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

 

[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?