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

Discuss Can we use the armour of an SWA as the CPC??? in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

spinlondon said:
This Regulation is aimed at mitigating the effects that are caused by unwanted magnetic fields (eddy currents).
They did not include multicore SWA cables in with the single core SWA cables, simply because multicore SWA cables are acceptable for a.c. circuits.
It is quite obvious that SWA is considered to cause unwanted magnetic fields, otherwise there would be no constraints against using the single core SWA cables.
They haven't specifically mentioned multicore SWA cables, just as they haven't specifically mentioned galvonised steel conduit, galvonised steel trunking or even maleable iron conduit as all of these are covered by the term 'ferromagnetic enclosure'.
Now you have stated that the practice of running a separate earth has been in use for generations.
Are you sure that it has been for that long, and do you know whether the separate earth is a CPC, or a bonding conductor?
The armour while acceptable for a CPC, is not acceptable for bonding, as such a separate conductor is required.
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I think you have basically said the same as me other than your last line.
Where does it say that you can't use the armour as a bonding conductor.
I've always thought that (assuming all the calcs allow) you can use the armour- as a cpc, bonding conductor, earth conductor or indeed all these at once.
In other words, if you were running a sub main you could use the armour as a combined cpc and bonding conductor.
 
Sorry that statement was in relation to TN-C-S systems and yes technically you could size an SWA cable to allow the armour to be acceptable as a bonding conductor.
However the practicalities would preclude it. To meet the requirements of Table 54.8 the armoured cable would have to have conductors at least twice the size of the supply neutral.
As such if your supply neutral is anything up to 35mm² which would require a 10mm² bonding conductor, your SWA cable would have to be 70mm² for the armour to have a 10mm² copper equivalance.
I wouldn't be able to justify using a 2 core 70mm² SWA cable instead of a 2 core 6mm² SWA cable with separate 10mm² bonding conductor
 
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Sorry spinlondon but I'm lost now.
I don't understand where you are getting 70mm from.
10mm 2 core pvc/swa cable has a armour equivalent of 17.7mm copper. Going by earthing nuts chart.
 
The ratio for electrical conductivity between copper and steel is about 8 or 9:1, depending on the make up of the materials.
The chart you are refering to probablly relates to the thermal resistivity.
 
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Sorry that statement was in relation to TN-C-S systems and yes technically you could size an SWA cable to allow the armour to be acceptable as a bonding conductor.
However the practicalities would preclude it. To meet the requirements of Table 54.8 the armoured cable would have to have conductors at least twice the size of the supply neutral.
As such if your supply neutral is anything up to 35mm² which would require a 10mm² bonding conductor, your SWA cable would have to be 70mm² for the armour to have a 10mm² copper equivalance.
I wouldn't be able to justify using a 2 core 70mm² SWA cable instead of a 2 core 6mm² SWA cable with separate 10mm² bonding conductor
Click to expand...

You have lost me too.....

if you use the adiabatic regarding a 35mm 4c for say a TP+N PME supply then the armouring has a resistance of 2 miliohms/metre, coupled with a CSA of 70mm.. i bet you find the armour is more than adequate....

have you read my previous post in this thread? its just your calcs dont seem to add up.... you dont need to consider ratios of conductivity between steel and copper when you know the resistance of the armour and you have the other info required to make the calc.....
 
35mm 4 core to bs 6346 is just about adequate for a 10mm bond
conductivity of copper is about 8 x more than swa
(pme only)
Click to expand...

BS 6346? not familiar with that. generally use cable to 5467 or 6724.

plus i dont see your point... copper is a great conductor, but as previously mentioned to follow regs use table 54.7 or use the adiabatic.... How can you say that cable is suitable when you havent stated it length, a Zs value etc...... every scenario is different....
 
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(pme only & bonding conductor )
35mm four core swa to bs 5467 is just about adequate for a 10mm bond
min size of swa which would equal a 10mm bonding conductor in copper

swa just cpc use adiabatic...
 
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ah sorry! im talking about using armour as an earthing conductor... not a bonding conductor.... and not specifically PME, where larger bonding conductors are required due to the nature of the earthing arrangements......

ha ha different wavelengths....
 
I'm not sure if there's a table but there's a conversion factor, if nobody else posts it, I'll do it tonight when I get back to the office.
 
hi josh,

if you go on to AEI cables website

you will find the information there

you have to open an account.... just a sign in thing, not a proper trade account

then all the info you want is specific to the product you select

the datasheets being the technical info, showing resistances etc....

or p166 O.S.G has a little info on it
 
I use both sometimes, if its outside and posible that it can be hit or perish I use the armour as a secondary defence at the sub main board and the third core cpc between the main dis board in the house and the sub main.
 
spinlondon said:
The ratio for electrical conductivity between copper and steel is about 8 or 9:1, depending on the make up of the materials.
The chart you are refering to probablly relates to the thermal resistivity.
Click to expand...

So, do we divide the tabulated figures in the 'earthing nuts' chart by the 8.8 resistivity factor of copper against steel or not ?

Are these figures purely a csa equivalent and not a resistivity comparison.

Dave's project example will not be correct, if that is the case.

Help !!
 
This looks like the answer ?http://www.------.org/forums/forum/...d=23890&STARTPAGE=1&FTVAR_FORUMVIEWTMP=Linear

Choose say 25 mm2 Cu and see that it has a resistance of 0.727 Ohms/Km

Look at the steel csa for say 4 c 25 mm2 and get 70 mm2 and this has 2.3 Ohm/Km.

Divide 0.727 by the 2.3 to give 0.316

Multiply the copper 25 mm2 by this 0.316 to get the copper equivalent of 70 mm2 steel ie 7.9 mm2.

Divide the original 70 mm2 of steel by 7.9 to get a figure of 8.86.
 
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It would depend on what it is you wish to achive.
If for instance you wish to determine whether the armour of an SWA cable is suficient to be used as a CPC, then I would either use the cable manufacturer's data or perhaps a chart that you feel is reliable, and then apply the adiabatic equation.
If however you wished to determine, whether the armour was suitable to be used as a bonding conductor, I probablly wouldn't bother making the attempt other than as an exercise to prove to someone that the armour is not sufficient.
 
Dave_ said:
Hello again sparkies, here is a snippet of my project submitted for 2391-20 qualification regarding minimum size of S= √(1966²×0.1)/46=13.5mm²

Using the table provided by AEI I can see that a 70mm² four core cable has a 131mm² armour CSA, therefore this is perfectly acceptable.
[/I]

This shows that the armouring can handle 10x the amount of fault current that is required....!! no need for seperate CPC ;-)

Please note: values for armour and conductor resistance are from AEI cables, vvv useful!!! as you dont find them in the regs book, OSG etc. (OSG up to 50mm and doesnt show armour resistance.... there is a way of calculating conductor resistance but i cant remember how.....

Was going to post link to AEI site but you need to be a member to view the datasheets.... if anyone wants to see them and needs assistance then let me know ;)

hope this helps AMP DAVID!!!
Click to expand...

The AEI table suggest that the 'effective csa of armouring' is 16.38 sq.mm. Which seems closer to the mark and still good enough but not 10 times as good.

I am not criticising the findings but trying to get this to sink into my bald/grey head and have found this thread very useful.

Thank You.
 
spinlondon said:
It would depend on what it is you wish to achive.
If for instance you wish to determine whether the armour of an SWA cable is suficient to be used as a CPC, then I would either use the cable manufacturer's data or perhaps a chart that you feel is reliable, and then apply the adiabatic equation.
If however you wished to determine, whether the armour was suitable to be used as a bonding conductor, I probablly wouldn't bother making the attempt other than as an exercise to prove to someone that the armour is not sufficient.
Click to expand...

Got it now.Thanks.
 
Just throwing this in because i'm sick of reading these threads about very basic electrical practise. If you have to ask questions like this then you shouldn't be touching any electrical installation above a 13amp plug!!!
 
madmac said:
Just throwing this in because i'm sick of reading these threads about very basic electrical practise. If you have to ask questions like this then you shouldn't be touching any electrical installation above a 13amp plug!!!
Click to expand...

I don't agree at all, this has been a good thread, especially the points relating to the adiabatic and copper-steel conductivity ratios.
 
madmac said:
Just throwing this in because i'm sick of reading these threads about very basic electrical practise. If you have to ask questions like this then you shouldn't be touching any electrical installation above a 13amp plug!!!
Click to expand...

I look forward to your future input with interest. :cool:

Try your spell checker on the more taxing issues. :rolleyes:
 
@Amp David: Thanks for that eloquent input into the discussion. After all, who knows? maybe i am a "nugget", or maybe i'm just some daft spark that's looking after his and his fellow electricians jobs! Now for the reality check........If guys on here think they are doing some sort of noble gesture by educating the ill-informed fast track sparks out there by giving away our trade secrets and experience, then don't complain when the next job you've priced for at competitive rates is blown out of the water by a fast tracked fake spark, undercutting everyone and saying a big thanks for this oppertunity!!! Just think about it for a second and you'll see who the real nuggets are and pray that you'll always have a secure and non threatened workplace. Oh and TAFF4, i don't need to be great at spelling on a site like this as it's not dangerous......unlike some of the question and answers i've had the misfortune to gaze on here!!!
 
madmac said:
@Amp David: Thanks for that eloquent input into the discussion. After all, who knows? maybe i am a "nugget", or maybe i'm just some daft spark that's looking after his and his fellow electricians jobs! Now for the reality check........If guys on here think they are doing some sort of noble gesture by educating the ill-informed fast track sparks out there by giving away our trade secrets and experience, then don't complain when the next job you've priced for at competitive rates is blown out of the water by a fast tracked fake spark, undercutting everyone and saying a big thanks for this oppertunity!!! Just think about it for a second and you'll see who the real nuggets are and pray that you'll always have a secure and non threatened workplace. Oh and TAFF4, i don't need to be great at spelling on a site like this as it's not dangerous......unlike some of the question and answers i've had the misfortune to gaze on here!!!
Click to expand...

madmac, I think you're missing the spirit of the forum, who are we to 'restrict' certain items of information?

The sizing of SWA for use as a CPC certainly doesn't slot into the 'trade secret' category, it's been the subject of many reports and studies by the IET and ECA!

Then the adiabatic equation, how many blank looks I get when I mention the adiabatic to 'time served' approved electricians, not because they are 'fast tracked fake sparks' but because they never have occasion to use it on a regular basis!

Of course we can all lose contracts to lesser qualified electricians but restricting forum information won't have a drastic effect on that....
 
Yes the SWA is a ferromagnetic enclosure. However there is apparently going to be an exception to the requirement for CPCs to be within the ferromagnetic enclosure for SWA in the first ammendment.
 
Yes the SWA is a ferromagnetic enclosure. However there is apparently going to be an exception to the requirement for CPCs to be within the ferromagnetic enclosure for SWA in the first ammendment.
Click to expand...

So i believe, think its due to test showing armoring spacing breaking down circulating currents
 
There is a report about from the ERA, produced for the ECA, I believe it's entitled 'ECA appendix 16' or similar.
This has information on the use of a separate CPC with SWA.
 
Just a little addition in relation to a previous point...
People coming from a 'fast track' scheme may not have covered this in enough detail to know for certain if the SWA armour is suitable for an installation - I know that I am / was unsure - This thread has helped me :)

I'm not quite sure on the point about loosing business to someone fresh (inexperienced) into the industry...
I will just state that the reason I'm here and looking for this info is because my company (not electrical) has an old boiler installation (late 80s) which I believe to be unsafe / questionable methods have been used on installations / modifications. The company has had electicians into and around the area before and no-one has commented on the installation - thus it will remain unchecked until it goes wrong. By carrying out calculations and checks, I hope to be able to get things rectified...
This kind of topic may end up generating jobs for the more experienced electrician rather than taking them away.
 
Well I have found all this a very interesting read... I for one would choose to use both the armouring and an extra core as a CPC.

I'm not quite sure on the point about loosing business to someone fresh (inexperienced) into the industry...
I will just state that the reason I'm here and looking for this info is because my company (not electrical) has an old boiler installation (late 80s) which I believe to be unsafe / questionable methods have been used on installations / modifications. The company has had electicians into and around the area before and no-one has commented on the installation - thus it will remain unchecked until it goes wrong. By carrying out calculations and checks, I hope to be able to get things rectified...
This kind of topic may end up generating jobs for the more experienced electrician rather than taking them away.
Click to expand...

I think it is important to remember that this forum attracts people from everyday walks of life. There are some very knowledgeable and experienced people on here, there are some very knowledgeable but inexperienced people on here and there are people that have very little knowledge or experience. But we are all here for a reason, either to share our knowledge, or to gain knowledge.

YEEE HAAAAAA!
 
Nice to see this thread is still going!

I always run a run a seperate cpc (boss says so!) although its very easy to prove its not a necessity (adiabatic). good practice. He has 30 years more experience than me. I just do as i am told.............................
 
I have a similar table that the niceic sent me. Another point is that although the armouring is suitable for the circuits cpc it would be important to remember that it my not satisfy the requirements for bonding of water pipes etc if the armoured was a sub feed. Rotting of the armouring can be an issue but that is why a periodic inspection is/should be carried out to catch such issues before it completly breaks down.
 
The armour of swa should always be connected to earth and that's why you don't have to rcd protect a swa because if you damage it you will hit an earth before a live conductor, thats why you don't see a lot of two core swa about because people stopped buying it and just using the old yellow or new black as the earth. But the cross sectional area of the swa is larger than the earth conductor inside the cable, but why not connect both.
 
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