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

[Amp David]

Show me the calcs Dave!

Seriously though lately i've been trying to gen up on using the adiabatic more lately as like you say the cost can differ massively on some jobs

Cheers

 

[Guest123]

Hello everyone just been reading this thread and i find it very interesting......

I must admit i´m a little surprised that only one person has mentioned using the adiabitc equation for calculating the size of a cpc... It doesnt take long and i have found i have never needed to run a seperate cpc when making calculations for distribution cables. As a general rule though, if the cable is to be run outside or where other external influences are present then i always run a copper earth (not a seperate single core cable, an extra internal core) due to corrosion.

I undertook the 2391-20 design course where a lot of cable calculations were made and i always used the adiabatic, as by using the table at the start of this thread you will generally find that you run in an earth that frankly is oversized. If you want to price a job competitively (im talking larger commercial, industrial jobs) then using the adiabatic is alot more accurate, if were talking about large SWA cables from say 95mm upwards you will find a seperate earth costs a fair bit... 50mm BS6491x say 150m.... NOT CHEAP! a simple calc.. oh its not required so why bother....?

If anyone wants a bit more clarity on this like exactly how the calculation is made (can show examples) then let me know as i would be more than happy to help....!

cheers Dave

An SWA will still have it's armour though.....which is what the thread was started about.

 

[Dave_]

hi Lenny! or Hola! (living in Barcelona! dont ask about the electrics over here, was just running my finger along my missus parents very shiny metal cooker hood today and i could feel a lovely 50hz... what do you mean earthing? ha ha.....)

I dont really understand what you mean by an SWA having its armour.... of course it will....? can you explain what you mean? bit lost as usual... ha ha!

cheers my friend

Dave

 

[Dave_]

Show me the calcs Dave!

Seriously though lately i've been trying to gen up on using the adiabatic more lately as like you say the cost can differ massively on some jobs

Cheers

Yeah of course no probs mate... Ill dig out some calcs from my project on distribution circuits and put up some nice detailed examples..... Watch this space....

 

[Deleted member 26818]

I'm supprised that no one has mentioned that running a separate CPC contravenes BS7671.
Specifically Regulation 521.5.2.

 

[Dave_]

Hello again sparkies, here is a snippet of my project submitted for 2391-20 qualification regarding minimum size of CPC, proving the armouring is sufficient....

Minimum Size of CPC

Here I will calculate the minimum size of CPC in accordance with regulation 541.1.3 of BS7671.
Primarily I will need to calculate the minimum size of CPC (or confirm the armouring is adequate) of the distribution cables. I already know that the external loop impedance value is 0.08Ω, so I will now focus on the distribution cable to the distribution board, and secondly the supply cable between the suppliers cut-out and the CCU.
I have used data from AEI cables (this can be found in the index) to ascertain the values for conductor and armour resistance.
Distribution circuit to DB3
I will use the equation: Zs = Ze(R1+R2)
Ze = 0.08
R1 = 0.342 mΩ/m (at 90°C, i will not need to apply a correction factor of 1.28)
R2 = 1.2 mΩ/m (armour resistance)
Circuit length = 24m
Therefore: 0.342 + 1.2 = 1.542 mΩ/m × 24 = 37mΩ or 0.037Ω
0.037 + 0.08 = 0.117Ω
I can now calculate the fault current using the following equation:

If= Uo/Zs
Where If = fault current, Uo = line voltage to earth and Zs = total loop impedance
If= 230/0.117=1966A
Using Amtech software at work I have sourced the relevant time current graphs for SQUARE-D BSEN 60947-3 MCCB’s which I will use in the installation (I will go into more detail in part 7, time current graphs can be found in the index)
Using time current graph number 1: t = 0.1s @ 1966A
This is perfectly acceptable, maximum disconnection time = 5 seconds (over 32A)

I will now use the adiabatic equation below to confirm that the armouring of the cable can withstand the level of fault current:
S= √(I²×t)/k

Values for k can be found on p129 BS7671 (in this case table 54.4, the armour of a cable)

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.


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!!!

 

[Deleted member 26818]

Here's a link to a site with relavant CSAs for both 70 degree and 90 degree armours:
steel wire armour as the earth conductor
The site has a number of downloads that may be usefull.

 

[IQ Electrical]

I'm supprised that no one has mentioned that running a separate CPC contravenes BS7671.
Specifically Regulation 521.5.2.

Indeed it does however IEE Guidance Note 8 (9.3.4) describes the procedure for sizing 'a separate green-and-yellow covered copper conductor'

While the Guidance Notes are not BS7671, they are published by the same organisation and the whole series should be treated with the same respect afforded to Guidance Note 3.

 

[Deleted member 26818]

I understand that the amendment out later this year may allow an exception to this Regulation for SWA, although nothing's definate yet.

 

[IQ Electrical]

These requirements do not preclude the use of an additional protective conductor in parallel with the steel wire armouring
of a cable where such is required to comply with the requirements of the appropriate regulations in Chapters 41 and 54. It​

is permitted for such an additional protective conductor to enter the ferrous enclosure individually.

We live in hope!

 

[Deleted member 26818]

I've always found it a bit odd, not allowed a separate CPC, but are allowed a supplementary bonding conductor.
As if the bonding conductor wouldn't carry any of the fault current.

 

[andyb]

I'm supprised that no one has mentioned that running a separate CPC contravenes BS7671.
Specifically Regulation 521.5.2.

Where does that reg state that you can't run a separate cpc.

It's referring specifically to single core armoured cable in an ac circuit.

 

[Deleted member 26818]

Where does that reg state that you can't run a separate cpc.

It's referring specifically to single core armoured cable in an ac circuit.

It doesn't just refer specifically to single core armoured cable.
If you read more than the first sentance you would note that it also refers to conductors in ferromagnetic enclosures, and that with an a.c. circuit; all Line conductors, the Neutral and the appropriate protective conductor have to be contained in the same enclosure.
Now unless you are suggesting that SWA is not ferromagnetic, and the armour does not meet the definition in Part 2 of an enclosure, then the Regulation applies.

 

[andyb]

I take your point sinlondon but I would see the armour as mechanical protection and not the enclousure. Also the cpc does not carry current in normal use so there would not be a problem.

It's not that clear is it, but I would say the words 'conductors of an ac circuit installed in a ferromagnetic enclosure suggests that the regs are referring to cable in trunking or conduit etc.

 

[Deleted member 26818]

Unfortunately whether the intention was to include SWA or not, the actual wording entails SWA is included.
SWA is ferromagnetic and meets the definition of an enclosure in Part 2, because it provides mechanical protection.
I believe IQ Electrical in post 40 has quoted the wording from the proposed amendment.
As you can see this is intended to remove this requirement from applying to SWA.
As yet the proposed amendment content and wording has not been finalised to my knowledge.

 

[IQ Electrical]

Yes, Spinlondon is right, ridiculous contradictions between BS7671 and Guidance Note 8.

The original reason for the reg was to nul circulating currents, following it to the letter would preclude the use of conventional pvc/pvc switch drops where the neutral would not be present.

 

[andyb]

I think you are both reading to much into this.
Running a separate earth is an industry wide practice that has been in use for generations. The reg you're quoting is written for single swa cable & ferromagnetic enclosures. This reminds me of the 15th edition when it came out and everyone bonding any piece of metal in the home.

As regards a switch drop, a length of conduit with a live & switch live in it has no current flowing through it in normal circumstances so there would not be a problem.

 

[IQ Electrical]

Andy, don't think that I'm saying 'don't run a separate cpc with swa' that's not the case, I'm merely agreeing with Spinlondon on the interpretation.

 

[andyb]

I know IQ, as I said it's not clear. I just think that in this instance the IEE are not meaning the armoure as an enclosure as stated. Which is why they say single armoured cable & ferromagnetic enclosures. Otherwise they would not say single core armoured, as it would apply to all armoured.
(if that makes sense)

 

[Deleted member 26818]

I know IQ, as I said it's not clear. I just think that in this instance the IEE are not meaning the armoure as an enclosure as stated. Which is why they say single armoured cable & ferromagnetic enclosures. Otherwise they would not say single core armoured, as it would apply to all armoured.
(if that makes sense)

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.

 

[andyb]

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.

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.

 

[Amp David]

Thought you could use it for bonding as long as the CSA checked out to be large enough.

 

[Deleted member 26818]

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

 

[andyb]

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.

 

[Deleted member 26818]

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.

 

[Dave_]

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

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.....

 

[Dave_]

ps when you say 'electrical conductivity' do you simply mean resistance?

 

[kmcgold]

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)

 

[Dave_]

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)

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....

 

[kmcgold]

(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...