Sub Main and seperate earth

[MDJ]

If I'm running two core 25mm SWA as the submain with a 16mm (non-armoured) earth seperately then will the earth require its own mechanical protection?

Also as regards earthing the armour wire - will 6mm (from the tag to the earth block) be sufficient as it now counts as extraneous metalwork?

Nothing wrong with it, but I use 10mm2 ---16mm2 TBH for the banjo link.

 

[D Skelton]

I was always taught to earth the armour at the supply end, no matter how it was installed. Obviousley, if the armour is being used as a cpc, then both ends. So, although it may not need to be earthed, I always do and I thought that was the norm??

It is the norm lol, I'm not saying it isn't, just that the norm isn't neccessarily required by regulation. All I did was explain the facts regulatory speaking, not neccessarily what we should do as professional sparks.

I would like to think that everyone here adhears to the norm, I include myself in this, but... Imagine an EICR scenario, you couldn't just immediately code an unearthed sheath as a C2 or even a C3 without a bit of thought because strictly speaking, there may be nothing wrong with it regulation wise! Maybe they just ran out of NYY and were to lazy to buy more lol.

 

[D Skelton]

So in effect your saying the armour of cable is 'not' an exposed conductive part?.
Well I neva lol

Yep lol

I meant an external fault on the cable...... as in a spade going through it lol.

Well the only risk of that is if the cable is buried, in which case as I have said three times now, there is a specific requirement to earth buried SWA!

If it aint buried, unless you are a complete moron, you're not going to be digging through your wall mounted SWA are ya haha!

 

[Outspoken]

For 25mm² cables the armouring needs to have a csa of 70.09mm² to be compliant.

So use the adiabatic equation, as already mentioned above but others, to calculate the minimum csa of your cpc and see where that leaves you..

The big question of course is the disconnection times...depending on the length of the sub-main will you achieve the required disconnection times for the circuit in the event of a fault.

I appreciate that in 98% of cases the armouring will meat all your needs, but if it is a long run it may be worth doing the calcs to confirm..

 

[44ALLAN]

If it aint buried, unless you are a complete moron, you're not going to be digging through your wall mounted SWA are ya haha!

I once called my mates sister a right slapper ......not realising she was standing behind me !
Does that count in the moron stakes? :sad3:

 

[D Skelton]

Naa, that makes you more of a legend in my book lol!

 

[Engineer54]

For 25mm² cables the armouring needs to have a csa of 70.09mm² to be compliant.

So use the adiabatic equation, as already mentioned above but others, to calculate the minimum csa of your cpc and see where that leaves you..

The big question of course is the disconnection times...depending on the length of the sub-main will you achieve the required disconnection times for the circuit in the event of a fault.

I appreciate that in 98% of cases the armouring will meat all your needs, but if it is a long run it may be worth doing the calcs to confirm..

For a 25mm SWA cable, the SWA needs to have a min CSA of 36.1mm to comply with the appropriate table in BS7671. A 25mm 2 core SWA cable will generally have SWA CSA of 60mm. This is for use as the CPC only, Where bonding is required on a PME installation, that CSA of 60mm of steel, will not comply!!

The armouring will only meet all your needs 98% of the time on a TN-S/TT installation, not on a PME installation where your extending the equipotential zone, as far as bonding requirements go!!

 

[Outspoken]

For a 25mm SWA cable, the SWA needs to have a min CSA of 36.1mm to comply with the appropriate table in BS7671. A 25mm 2 core SWA cable will generally have SWA CSA of 60mm. This is for use as the CPC only, Where bonding is required on a PME installation, that CSA of 60mm of steel, will not comply!!

The armouring will only meet all your needs 98% of the time on a TN-S/TT installation, not on a PME installation where your extending the equipotential zone, as far as bonding requirements go!!

I think the last part of my post covered that by simply advising to do the calculations.....Also, I am unconvinced that the CSA of the steel armouring is as big as 60mm², perhaps the overall cable may have that as a csa, but the steel will be significantly smaller, although I admit I have never measured it so stand to be corrected on my last thought.

 

[Engineer54]

For your convenient information Outspoken....



Blue = min CSA of cables SWA to comply.

(Red) = Cable does not comply.

Table for 70[SUP]0[/SUP]C Thermoplastic PVC SWA cables.

Conductor CSA​	Minimum CSA of SWA to meet 54G​	CSA of armour 2 core​	CSA of armour 3 core​	CSA of armour 4 core​
1.5​	3.4​	15​	16​	17​
2.5​	5.7​	17​	19​	20​
4​	9.0​	21​	23​	35​
6​	13.6​	24​	36​	40​
10​	22.6​	41​	44​	49​
16​	36.1​	46​	50​	72​
25​	36.1​	60​	66​	76​
35​	36.1​	66​	74​	84​
50​	56.4​	74​	84​	122​
70​	79.0​	84​	119​	138​
95​	107.2​	122​	138​	160​
120​	135.3​	(131) ​	150​	220​
150​	169.2​	(144) ​	211​	240​
185​	208.6​	(201) ​	230​	265​
240​	270.6​	(225) ​	(260)​	299​
300​	338.3​	(250) ​	(289)​	(333)​
400​	403.9​	(279) ​	(319)​	467​

Table for 90[SUP]0[/SUP]C Thermosetting SWA cables operating at 70[SUP]0[/SUP]C.

Conductor CSA​	Minimum CSA of SWA to meet 54G​	CSA of armour 2 core​	CSA of armour 3 core​	CSA of armour 4 core​
1.5​	3.4​	16​	17​	18​
2.5​	5.7​	17​	19​	20​
4​	9.0​	19​	21​	23​
6​	13.6​	22​	23​	36​
10​	22.6​	26​	39​	43​
16​	36.1​	41​	44​	49​
25​	36.1​	42​	62​	70​
35​	36.1​	62​	70​	80​
50​	56.4​	68​	78​	90​
70​	79.0​	80​	90​	131​
95​	107.2​	113​	128​	147​
120​	135.3​	(125) ​	141​	206​
150​	169.2​	(138)​	201​	230​
185​	208.6​	(191) ​	220​	255​
240​	270.6​	(215) ​	(250)​	289​
300​	338.3​	(235) ​	(269)​	(319)​
400​	451.0​	(265) ​	(304)​	452​

 

[Engineer54]

I think the last part of my post covered that by simply advising to do the calculations.....Also, I am unconvinced that the CSA of the steel armouring is as big as 60mm², perhaps the overall cable may have that as a csa, but the steel will be significantly smaller, although I admit I have never measured it so stand to be corrected on my last thought.

Taking a look at the tables you could be right, as most SWA cables these days are of the thermosetting insulation, so a modern 25mm cable will have a CSA of SWA of 42mm, not the 60mm of the thermoplastic variety!! lol!!