Shed straight concentric supply query

[JohnW2]

I just thought it might be worth making sure that it really is straight concentric, and not single core SWA.

Fair enough - but in view of the discussion I've been having with EFLI, would it actually make any difference?

Kind Regards, John

 

[ban-all-sheds]

I stopped reading the back-and-forth between you two when it became apparent that it was pointless.

 

[JohnW2]

I stopped reading the back-and-forth between you two when it became apparent that it was pointless.

You should have read the end of that back-and-forth, If you had, you will have seen the agreed conclusion that, as far as the regs are concerned, it would seem that any cable with an earthed metal covering is seemingly deemed OK for burying, provided only that the metal covering is "adequate as a CPC", seemingly regardless of the material or mechanical properties of that covering.

Kind Regards, John

 

[John D v2.0]

Having just measured, a hair from my head is bout 40 microns

That's what I love about this forum, there's always innovate ways to discuss things!

 

[ban-all-sheds]

I don't know. Is there a definition of a minimum level of mechanical protection?

I don't know.

Read 521.6 and wonder if BS EN 61386 or BS EN 50085 have anything to say about that.

And what about cable standards? For a cable to legitimately describe itself as "armoured", does it have to meet any minimum level(s)?

Ditto "metal sheathed"?

Incidentally, does anybody know of any "metal sheathed" cables (in a UK context) other than MICC/MIMS?

 

[JohnW2]

Read 521.6 and wonder if BS EN 61386 or BS EN 50085 have anything to say about that.

That's about conduit, so I doubt it has any direct relevance to the outer coverings of cables.

And what about cable standards? For a cable to legitimately describe itself as "armoured", does it have to meet any minimum level(s)?

Possibly, but I suspect not. Let's face it, SWA has a very long history of using that description but, as EFLI and I were discussing, give me a spade and I'll get through any modest-CSA SWA for you So, even if "armoured" has some sort of definition/requirements, that probably has little practical meaning.

Ditto "metal sheathed"?

Again who knows, but I would doubt that such a description means more than it says, hence with no requirement for mechanical properties.

Incidentally, does anybody know of any "metal sheathed" cables (in a UK context) other than MICC/MIMS?

What about the 'ali-tube' cable we're always hearing about here (but which I have never even seen)?

... and how does SY fit into this spectrum, I wonder? It has the same sort of structure as SWA, and I think the sheathing is usually (always?) made of (pretty thin) steel?

Kind Regards, John

 

[ban-all-sheds]

Considering , say, an ~12mm diameter earthed covering, it would only have to have a thickness of about 0.08mm (80 microns) to have a CSA of 1.5mm², or a thickness of about 0.135mm for a CSA of 2.5mm² or a thickness of about 0.27mm for a CSA of 4mm².

Those thicknesses are all about (but not exactly ) twice what I get.

What value did you use for the circumference of your 12mm diameter cable?

 

[JohnW2]

Those thicknesses are all about (but not exactly ) twice what I get. What value did you use for the circumference of your 12mm diameter cable?

12mm OD ... but you have made me realise an error in what I did....

... I took the ID being OD-thickness, whereas it should have been OD-(2*thickness). So, yes, doing it properly (with 12mm OD) I get (roughly) ...

1.5mm² ... 0.0399 mm thick
2.5mm² ... 0.0667 mm thick
4.0mm² ... 0.1070 mm thick

Hope I've got it right this time Sorry about that, and thanks for checking, particularly since it makes my point (about pretty thin 'sheathing' having adequate CSA for a CPC) even stronger! (my hair thickness would be about right for 1.5mm²) I'll correct the originals!

Kind Regards, John

 

[ban-all-sheds]

That's about conduit, so I doubt it has any direct relevance to the outer coverings of cables.

And ducting.

Direct relevance? No - it's true that the equivalence in 522.8.10 goes the other way, but it could be considered relevant if wondering "Is there a definition of a minimum level of mechanical protection?"

Possibly, but I suspect not. Let's face it, SWA has a very long history of using that description but, as EFLI and I were discussing, give me a spade and I'll get through any modest-CSA SWA for you

Well yes - as you go up the scale, spade, JCB, shaped charge..., it of course becomes harder to defend against an "attack".

So, even if "armoured" has some sort of definition/requirements, that probably has little practical meaning.

Au contraire.

Instead of looking at ever more forceful impacts on the cable, go the other way.

There are all sorts of real-world impacts on buried cables which are less serious. A glancing strike from one tine of a fork, or the edge of a spade. Someone using a small one-handed fork or trowel. The getting of a fork under a cable and lifting.

What if there were some sort of definition/requirements, and they were not to resist the mad axeman, but to resist smaller forces, and SWA could do that but another "lesser" cable could not?

Again who knows, but I would doubt that such a description means more than it says, hence with no requirement for mechanical properties.

That might also be incorrect.

AIUI there is/are BS EN standard(s) for MI cables, so if there are no "metal sheathed" cables apart from MI, we might know what "metal sheathed" meant re mechanical properties.

What about the 'ali-tube' cable we're always hearing about here (but which I have never even seen)?

That is not a metal sheathed cable.

... and how does SY fit into this spectrum, I wonder?

That can be your assignment.

Are there any official definition(s) or standard(s) for SY cable? Is it classed in anything de jure or de facto as "metal sheathed"?

Spoiler: My [I][B]guess[/B][/I]

No.

 

[JohnW2]

I think the point that might be getting missed here is that (as with cables buried in walls) the reg is offering an earthed metal covering as an alternative to mechanical protection, so one wouldn't necessarily expect the earthed metal to be required to provide any 'mechanical protection'.

As above, the same presumably applies to cables buried in walls (<50mm) as well as underground.

Kind Regards, John

 

[ban-all-sheds]

12mm OD ... but you have made me realise an error in what I did....

... I took the ID being OD-thickness, whereas it should have been OD-(2*thickness). So, yes, doing it properly (with 12mm OD) I get (roughly) ...

I considered the difference between OD & ID as insignificant, i.e. I took the circumference of the metal layer to be 12mm in all cases, even though on the inner surface it would have been slightly less.

1.5mm² ... 0.0399 mm thick
2.5mm² ... 0.0667 mm thick
4.0mm² ... 0.1070 mm thick

Much closer to what I got

• .0398
• .0663
• .1061

So close that the differences are explained by your more precise consideration of the different diameters involved, and so close that I was right that using a constant diameter of 12mm was perfectly OK.

But come on, John - there's no way that your original figures can be explained by using OD-thickness instead of OD-(2*thickness)

12mm dia = 37.7mm circumference.

Your original figures imply mean circumferences of:

• 18.75mm
• 18.52mm
• 18.87mm

You were using πr instead of πd, weren't you...

 

[ban-all-sheds]

I think the point that might be getting missed here is that (as with cables buried in walls) the reg is offering an earthed metal covering as an alternative to mechanical protection, so one wouldn't necessarily expect the earthed metal to be required to provide any 'mechanical protection'.

Again, it aint necessarily so.

The reg is offering earthed armour or earthed metal sheath.

And it is unknown whether, but far from inconceivable that, the terms 'armour' and 'metal sheath' do mean defined level(s) of mechanical protection.

 

[davelx]

Are there any official definition(s) or standard(s) for SY cable? Is it classed in anything de jure or de facto as "metal sheathed"?

This is what Eland Cables have to say about it:

"SY cables, also known as armoured flexible control cable, shielded flex, or simply armoured flex, are strong and durable cables suitable for internal dry, damp or wet environments (including oil-water mixtures), and can be used outdoors when protected against direct sunlight. They are also referred to by their constructional reference: YSLYSY cables for the PVC sheathed variation.

SY control cables are not suitable for fixed wiring applications requiring compliance with the requirements set out in BS7671".

 

[Furball]

I just thought it might be worth making sure that it really is straight concentric, and not single core SWA.

Definitely not swa, gotta plenty of 3 core running round the lights in the garden. At my best guess id say it's around 30mm2 on the Alu cores and slightly less on the copper outers

 

[JohnW2]

Apologies for delay - I've been out for much of the day ....

I considered the difference between OD & ID as insignificant, i.e. I took the circumference of the metal layer to be 12mm in all cases, even though on the inner surface it would have been slightly less. .... Much closer to what I got ..... So close that the differences are explained by your more precise consideration of the different diameters involved, and so close that I was right that using a constant diameter of 12mm was perfectly OK.

I'm sure that it was perfectly OK for the purpose. However, using the method I used (see below), if I had assumed that the inner and outer diameters were the same, I would have got a CSA of zero !

But come on, John - there's no way that your original figures can be explained by using OD-thickness instead of OD-(2*thickness) .... 12mm dia = 37.7mm circumference. .... Your original figures imply mean circumferences of: .... You were using πr instead of πd, weren't you...

Nope. In fact, I was not using circumference at all.

To calculate CSA, I was subtracting the area of the 'inner circle' from the area of the 'outer circle'. Hence, if D is the outer diameter and T the thickness of the sheath (i.e. inner diameter = D - 2T), then:

CSA = πD²/4 - [π(D-2T)² / 4]

Since I couldn't be bothered to solve that for T, I just threw it into a spreadsheet with very small increments in T and looked to see which values of T resulted in CSAs close to 1.5mm², 2.5mm² and 4mm² respectively. I didn't save the worksheet, so can't check to see if I typed everything correctly. I may possibly have made mistakes either in typing the formulae or manually transcribing the answers into my post.

Anyway, now doing it 'properly', using that equation (but, again, not bothering to solve it), the following increases T by very small increments and outputs the first values of T which give CSAs ≥1.5mm², 2.5mm² and 4mm² (note that ** indicates exponentiation) ....

data DIYnot1 (keep = thickness csa) ;
  OD = 12 ;
  pi = constant("pi") ;
   do thickness = 0.01 to 0.3 by 0.00001 ;
     CSA = ((pi * OD**2) / 4) - ((pi * (OD-(thickness*2))**2) / 4) ;
      if CSA >= 1.5 and CSA_prev < 1.5 then output ;
      if CSA >= 2.5 and CSA_prev < 2.5 then output ;
      if CSA >= 4 and CSA_prev < 4 then output ;
      CSA_prev = CSA ;
   end ;
run ;

.... which gives output ...

thickness CSA
0.03993 1.50032
0.06669 2.50018
0.10706 4.00006

When I made the mistake of taking the inner diameter as (D-T), rather than (D-2T), I should have got ...

thickness CSA
0.07985 1.50013
0.13338 2.50018
0.21412 4.00006

... which is broadly the same as what I initially reported, other than for 4mm² - I can but presume that there was a 'transcription/typing error' in relation to that (it was late at night ), since what I reported was "about 0.27mm" - I probably had intended to type "about 0.21mm", and maybe didn't have my glasses on .

Kind Regards, John