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Secondary CPC's or Overlays as they are commonly known are another belt and braces practice set by the DNO about 30 years ago. They weren't convinced that the steel armouring was capable of taking the fault current. There is only one cable that I've come across where this was possibly the case and that was 185 mm 3 core. 


Interesting, what influence did the DNO have over installations at the time? I don't suppose you've got any information relating to this? 

 
I'll see what I can dig out, the local electricity boards when were a law unto themselves started refusing to connect sub mains etc unless an overlay was installed alongside the supply cable.

i used to have a table with the cable / armour info on relating to steel equivalent CPC size.

It was about 1982/3 when it first reared its head.

 
Secondary CPC's or Overlays as they are commonly known are another belt and braces practice set by the DNO about 30 years ago. They weren't convinced that the steel armouring was capable of taking the fault current. There is only one cable that I've come across where this was possibly the case and that was 185 mm 3 core. 


Thirty years ago the electricity boards had embraced CNE WaveCon. Prior to that TN-S PILC-SWA ruled the roost. No need for an external CPC with either.

Newer MV cables sometimes had an external CPC but normally only short runs.

Any links to the information you have?

 
Thirty years ago the electricity boards had embraced CNE WaveCon. Prior to that TN-S PILC-SWA ruled the roost. No need for an external CPC with either.

Newer MV cables sometimes had an external CPC but normally only short runs.

Any links to the information you have?
Eastern Electricity Board in the early 80's insisted on overlay cpc's on supplies connected to their network ie from a LV cubicle on their transformer

 
Eastern Electricity Board in the early 80's insisted on overlay cpc's on supplies connected to their network ie from a LV cubicle on their transformer


Would that not be more to do with the exceptionally high fault currents available and the possible interconnections between LV and MV earths. Or was it to satisfy their requirements for a main bond, which the armour would almost certainly not be suitable for?

 
All I can vaguely remember was our contracts manager at the time stating that all swa supplies above 16mm had to have separate overlays run alongside, irrespective of what was written in the 14th / 15th edition of the regs or any other criteria. The local electricity board insisted on it and that was that. 

 
OK, so no matter what, practical research & experimentation has since been undertaken, which has been backed up by mathematical modelling, which has been undertaken by a respectable body (NOT one of the SCAMS!) that has shown that no matter which way you do it, if, you have an SWA & a conductor in parallel, inside or outside the SWA, they both MUST be sized for full fault current.

Thus, if the SWA MUST be sized for full fault current, as it MUST be adequately connected to earth at the source end, thus it is adequate as a CPC, thus, there is never a requirement for an additional CPC, the only requirement can be for a bonding conductor.

 
All I can vaguely remember was our contracts manager at the time stating that all swa supplies above 16mm had to have separate overlays run alongside, irrespective of what was written in the 14th / 15th edition of the regs or any other criteria. The local electricity board insisted on it and that was that. 


So where did he get the rule from?


It's not the first time I've heard that, it's something my mentor told me when I was but a young apprentice. He never could give a good reason for it though, so after a long discussion we came to the conclusion he had been misinformed. 

 
And i would add that in cases where you are required to have a bonding conductor larger than the cable conductors, the additional one used as the bonding conductor must be large enough all on its own. What i mean is, say the requirement was for a 70mm bond, you could not make this up by having, say, 25mm in the cable [or the copper equivalent in armour] and the balance separate, you would need a 70mm cable for the bond..

john...

 
OK, so no matter what, practical research & experimentation has since been undertaken, which has been backed up by mathematical modelling, which has been undertaken by a respectable body (NOT one of the SCAMS!) that has shown that no matter which way you do it, if, you have an SWA & a conductor in parallel, inside or outside the SWA, they both MUST be sized for full fault current.

Thus, if the SWA MUST be sized for full fault current, as it MUST be adequately connected to earth at the source end, thus it is adequate as a CPC, thus, there is never a requirement for an additional CPC, the only requirement can be for a bonding conductor.


Are you able to give a reference, or a point in the right direction, of where I could read up further on this? 

 
It's not the first time I've heard that, it's something my mentor told me when I was but a young apprentice. He never could give a good reason for it though, so after a long discussion we came to the conclusion he had been misinformed. 
As I said earlier there was no reason for it, it was simply the local electricity board stating that you needed to run a separate CPC with any swa 16mm and over. Like another myth which gets discussed on here occasionally we know it's not needed but most still install one 

 
Found this on my computer, but i am not sure where i got it from!!!!

4.  Where the csa of the armouring is not sufficient

Where the csa of the armouring of a cable is not sufficient to meet the requirements of Regulation 544.1.1 referred to in item 2 or item 3.2, as applicable, it is necessary to use an alternative main bonding conductor. The conductor must have a csa sufficient to meet the applicable requirements of Regulation 544.1.1 and may be provided by one of the following.

(i)

 A separate conductor, such as a green-and-yellow covered copper cable.

(ii)

Selecting a cable with an extra internal core, and using that core as the main bonding conductor (not forgetting that all the cores must be suitably identified Regulation 514.3.1 refers). Note, however, that this alternative may be unsuitable where PME conditions apply, due to heating effects associated with diverted neutral current in the extra internal core (item 3.1 penultimate paragraph refers).

(iii)

Selecting a larger size of cable, having not less than the required csa of armouring, and using its armouring as the main bonding conductor.

It is stressed that, where alternative (i) or (ii) is chosen, the conductor still has to be of the full csa needed to meet the applicable requirements of Regulation 544.1.1, even though the conductor will effectively be in parallel with the armouring.

It is not permissible to ‘make up’ the csa of the armouring to the necessary size with an alternative conductor, because it cannot be accurately predicted how the current will divide between that conductor and the armouring, due to the magnetic effect of the armouring.

So, there you go!!

john......

 
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In 2007 the ECA commissioned the ERA (Electrical Research Authority) to carry out tests on armour cables and additional CPC's.

Initially only two cables failed under the requirements for BS7671, however when tested under realistic fault currents they also passed.

I have the full report, but will just say that the findings are that if an external CPC is to be included in parallel to the armour, then that Cable must be at least 1 quarter of the size of the line conductors.

If anyone is interested in the tests and results I will see if I can upload the full document later.

 
I have just uploaded the research paper for anyone to download. It is in detail and contains some very good formula for calculations.

 
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