swa

[switched on]

I've always used a 3 core armoured so it has its own internal earth. I always earth up the armoured to use the steel wire with a gland and banjo but i worry that if i don't use a 3 core and just a two core and the steel armoured as the earth is this ok?
Obviously safety is the key feature but when can you use the steel and just run a two core?? Just a funny subject i feel. Thanks alot

 

[pevvers]

Check guidance note 1, they have tables in there for all the different types of armoured cable which show which ones satisfy the adiabatic equation. Simples!

 

[switched on]

ideal i will check that out! just wandering if everyone else uses a seperate earth or just the armoured itself?? thanks

 

[pevvers]

I think a lot of people use it as they're not sure whether they can use the armour or not. It's easier to get a three core and use the extra core as your CPC than wonder whether the armour is adequate or not. You're normally stuffed if you're putting in 3 phase supplies though, and have to run the extra CPC, as I've only come across a 5 core once in the last 7 years. That was the only good part of the installation too, the rest was pants!!

 

[Bromd123]

is there a definative annswer on this as my lecturer at college has said that the armouring WOULD be used as the CPC through glanding and banjo's is this right?

 

[allsparkjonny]

there is a table somewhere on the internet that tells you....its to do with the csa and number of cores in the cable itself,you can go to a decent sized cable before the armourings will not actually have a combined csa large enough to be able to be used as an earth and a seperate earth would have to be put in.sorry cant find the table at the moment but hope this helps.

 

[allsparkjonny]

I think a lot of people use it as they're not sure whether they can use the armour or not. It's easier to get a three core and use the extra core as your CPC than wonder whether the armour is adequate or not. You're normally stuffed if you're putting in 3 phase supplies though, and have to run the extra CPC, as I've only come across a 5 core once in the last 7 years. That was the only good part of the installation too, the rest was pants!!

the smaller 3 phase supplies you can use the armourings no problem at all.its only when you get to the bigger stuff the combined csa of the armourings is insufficient.

 

[cornburn]

I was always told it's fine to use, but would need to check with the manufacturer of the cable (or their literature) for an equivalent CSA. For the price difference on new installs I can't imagine it's ever worth it, just use a 3 core and then you've got 2 earths, even better!

 

[Bromd123]

I was always told it's fine to use, but would need to check with the manufacturer of the cable (or their literature) for an equivalent CSA. For the price difference on new installs I can't imagine it's ever worth it, just use a 3 core and then you've got 2 earths, even better!

I suppose it would only start to be a problem when you get to the REALLY big ones (costwise)

 

[pevvers]

the smaller 3 phase supplies you can use the armourings no problem at all.its only when you get to the bigger stuff the combined csa of the armourings is insufficient.

Ah yes, I know this. But many people put in a 4 core, then think that they have to run a separate earth as they don't know that there are tables available to show you whether the armour is sufficient for the CPC.

is there a definative annswer on this as my lecturer at college has said that the armouring WOULD be used as the CPC through glanding and banjo's is this right?

Tables D.9, D.10a and D.10b of GN1, starting at page 147 are where you need to look.

If you're using PVC or XLPE cables up to a max operating temp of 70 degrees C, then you can go up to a 95mm using the armour as the CPC before you have to start running in an extra CPC.

BUT! If you're using XLPE SWA at max operating temperature of 90 degrees C, then things get a bit more interesting. You would not be able to use the armour as a CPC on the following cables: -

2C 10mm
2C, 3C and 4C 16mm
2C 25mm
2C 50mm
2C and 3C 70mm
2C, 3C and 4C 95mm
2C and 3C 120mm
And from 150mm all the way up to 400mm are not allowed.

I hope this helps!

 

[Bromd123]

Pevvers your a legend matey thanks!

 

[benji]

I have often wondered what would happen in a earth fault condition ?would it travel down the armour or would it travel down the additional cpc, but we know that it would go down the path of least resistance so if the SWA can,t carry the fault current the additional cpc must be able to carry the full fault current .Iv,e known very highly qualified elec.designers add a 6.0mm cpc as a suplimentary cpc because due to length of 50mm 4core the armour was just wasn,t big enough add the 6.0mm and it was ok .I could never work out how a fault current would decide which to go down .


yours benji

 

[pushrod]

is there a definative annswer on this as my lecturer at college has said that the armouring WOULD be used as the CPC through glanding and banjo's is this right?

even if you are using 3 core swa with one of the cores as the cpc you still need to earth the armouring at the supply end. ( i believe it is because it counts as an exposed conductor)

So yes the armouring would be earthed through the gland and "banjo" even if it was not being used as the means of earthng.

 

[pevvers]

I've known very highly qualified elec.designers add a 6.0mm cpc as a suplimentary cpc because due to length of 50mm 4core the armour just wasn't big enough

I would tend to disagree with adding the extra 6mm in there for these purposes. I have always been under the impression that if the armour cannot take the full fault current, then the additional CPC should be able to take it all.

I could never work out how a fault current would decide which to go down

The current would split in respect of the differing impedances. More current would flow down the path of least impedance. Just think of resistances in parallel, Ohms law and Kirchoffs theorem... Although it's been quite a while since I've done current in resistors in parallel myself, it's not hard!

 

[Sintra]

Check out the link for swa as cpc.

http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

 

[martysparky]

I would be inclined to think that the additional CPC should carry the whole load.

Reason: if the SWA was severed to a point and angle that the Live was the only conductor intact there would still be a full escape route for the current.

When I did my training mt instructor said that the armour did not need to be connected to the earth bar at ccu end because the 3rd core as cpc would be sufficient.
if a fault current is travelling in the armour why send it away to then return.

Does that make sense or is it too late?

 

[pushrod]

When I did my training mt instructor said that the armour did not need to be connected to the earth bar at ccu end because the 3rd core as cpc would be sufficient.
if a fault current is travelling in the armour why send it away to then return.

Does that make sense or is it too late?

Have been told at college that if you are running swa to a shed that regardless of whether you have a third core for a cpc the armouring should be earthed at supply end. Was told the reason is that there is a theoretical chance that if you didn't a spade/pickaxe etc could puncture the armour and just the line. If the armour was not earthed then the supply would not necessarily disconnect.

 

[pevvers]

The armour needs to be connected to earth as it's an exposed conductive part. This is for both the reason stated above, and to stop induced voltages... One of my first jobs was to put in a load of SWA to supply garages, and I didn't continue the earthing of the armours along the run, and you could get little shocks off it for this very reason.

The question would have to be why WOULDN'T you connect the armour to earth, not the other way around.

 

[pevvers]

if a fault current is travelling in the armour why send it away to then return.

How is the fault current going to return, unless you have another equally low impedance connection to earth at the remote end?!

Current wants to flow to earth, once it's gone to earth it's gone. And there should be only one earth return path for the system, which would all be going through the MET back to the transformer. By either the neutral (TN-C-S), metallic sheath of the cable (TN-S) or earth rod, tapes etc., (TT).

 

[martysparky]

What I meant was that the armour would be connected and earthed through the banjo at the accessory ? adaptable box at the away end. If there was a cause of current nearer the CCU end and the armour was only earthed at the far end it would travel away to the remote end and return via the 3rd core. Away to then return.

Instructors view that if there was a fault current in the armour then it would be earthed via the internal 3rd core as cpc.

I would think it should be earthed at both ends because if the cpc core was severed by a pick ax and the live was still in tact. need I say any more.

Instructors argument was there was nor spare slots in the CCU.
Why not put the 3rd core and armour fly lead from banjo in the same number slot on CCU for that extra safety.

He agreed but still said don't do it??

 

[pevvers]

What I meant was that the armour would be connected and earthed through the banjo at the accessory ? adaptable box at the away end. If there was a cause of current nearer the CCU end and the armour was only earthed at the far end it would travel away to the remote end and return via the 3rd core. Away to then return.

Oh, I get you now. I can see the logic behind it, but with the tiny magnitudes of time we are talking about for the current to get to earth, the fact that it's flowing to somewhere further than the supply wouldn't really be an issue.

I would think it should be earthed at both ends because if the cpc core was severed by a pick ax and the live was still in tact. need I say any more.

If it's not the primary CPC, then there is no need to bond it at both ends, as to get through all the armour in one go with a pick axe without also severing the line conductor at the same time causing a short circuit would be some feat of unluckiness!!

Instructors argument was there was nor spare slots in the CCU.
Why not put the 3rd core and armour fly lead from banjo in the same number slot on CCU for that extra safety.

He agreed but still said don't do it??

I would question his arguments about why not to do it then. If there are no spare slots at the CU, then maybe you would have a spare terminal on your MET which would probably be fairly close. Or if not, get a bigger MET....

 

[martysparky]

pevvers
If it's not the primary CPC, then there is no need to bond it at both ends, as to get through all the armour in one go with a pick axe without also severing the line conductor at the same time causing a short circuit would be some feat of unluckiness!!


Thats my point exactly (not all Armour severed) if the pick ax severed neutral and internal cpc and the armour was not bonded to CCU at the CCU end where is the earth current going to go. It's not goint to go down the armour to thr remote end and return as the internal return path (cpc) has gone in this case unless of course there has been a rod driven. The armour still has a means of earth because only 2 3rds for example is still in tact containing the Live. Forb that reason I think the armour makes a better cpc. But I guess sometimes it has to be topped up with an external and I think if the run took an impact the external would be wiped out before the armour. Thus my reason to always bond armour at CCU end.

So, yes I should have stood my ground I guess..

EDIT: to get to the bottom of it

Thanks, No spare ways and MET

 

[pevvers]

Thats my point exactly (not all Armour severed) if the pick ax severed neutral and internal cpc and the armour was not bonded to CCU at the CCU end where is the earth current going to go.

I think that you would have to extremely unlucky for that to ever happen and leave the line condcutor completely untouched but still uninsulated.

I don't think these sorts of circumstances can ever be covered by regs as they can only look at likely scenarios. If not, we would all be wandering round in 1000V rated rubber suits (kinky! ) every time we touched something electric...

Just my thoughts though. But I would still bond at the CU end every time.

 

[pennywise]

Bond it both ends, you never know someone in the future may break into the run and not complete the continuity of the armoring (only joking) It's always fun having what if moments which i suppose the regs are based on however ridiculous

 

[martysparky]

lovin it!

 

[martysparky]

I think that you would have to extremely unlucky for that to ever happen and leave the line condcutor completely untouched but still uninsulated.

I don't think these sorts of circumstances can ever be covered by regs as they can only look at likely scenarios. If not, we would all be wandering round in 1000V rated rubber suits (kinky! ) every time we touched something electric...

Just my thoughts though. But I would still bond at the CU end every time.

Double bondatration and a rubber suit kinky???


Is it that unlikely really and I suppose it depends on the impact of the blow and what makes it

 

[pevvers]

Is it that unlikely really and I suppose it depends on the impact of the blow and what makes it

I reckon it's incredibly unlikely.

If you think you could get a pick axe through a cable, and manage to short the line against the armour and before the current gets round the earth fault loop path and trips the protective device you've cut through everything but the live. I don't think it can be done, as electric is pretty darn quick!

 

[martysparky]

I was wondering when someone would mention the protective device.

It would be even more incredible if the was malfunctioning

 

[pevvers]

If you ever come across a situation this unlikely, you must be the unluckiest man since Frank Spencer...

You just can't write regs for that kind of situation! MCB as far as I am aware rarely fail, if they do it's because they trip to easily, RCD's fail, but there would still be a MCB or fuse to back it up.

 

[martysparky]

How many times do you hear the expression 'Drove a rod in for good measure'.

Without checking I thought, thats something to do with the regs instructing a remote building becoming a TT system.


Is there any reason why the armour should not be earthed both ends?

 

[martysparky]

How many times do you hear the expression 'Drove a rod in for good measure'.


Is there any reason why the armour should not be earthed both ends?

apart from what has already been stated

 

[pushrod]

Is there any reason why the armour should not be earthed both ends?

If you are exporting an earth with 2 core SWA there would be nothing to attach it to at the earth rod end.

 

[kevben]

I've got quite a few submains to install soon (25mm 3 core SWA) and have always glanded and banjo'd at both ends. I usually terminate the gland through a metal adaptable box before entering CU.This is unsightly and usually means the builder needs to build a cupboard to conceal equiptment.

Can i just tape up the armouring at the CU without glanding as it would be bonded at the Origin of the supply through a Switch Fuse (100A) ?
If NOT can i install adaptable box with gland termination inside a stud wall ,behind CU,(wouldn't be accessible after plastering)?
Would make my life a lot easier if there is no problem with either question.

 

[martysparky]

Connections & Terminations

Connections must not be subject to any mechanical stress or strain. 526.6, 530.4.1
All screw connections must be accessible. 526.3 Through crimp connections do not have to be accessible. 526.3
Stranded cable ends must be fitted with appropriate terminals or suitably treated. 526.8.1
Every termination, connection or joint between live conductors including the neutral conductor must be made in a suitable accessory or enclosure. 421.7, 526.5

 

[kevben]

Thanks Marty, Not sure if that has answered my questions. I guess what you are saying is that option 2 would be ok , as i was proposing to just terminate the gland into the metal box and install the cores into the CU through the back with No joints.

 

[pushrod]

Marty, in reponse to an earlier question about earthing both ends of the swa have a read thru reg 542.1.8. Basically, if there are separate earthing arrangements the cpc must be able to carry the max fault current or be earthed at only the end with the protective device.