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

[johnboy6083]

I'm sorry johnboy but I don't get it, it states a copper equivalent Cu, if this isn't meant to be the size of a copper conductor having the same conductance as the swa, then whats the point .
From that chart how would I determine if the swa armour was equivalent or better than a 10mm copper conductor , for the sake of complying with TNCS minimum main bond size ?

the ciopy of the tabvle i have says that : "this table is a guide only, and is for disconnection times upto and including 5 secs. For other times consult the cable manufacturer"

SWA will be able to dissipate heat from fault currents easily because it has more surface area, as the armour is not bunched together.
In the case of the worst type of electrical fault, a long duration, low magnitude fault (like the TNC-s scenario as lenny mentioned), this dissipation will mean that heat is transferred into the inner cores of the cable.

So in other words, the k values which are used to calculate the copper equivelants, and are used in the adiabatic equation are only suitable for short duration faults.

 

[Skittles]

Are you saying that the for swa , for example on 6mm 2core swa cable , the armour has twice the resistance of the 6mm cores, nevertheless because of heat dissipation it is equivalent to a 6mm copper core as stated in the chart ?

 

[johnboy6083]

That's about it, but only for the purposes of Handling fault current for upto 5secs

 

[malcolmsanford]

Times by 8

Wouldn't it be good if more guys bought the GN 8 ...............

 

[Skittles]

Ah so now I see, it is for calculating the armour size suitable for fault currents, but not to be used for calculation of bonding conductors. Looking back through the thread I see I am not alone in being puzzled by this chart.
wouldn't it be good if more guys were perfect

 

[johnboy6083]

Nobody I've ever met is perfect. GN8 is an interesting read though.

 

[Engineer54]

Ah so now I see, it is for calculating the armour size suitable for fault currents, but not to be used for calculation of bonding conductors. Looking back through the thread I see I am not alone in being puzzled by this chart.
wouldn't it be good if more guys were perfect

I don't quite understand why or where you got the idea that these tables were for use in calculating main bonding conductors?? Neither of these tables mentions anything about there use for bonding conductors. The clue is also in the thread title.

The original table posted, can be very confusing to both experienced and novice electricians, expecting far more compliance calculation to many cable sizes than is necessary. The table that Spinlondon first posted fairly early on (around post 34), and later by myself (post 155) and which i have been using for a number of years now, is far more precise for confirming or otherwise, a SWA cables CPC compliance...

 

[johnboy6083]

the way you work out if SWA is appropriate for use as a main bonding conductor is to multiply the mkinimum copper size required by 8.5. The actual steel armour CSA has to be at least that size.

so the minumum size of SWA that can be used for the minimum size of main bonding on TNS (6mm2) is 25mm2. For the 10mm2 minimum copper size of main bonding for TNC-S, the minimum SWA size is 50mm2 4core and 70mm2 3 core onwards. 70mm2 2 core does not comply.

The table posted can be used to find the actual steel armour size, but you cannot use the Copper equivelant CSA from thees tables for main bonding calculation/selection. If you deviode the steel armour CSA by 8.5, it will give you the copper equivelant for main bonding sizes.

As Lenny mentioned, you have to take account of the heating effect that the diverted neutral scenario may produce. In many cases, this may mean that using the SWA as a main bonding conductor is not appropriate.

 

[gonefishing]

Can we use the armour of a SWA as a CPC? you should consider the construction of the cable 1, It's armour protected, bit of a give away, so if it's cut in any way and the armour isn't connected to earth what happens? (that tingling feeling appears).
If the cable route is properly designed and installed correctly and the ........cable is chosen correctly .........underground etc. with an extra core for a CPC and the CPC sizing is correct the jobs good and will last years. I have seen so many SWA installs not earthed in anyway and no CPC whatever and additionally not terminated correctly. I replaced SWA's that were downright dangerously installed after just 4 years, no glands fitted, no armour connected and BS 951 clamps used underground for earth continuity. All this was installed by a well respected company in a millionaires driveway lighting. It's not always "you get what you pay for" And the quick answer is if you have to ask you shouldn't be doing it.... should you?

 

[bryanh]

Running a separate CPC is stupid + it is not practical from a protection of the cpc point of view is it and also it looks bad having a G/Y cable strapped to it in a surface installation,

wouldnt it just be better for the companys who make swa to start including a extra core that brings the swa up to and better then it needs to be, we nearly alway run a 3 core in instead of 2 cores and 5 cores just to make sure that we have a great earth it costs a bit more yes but we think its worth it.

 

[somersetsparks]

Bryanh - I don't really get your post, 3 core and 5 core are available just not necessary most of the time. Using the armour as the earth is cheaper and more practical in most cases

 

[bryanh]

The point was some were saying they dont like using the SWA as a CPC, What I was saying was they could include a CPC within the SWA cable just as they do in T&E etc 2.5+1.5 not a full size CPC then you are covered both ways

 

[somersetsparks]

Ahh I understand, however it would be pointless - The armour provides this function, the only time it would be useful is on cables which dont meet the minimum CSA of armourings. I don't understand the covered both ways bit though - You are covered if you install a proper and continuous compliant earth, in most cases the armour provides this, in fact I don't see how people are happy to trust a bit 1.0 cpc in a screw terminal which has probably chewed it down to 0.5mm on a lighting circuit, but frown upon using the mass of armourings, terminated in proper glands? beats me

 

[johnboy6083]

the only time i wouldnt use Armour as CPC was a) i had lots of 3 core spare, b) it was going to be installed in an environment where if the sheath were damaged, excessive corrosion would cause failure iof the Armour as CPC, C) in a TNC-S scenario where i could see the armour carrying diverted neutral currents, i would assess this ti the best of my knowledge and then decide.

 

[KAS1]

Nothing wrong with using the SWA as the cpc, although most companies I've worked for over the years always use a three core for single phase and four core for motor cables so as to use the spare core as earth together with the armour.

 

[aleksandar]

Yes, you can use SWA armour as a CPC. If you use the adiabatic formula you can check if the armouring is suitable or not.

 

[MDJ]

Of course it can be used and will probably be adequate, have a look at the table engineer put up recently, however If I was doing the job I would run a 3 core rather than 2 core and use both the armoured and the third core together, and banjo the ends as well with fly leads to the earth terminals, I really cannot see the problem with this sytem and providing it is allowed for in the fixed price quotation why wouldn't it be a good idea not to use a 3rd core as well as the sheath?

Mike

 

[Engineer54]

Never a good idea to rely on the armour to make up any deficiencey in the size of a 3rd core CPC. Being as there is a big difference in K values and therefore will not divide equally to carry a fault current to operate a protective device. Where a 3rd core is employed as the CPC, it should be of a sufficient size to provide the Zs required, to trip the protective device and carry the full fault current of that circuit.

 

[MDJ]

The third core would probably be adequate in most circumstances of course, the sheath would be connected to it via a fly lead to the banjo for professional reasons, I can't imagine running in an armoured cable and not connecting it to an earthed point, so having both of them connected together at an earthed point would be normal and not to ensure a correct sized cable, as you said the cpc should sufficient in size, but they are bound to be connected together.

 

[SLSpark]

ARMOUR FOR EARTH OF COURSE THATS WHAT ITS THERE FOR(and underground protection etc) IF TERMINATED TO THE CONTAINMENT (trunking) and that should be earthed anyway then you have just made the armour part of the earthing system common practice to run seperate earth along with armour , if actually seen this done with a 5 core armour which gives a 3rd earth overkill maybe but the earth is the most important. ever heard the saying "ITS JUST AN EARTH" without a earth it shouldn't be connected