Can we use the armour of an SWA as the CPC???

[ojchurch]

A guy I worked with last year reckoned he'd read an article about some studies done on the actual routes taken by the fault current for SWA distribution cables with & without additional cpcs. I never managed to get them off him before he left the job. He thought it said that very little current travelled through a separate cpc irrespective of this or the SWA csa. He thought it would alter the regs in the near future. Unfortunately I didn't get the publication either. Anybody hear of this? Cheers.

 

[pactrol]

Hi ojchurch

yes thats right it was a study done by the ERA where it was shown that the fault current is shared fairly equal between the swa & seperate cpc therfore negating the need for the seperate cpc to be fully rated for the fault current the same report also shows that the swa is suitable as the cpc in nearly all cases
I will see if I can find it but try googling ERA cable study.

 

[ojchurch]

Cheers for that, just tried a couple of combinations in google and got nothing.

Was on a job briefly recently where they had what seemed to be under sized additional/external cpcs with SWA cables. Didn't manage to get hold of enough info to do the calcs myself at the time before I left. I think one example was 25mm2 4c feeding a final DB, they added a 2.5mm2 cpc. The only comment I made without all of the info was that it needed to be 4mm2 minimum without mechanical protection. I always used to be under the impression that an additional cpc needed to be sized to take the full fault current. I'm thinking maybe this study has filtered down to the design companies. Although how they get round non compliance with the regs I don't know?

 

[Silly Sausage]

Hi ojchurch

yes thats right it was a study done by the ERA where it was shown that the fault current is shared fairly equal between the swa & seperate cpc therfore negating the need for the seperate cpc to be fully rated for the fault current the same report also shows that the swa is suitable as the cpc in nearly all cases
I will see if I can find it but try googling ERA cable study.

Is this it...?
http://www.wiley.com/legacy/wileychi/eca_wiringregulations/supp/Appendix_16.pdf

 

[ojchurch]

Looks good, I'll have a read later cheers.

(What exciting lives we lead!)

 

[pactrol]

Is this it...?
http://www.wiley.com/legacy/wileychi...ppendix_16.pdf

Yes thats the boy

Pity that The writers of BS7671 havent read it

 

[theroadblock]

haha this old chestnut! many a time ive been preached to by other electricians that you are to run a seperate earth alongside your swa and not just use the armouring, hell it states in the regs that the armouring on swa is adequate means of earthing! from what i have read so far in various places the idea of running a seperate earth came from the older types of SWA cable now not in production, where over a certain size of csa the armouring was not adequate means of earthing, but nowadays i believe that this has been resolved ( evidently from the 17th's section on earthing ).

 

[Des 56]

http://www.electriciansforums.net/e...ulations/23288-can-we-use-armour-swa-cpc.html

 

[Silly Sausage]

http://www.electriciansforums.net/e...ulations/23288-can-we-use-armour-swa-cpc.html

I suspect he already knows that. T & E54 might have a new brother here

 

[adiewadie247]

Surely running a separate CPC on the outside runs into all the same (and probably worse) problems that arise when using the armour. Given that the cost differential between 4-core/3-core/2-core is small (compared to the total cost of the job) why not just use an extra core for the CPC? As the CPC is so important it needs to benefit from the protection that the PVC/armour provides...

 

[Silly Sausage]

Unless you are installing big boy cables where the armour isn't sufficient, why waste money on an extra core for the earth when you've got a perfectly good enough one in the armour, assuming one buries it correctly (no potential sheath damage)

 

[adiewadie247]

I agree. Using the armour is the way to go but my point related more to the suggestion of attaching a separate CPC on the outside which in my humble opinion is a little pointless. Why have SWA if you going to compromise on the protection of the CPC?

 

[somersetsparks]

I agree. Using the armour is the way to go but my point related more to the suggestion of attaching a separate CPC on the outside which in my humble opinion is a little pointless. Why have SWA if you going to compromise on the protection of the CPC?

The armouring prevents the live conductors from getting damaged etc. even if the cable was crushed it would just short to earth, hence no shock risk. Therefore the earth even if it was squashed wouldn't shock anyone.
Even if you repeatedly drove over a 25mm single cable, you would probably only damage the insulation, not the copper.

 

[Barry Rathbone]

If you even thinking of possible damage to armouring on a SWA cable install a 3 Core for 1 Phase installs and
a 5 core for 3PH installs... Life is more important than money ....Labour costs etc etc on installing seperate earthing. IHMO

 

[Engineer54]

Here is an updated table that i have been using for several years now on SWA compliance, for using the armour as your CPC

Figures in brackets, ... DO NOT COMPLY.



Table for 70[SUP]0[/SUP]C Thermoplastic PVC SWA cables.

Conductor CSA​	Minimum CSA of SWA to meet 54G​	CSA of armour 2 core​	CSA of armour 3 core​	CSA of armour 4 core​
1.5​	3.4​	15​	16​	17​
2.5​	5.7​	17​	19​	20​
4​	9.0​	21​	23​	35 ​
6​	13.6​	24​	36​	40​
10​	22.6​	41​	44​	49​
16​	36.1​	46​	50​	72​
25​	36.1​	60​	66​	76​
35​	36.1​	66​	74​	84​
50​	56.4​	74​	84​	122​
70​	79.0​	84​	119​	138​
95​	107.2​	122​	138​	160​
120​	135.3​	(131) ​	150​	220​
150​	169.2​	(144) ​	211​	240​
185​	208.6​	(201) ​	230​	265​
240​	270.6​	(225) ​	(260)​	299​
300​	338.3​	(250) ​	(289)​	(333) ​
400​	403.9​	(279) ​	(319)​	467 ​

Table for 90[SUP]0[/SUP]C Thermosetting SWA cables operating at 70[SUP]0[/SUP]C.

Conductor CSA​	Minimum CSA of SWA to meet 54G​	CSA of armour 2 core​	CSA of armour 3 core​	CSA of armour 4 core​
1.5​	3.4​	16​	17​	18​
2.5​	5.7​	17​	19​	20​
4​	9.0​	19​	21​	23​
6​	13.6 ​	22​	23​	36​
10​	22.6​	26​	39​	43​
16​	36.1​	41​	44​	49​
25​	36.1​	42​	62​	70​
35​	36.1​	62​	70​	80​
50​	56.4​	68​	78​	90​
70​	79.0​	80​	90​	131​
95​	107.2​	113​	128​	147​
120​	135.3​	(125) ​	141​	206​
150​	169.2​	(138) ​	201​	230​
185​	208.6​	(191) ​	220​	255​
240​	270.6​	(215) ​	(250)​	289​
300​	338.3​	(235) ​	(269)​	(319) ​
400​	451.0​	(265) ​	(304)​	452 ​

 

[aprillove1719]

In addition to that:
Steel wire armoured cable, commonly abbreviated as SWA, is a hard-wearing power cable designed for the supply of mains electricity. It is one of a number of armoured electrical cables – which include 11kV Cable and 33kV Cable – and is found in underground systems, power networks and cable ducting. Steel wire armour is only used on multicore versions of the cable. A multicore cable, as the name suggests, is one where there are a number of different cores. When SWA cable has only one core, aluminium wire armour (AWA) is used instead of steel wire.

 

[malcolmsanford]

^^^^^^^^^^^^^^^^^^^^ Wouldn't it have been easier to just post a link to

Steel wire armoured cable - Wikipedia, the free encyclopedia

 

[johnboy6083]

Here is an updated table that i have been using for several years now on SWA compliance, for using the armour as your CPC

Figures in brackets, ... DO NOT COMPLY.



Table for 70[SUP]0[/SUP]C Thermoplastic PVC SWA cables.

Conductor CSA​	Minimum CSA of SWA to meet 54G​	CSA of armour 2 core​	CSA of armour 3 core​	CSA of armour 4 core​
1.5​	3.4​	15​	16​	17​
2.5​	5.7​	17​	19​	20​
4​	9.0​	21​	23​	35 ​
6​	13.6​	24​	36​	40​
10​	22.6​	41​	44​	49​
16​	36.1​	46​	50​	72​
25​	36.1​	60​	66​	76​
35​	36.1​	66​	74​	84​
50​	56.4​	74​	84​	122​
70​	79.0​	84​	119​	138​
95​	107.2​	122​	138​	160​
120​	135.3​	(131) ​	150​	220​
150​	169.2​	(144) ​	211​	240​
185​	208.6​	(201) ​	230​	265​
240​	270.6​	(225) ​	(260)​	299​
300​	338.3​	(250) ​	(289)​	(333) ​
400​	403.9​	(279) ​	(319)​	467 ​

Table for 90[SUP]0[/SUP]C Thermosetting SWA cables operating at 70[SUP]0[/SUP]C.

Conductor CSA​	Minimum CSA of SWA to meet 54G​	CSA of armour 2 core​	CSA of armour 3 core​	CSA of armour 4 core​
1.5​	3.4​	16​	17​	18​
2.5​	5.7​	17​	19​	20​
4​	9.0​	19​	21​	23​
6​	13.6 ​	22​	23​	36​
10​	22.6​	26​	39​	43​
16​	36.1​	41​	44​	49​
25​	36.1​	42​	62​	70​
35​	36.1​	62​	70​	80​
50​	56.4​	68​	78​	90​
70​	79.0​	80​	90​	131​
95​	107.2​	113​	128​	147​
120​	135.3​	(125) ​	141​	206​
150​	169.2​	(138) ​	201​	230​
185​	208.6​	(191) ​	220​	255​
240​	270.6​	(215) ​	(250)​	289​
300​	338.3​	(235) ​	(269)​	(319) ​
400​	451.0​	(265) ​	(304)​	452 ​

can i just add, that they do not comply with table 54G, but may comply by calculation, using the adiabatic. I seem to remember that the only size that doesnt comply with the adiabatic is 400mm2 2c. This will need you to verify this though, dont take my word for it.

 

[Tonkatoy27]

I have a job coming up in the school hols to supply 2 temporary portacabin type classrooms which will be set up as technology workshops untill xmas. 60amp 60898 3 phase protective device. It's a 150m run which means that due to volt drop I'll have to use 50mm SWA. Now I'd like to use 5core and include an integral cpc but theres about £1200 difference between 4 and 5 core so thats out. I could run a seperate earth if I specially ordered 150m of 25mm which would be better value but is it necassary? From the previously listed tables it's not but as it's a school I'd like to have 'belt and braces'
So my question is, who would run a seperate cpc and who wouldn't?

It'll probably be nicked by pikeys in 10 minutes anyway, they had 100m x 6m of lead off a roof at the same school last year...

 

[malcolmsanford]

Can I ask Tonka first off why the 60amp load, will that be for heating these cabins?

I'm assuming that your taking the VD from the In rather than the Ib ?

Why the 5 core is it a 3phase supply?

Are you going to be classing these as a temporary structure and therefore under section 717. I would think they should be and therefore if you have a PME system you need to make sure you can export it as per reg 717.411.4, which should be ok, just want to make you aware of it.

Personally that long a run I would be looking very closely at the design load, to try and use a smallish cable as possible, especially as it is temporary. I certainly would not be looking at running a separate CPC for it . Is there no design criteria for this by the customer or their representative