegant, but cables with enough cores are not always readily available, in all sizes, so  the external  CPC version is then the next neatest solution.  The magnetics are not in your favour then, but the steel armour wires of SWA  do not form an unbroken ring of steel around the cores, but rather  a core with lots of air gaps, so it is not as bad as an unbalanced steel conduit - the problem the excess inductance is not accurately calculable in advance, nor is it constant between apparantly quite similar cable lay-ups. But again, at high enough currents the problem dissappears - a quick consideration of the currrnt needed to fully saturate the solid steel band case shows this.

(numbers for the rusty of memory - take the amps the middle - perhaps 100, and divide by the circumference around the SWA - say pi times diameter of the core bundle, so 6cm for a 2cm dia cable,  to get the H and convert to amps/metre
(in this case 100/ 0.06 ~  2000 amp -metres )    B = u0ur H. well u0 is 1.2 E-6 and uR for steels is in the range 500- 10000. But with air gaps of 10% , probably less than 50. and saturation occurs at say 1Tesla. )

M.

It all rather depends what you want to happen during a fault,

Also whether the c.pc. has to act as a bonding conductor as well - if it does then you may well be looking at a much larger protective conductor than you'd often need for the live conductors, and as cables don't usually come with an option for one enlarged core, the implication would be for a uneconomic increase in cable size. Plus for PME (or possibly railway systems where LV bonding conductors may end up carrying portions of the traction current as well) you may well want to keep the heating effect from standing currents in the bonding conductors a little further away from your live conductors.

    - Andy.