Hot tub - 2 core or not 2 core

[JohnW2]

I always thought the IET or someone like that had calculated that the armour of 2 core is always suitable unless you delv into larger size swa ....

I'm not sure what you mean by suitable ....

.... with any type of cable, there is (in relation a particular installation) always going to be a length beyond which a cable of a particular CSA does not result in a low enough Zs for ADS. That's no less true of SWA than any other type of cable, but with SWA (using armour as CPC) there is the added complication that if Zs is 'too high' (i.e. cable too long), not only will the requirements for ADS not be satisfied in relation to L-CPC faults in the load, but also L-armour faults (e.g. garden fork through it!) in the more downstream parts of the cable - in other words, the 'earthed armour' will not do its job properly (won't necessarily result in disconnection as quickly as the regs require).

Kind Regards, John

 

[EFLImpudence]

but with SWA (using armour as CPC) there is the added complication that if Zs is 'too high' (i.e. cable too long), not only will the requirements for ADS not be satisfied in relation to L-CPC faults in the load, but also L-armour faults (e.g. garden fork through it!) in the more downstream parts of the cable - in other words, the 'earthed armour' will not do its job properly (won't necessarily result in disconnection as quickly as the regs require).

I am still not sure why you keep saying "if Zs is too high" - because it is armour.

If Zs is too high, then the circuit is not compliant - the same as any other circuit.

 

[333rocky333]

some time back it became the trend to run a seperate external earth core with 2 core swa, tests were done comparing the resitivity of the armour compared with using a 3 core with one used as earth, and in a majority of cases the armour was nigh on the same as the copper core, i assume because of the amount of steel.
Obviously theres a limit to the length as you say whether copper, ally or steel.
How i read the proceeding posts especially Winstons was that Armour is NEVER suitable and that 3 core MUST be used and that he now has to dig it up

 

[JohnW2]

I am still not sure why you keep saying "if Zs is too high" - because it is armour. .... If Zs is too high, then the circuit is not compliant - the same as any other circuit.

Quite so. That's why I just wrote ...

.... with any type of cable, there is (in relation a particular installation) always going to be a length beyond which a cable of a particular CSA does not result in a low enough Zs for ADS. ....

My point was that if one is using armour as the CPC, 'too high a Zs' is, in some senses, 'doubly-non-compliant' - not only will ADS inadequately protect exposed-c-ps of the load(s), but the 'earthed armour' of the SWA will also not (adequately) achieve what it is meant to achieve.

When I mentally agreed with everyone who was saying that an on-site electrician was needed to make sure that the cable (using armour as CPC) was 'suitable', I was thinking in terms of ensuring that, with that cable, used in that way, the circuit's Zs would be compliant (i.e. low enough for ADS). Was there something else that you feel would need to be 'checked' (as I said before, I think that adiabatic calculations are probably a bit of a red herring in this situation).

Kind Regards, John

 

[JohnW2]

How i read the proceeding posts especially Winstons was that Armour is NEVER suitable and that 3 core MUST be used and that he now has to dig it up

As I have acknowledged, there are some people who believe that one should not rely on armour as the only CPC, not because of considerations of resistivity or calculations, but because an armour CPC is more likely to fail (due to corrosion) than a CPC which is a core.

Kind Regards, John

 

[333rocky333]

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

Found the table here

 

[JohnW2]

some time back it became the trend to run a seperate external earth core with 2 core swa, tests were done comparing the resitivity of the armour compared with using a 3 core with one used as earth, and in a majority of cases the armour was nigh on the same as the copper core ....

'Tests' are not really required - look in our famous wiki (click here) . That shows, for example that the armour of 10mm² 2-core XPLE is not necessarily (needs an adiabatic calculation) adequate as a CPC.

I don't think it's just about resistance/resistivity - different materials behave differently when heated, so things like melting point are presumably relevant, and I presume that the 'k' values for different materials take that into account.

Kind Regards, John

 

[JohnW2]

http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdfFound the table here

Ah - so that's where the wiki 'stole' the tables from

Kind Regards, John

 

[EFLImpudence]

My point was that if one is using armour as the CPC, 'too high a Zs' is, in some senses, 'doubly-non-compliant' - not only will ADS inadequately protect exposed-c-ps of the load(s), but the 'earthed armour' of the SWA will also not (adequately) achieve what it is meant to achieve.

That's what I don't understand. What else it is meant to achieve?

When I mentally agreed with everyone who was saying that an on-site electrician was needed to make sure that the cable (using armour as CPC) was 'suitable', I was thinking in terms of ensuring that, with that cable, used in that way, the circuit's Zs would be compliant (i.e. low enough for ADS). Was there something else that you feel would need to be 'checked'

No, I don't know what the difference is between the two thing you are considering.

(as I said before, I think that adiabatic calculations are probably a bit of a red herring in this situation).

Why are they red herrings?
Do you mean that the armour is large enough without needing to check?

 

[EFLImpudence]

That shows, for example that the armour of 10mm² 2-core XPLE is not necessarily (needs an adiabatic calculation) adequate as a CPC.

PVC is a lot bigger.

10mm² T+E only has a 4mm² copper CPC.
The XPLE has an armour copper equivalent of 8.36 and the PVC 17.7.

 

[JohnW2]

No, I don't know what the difference is between the two thing you are considering.

I'm only considering one thing - that an electrician needs to confirm (or otherwise) that the circuit would be compliant, Zs- wise, using 2-core 10mm² SWA. As I asked, were you (and others) thinking of something else that he/she [you don't seem to like my usual "(s)he" !] needs to check?

Why are they red herrings?

Probably a bad choice of phrase on my part. I really meant that I doubted that it would be the limiting factor.

Kind Regards, John

 

[JohnW2]

PVC is a lot bigger.

Indeed it is.

10mm² T+E only has a 4mm² copper CPC. The XPLE has an armour copper equivalent of 8.36 and the PVC 17.7.

Are those 'copper equivalents' in terms of just resistivity? If so, as I recently wrote, the 'k' values for a steel sheath and a copper core are very different.

Kind Regards, John

 

[EFLImpudence]

I'm only considering one thing - that an electrician needs to confirm (or otherwise) that the circuit would be compliant, Zs- wise, using 2-core 10mm² SWA.

Of course - as with any cable - but you seem to be making some distinction because it is swa.

As I asked, were you (and others) thinking of something else that he/she needs to check?

No, but you were:

My point was that if one is using armour as the CPC, 'too high a Zs' is, in some senses, 'doubly-non-compliant' - not only will ADS inadequately protect exposed-c-ps of the load(s), but the 'earthed armour' of the SWA will also not (adequately) achieve what it is meant to achieve.

[you don't seem to like my usual "(s)he" !]

Neither method is necessary.
If you must, then do as Bas and just use 'she'; the men don't care.

 

[EFLImpudence]

Are those 'copper equivalents' in terms of just resistivity? If so, as I recently wrote, the 'k' values for a steel sheath and a copper core are very different.

No, they are temperature related for ADS. K2/K1 x csa

That's why larger csa is needed for Bonding Conductors.

 

[JohnW2]

Of course - as with any cable - but you seem to be making some distinction because it is swa.

No - as I have said (and 're-said') my comments apply to any type of cable.

The implication of what some people were saying seemed to be that using the SWA armour might result in the circuit having a non-complaint Zs. Without knowing the length of the cable and other components of the total Zs, we obviously cannot know if that would be the case - but, if it were, then putting a CPC core in parallel with the armour would obviously reduce the Zs, quite possibly to the extent of achieving compliance.

Kind Regards, John