A handy guide, but it does not seem to be displaying the tables correctly with the sizes that are a problem in red.

Using SWA as a CPC the guide for your reference. (gadsolutions.biz)

we are all struggling with algebra here ;-)

The current in the armour must go up with increasing external CPC reistance, and in the report it does.

Note that there  is a problem with this graphic, all the lower bracket is squared, and the root should extend over both the bracket and the 0.2^2

And the current in the external CPC must go up with rising armour resistance and indeed in the report it does though for reasons unclear the reactance does not appear in this one.

And these are still approximations that are only a "reasonable"  fit to 85mm cable.

Mike

Mike

Mike your equation for Ia is taken from page 26 of the ERA Report (in the format I have) which is used to produce Table 13. This differs from the final equation proposed in the ERA Report (page 29) in that the 0.7 factor becomes 1.1. This final equation has apparently been based on work in IET GN6 rather than the experimental data. The square root symbol extends as you said.

Don't ask me why!!

Regards

Geoff Blackwell

taken from page 26 of the ERA Report

Indeed it is..

Arghh but it can't need to be both rounded up and also rounded down to be a better empirical 'fit' to the same results. If I was not working silly hours at the  moment I'd try and find the time to crash it through from their reported amplitudes and time delays because (at least) one of them must be wrong.

The other secondary lesson here says something about measuring with a micrometer something that gets tested by an axe.

What should be said is that the PSSC will be large, and it will not be quite what any of these formulae say, indeed even what we really mean by RMS current for an imulse waveform where no two half-cycles are quite the same is unclear.

Nine out of ten SWAs do not need a parallel CPC, unless you employ folk who cannot use a set of spanners and do not understand where not to paint and which side to put star washers.

Mike.

I agree with most of that.

There are times that a parallel cpc can be needed such as bonding in some situations, disconnection times, etc – but, in general, they not required provided the cables are properly installed. I would add a copper connection to the earth terminal via a banjo washers as well as the other measures to ensure a good connection.

As to the micrometer and the axe, I definitely agree. We should bear in mind that cables come in discreet sizes so a calculation that indicate that an 82.345 mm² cable is required are a waste of time – a quick calculation using a simple model might lead you to a 95 mm² cable in one hit .

Regards

Geoff Blackwell

GeoffBlackwell said:

As to the micrometer and the axe, I definitely agree. We should bear in mind that cables come in discreet sizes so a calculation that indicate that an 82.345 mm² cable is required are a waste of time – a quick calculation using a simple model might lead you to a 95 mm² cable in one hit .

Yes but if you are a contractor and a simple model says the cable should be 97.65mm2 and requires you to use 120mm2, you might be grateful for the micrometer approach that confirms you can use 95. 

Graham, setting aside Fowlers statement on pedantry, I believe this is where the 60C is justified;

No you just tweak one of the parameters so the answer becomes 95!!!!!!!!!!

lyledunn said:

Graham, setting aside Fowlers statement on pedantry, I believe this is where the 60C is justified;

A little care, perhaps, is needed here ... these are tables for K for the stated values of initial and final temperature (see top of page 200, in 543.1.3, 'The values are based on the initial and final temperatures indicated in each table.'

That does not mean that every cable has those initial temperatures ... it may be a "good enough" assumption (the tables say they are "assumed" initial and final temperatures) for the adiabatic criterion, but that doesn't mean one shouldn't err on the side of caution when trying to establish whether an impedance is low enough to operate a protective device effectively?

What would the examiner expect if the cable was bunched, or installed in an underground conduit with other cables?

In the particular case we are talking about - parallel copper cpc - these are often bunched with the cable, the assumed temperatures in Table 54.2 may not be valid.

In the case of the example of CPEVCEI 5th Edition, the temperature correction factors used in the example for the armour are actually stated (in another table) so the reader doesn't have to work it out for themselves.

Using SWA as a CPC the guide for your reference. (gadsolutions.biz)

"The use of Jubilee clips to terminate the armour is an absolute bodge"

But what about lighting column cut-outs? https://www.lucyzodion.com/product/thm0028975-titan-single-fuse-cut-out/
Granted it includes a brass tube to support the inside of the armour, but presumably have proved to be satisfactory.

   - Andy.

Graham, yes, I hadnt thought of it that way and had just slavishly followed Chief Examiner comments (and some of the older texts like BD Jenkins who shows calcs to 60 for the wire armour, albeit as an average of initial and final temps). It all becomes  unnecessarily arcane when one considers the current practice of using 218.5v that used to be 240v then 230v to calculate Zs not to mention the uncertainty of the value of Ze. I believe GB and Mike are probably spot on with that micrometer and axe analogy. 

I just spotted a howler in my own Excel calcs regarding some swa values. I have been extracting data from this table from Paul Cooks wonderful commentary on the 17th. 

I couldnt understand why my calculations using old fashioned approach of separate r and x value was just so far apart from the one in the CPEVCEI 5th Edition until I twigged the r value for the 70C swa was significantly different to the value for 90C thermosetting given in IETEIDG. One is 3.9 and the latter 6. There must be significantly less csa of wire armour in the 90C cable?

Graham, yes, I hadnt thought of it that way and had just slavishly followed Chief Examiner comments (and some of the older texts like BD Jenkins who shows calcs to 60 for the wire armour, albeit as an average of initial and final temps). It all becomes  unnecessarily arcane when one considers the current practice of using 218.5v that used to be 240v then 230v to calculate Zs not to mention the uncertainty of the value of Ze. I believe GB and Mike are probably spot on with that micrometer and axe analogy. 

I just spotted a howler in my own Excel calcs regarding some swa values. I have been extracting data from this table from Paul Cooks wonderful commentary on the 17th. 

I couldnt understand why my calculations using old fashioned approach of separate r and x value was just so far apart from the one in the CPEVCEI 5th Edition until I twigged the r value for the 70C swa was significantly different to the value for 90C thermosetting given in IETEIDG. One is 3.9 and the latter 6. There must be significantly less csa of wire armour in the 90C cable?

Be wary of modern cables claiming to be BS6346 (the PVC clad SWA) as that standard was withdrawn over ten years ago, and the XPLE standard ' 5467 is the more common.

Worth a read of this wiring matters article Of course there is plenty of old cable about the place. But the diameter and no of the steel wires is not guaranteed the same and can be a source of confusion.

Except for BATT cables data, where their values are not always quite the same as those tables either. but the variation of armour resistance between their XPLE and PVC offerings is shown side by side, so is easier  to see.

Mike