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Sparkydan

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Does anybody know what table in the regs gives the size of armouring in mm when using the SWA as a CPC ? so i can write this in the circuit details on the installation cert.
 
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uksel

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  • #2
as far as i'm aware you are not supposed to rely on the armourings for the earth connection of SWA for installations. if there is no provision for earth in the cable you have run then you should run a seperate earth along the length of cable.
 
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randyrat

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  • #3
Agreed,
it is good practice to wire the armour into the earth each end for additional protection, but you should use a core cable for the "proper earth"
Randyrat:D
 
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Ross Trician

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  • #4
evening all,,,

although I always use an additional core for earth I always thought that you could use a 2core swa and then the armourings as the cpc...

when I use saw I use three core for single phase supplies and also earth the armourings at the supply end for additional protection.

as for where in the books you find the csa of armourings etc I havnt a clue.....
 
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KNIPEX

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  • #5
on that note, would you need to bond both ends of the bandjo if you were using a 3 core swa with one core being the earth? or just the one end?
 
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randyrat

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  • #6
If in doubt...
BOND IT...
It will never be the wrong thing to do.
Randyrat
 
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Sparkydan

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  • #7
I have also seen alot of street lighting done in 2 core using swa as CPC and thought is was ok.
 
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uksel

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  • #8
if you aren't familiar with SWA, then some things to consider

depending on what you're connecting the cable to at both ends, if it's a plastic/PVC enclosure then make sure you earth from the banjo (little brass ring with a smaller brass ring) to somewhere suitable in the CU or enclosure, if the CU or enclosure metallic i like to use serrated washers to give a good connection between the gland and the metallic enclosure.

a factory unit i stripped out the other day only had 4 short lengths of SWA supplying some 3 phase outlets, i didnt need to get my adjustable spanner out of my box, i got every single gland off by hand!! so what i'm saying is, make sure your glands are tight.

i would imagine that street ligthing tends to be mettalic throught, and i imagine that the armourings bonded to the frame of the lighting would be considered adequate

the steel armourings of SWA are notoriously rubbish at withstanding the elements, if everything isn't sealed of or shrouded properly and water gets in the armouring will only last a few short years before it seriously degrades. rotten armourings = high resistance = tripping times will suffer.
 
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S

Spudnik

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  • #9
If the swa has three cores and one is used as an earth then it should be banjo'd at the SUPPLY end. If the cable was damaged or cut then the live side would still have protection, kind of.

However, nothing wrong with doing both ends.

You are permitted to use the swa as an earthing conductor, provided various conditions in the regs are met, and obviously both ends are banjo'd, although personally i wouldn't use this method.

You can check the cable manufacturers web site for the CSA's of the armour of the type of cable you used.
 
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Paul_Rawlinson

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  • #10
as far as i'm aware you are not supposed to rely on the armourings for the earth connection of SWA for installations. if there is no provision for earth in the cable you have run then you should run a seperate earth along the length of cable.

So what happens if you cut though thw SWA underground??

You can potentially cut though without causing a Fault. The Armour is designed to be used as the MAIN CPC for SWA - so that in the event of it being cut/damaged from any side there will be a fault to earth and therefore trip the MCB/RCD. (Hence why there is no specific CPC ran in a SWA cable).

Where possible i always use a spare core as a cpc aswell (double protection) but in many cases just use the Armour of the SWA.
 
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uksel

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  • #11
NO NO NO - i believe you are very wrong there!

the armouring is not designed to be relied on as an earth connection!

the armourings reason for being there is to offer mechanical protection, end of - the earth connection is a benefit. the armourings are there to try and protect the cable from being cut through, which is why you use ARMOURED cable! armour is armour, earth is earth
 
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Sparkydan

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  • #12
This is from the nic eic technical manual

"Whilst the primary function of the metallic armouring of an armoured cable (Fig 1) is to provide overall strength, it may be used as a circuit protective conductor (cpc), main protective bonding conductor or, where practicable, an earthing conductor.
Whichever of these three functions is to be performed by the armouring of a cable, all the relevant requirements of BS 7671 need to be satisfied. The armouring of a cable may be used for more than one of the functions if all the appropriate requirements for each function concerned are met."
 

benji

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Arms
contract i'm on has swa from 2.5 2c upto 185 4c & 800mm 1c, only 3 cables have seperate cpc all of them are 50mm 4c
yours benji
 
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uksel

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  • #14
i'm not saying it can't be used, it is (i believe) good practice to ensure if there is no core to be used as an earth then a seperate earth will be run. the csa of the armourings meets the requirements of the regulations (unless you start using single cores or two core SWA with conductor CSA's of greater than 120mm)

whilst it may be adequate on installation, with 5 years of degredation (depending on the environment) it could easily break down/rust at the glands or any breaks or water ingress into the pvc sheath.

if it's possible i'd always run a seperate earth, and it is in most circumstances i think reasonable to do so

contract i'm on has swa from 2.5 2c upto 185 4c & 800mm 1c, only 3 cables have seperate cpc all of them are 50mm 4c
yours benji
4 core 50mm SWA, the armouring (according to table 54G) is 122mm, where the minimum requirement would be 56.4mm, well within the requirements

i would personally still run a seperate earth, or the best idea is to always run SWA with enough cores for the provision for an earth conductor
 
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Paul_Rawlinson

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  • #15
I take your point, but if you run a SWA underground (say 3 core and each core is used as a line conductor) and someone goes though that with a saw, (seen it a few times when people have foundations dug out for a conservatory) then that cable will still be live, not trip the Protective device and becomes a hazzard to the poor bugger you grabs the end of it (if not already had a shock from saw). Where as if the Armour was earthed then the protective device would trip preventing the risk of shock.

The seperate earth would only provide an earth at the other end, but so does the armour. if the ends are terminated properley then there shouldnt be any moisture ingress, and the cable (inc earth continuity) should has as long as the installation
 
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