SWA test results

[sparksburnout]

If the armour is only connected at the supply end then its not a parallel path, and nothing to do with the R1+R2 values. However, it is a good point, that it needs to be of a good enough value to operate the OCPD when someone shoves a spade through the cable?

 

[westward10]

I agree that if you use a separate core as the cpc the armour still becomes a parallel earth or cpc

and what about if the armour is only earthed at the supply end to comply with the fact that it must be earthed, if not used as cpc? it's not a parallel cpc then.

Depending on armour length/csa it should be earthed both ends to prevent any potential difference.

 

[Simonslimline]

With High integrity circuits, you need two separate readings, one for each CPC.
Some people just use the highest reading, others try to squidge two readings into one box and others use a second line for the second CPC.
Not come across anyone using the R2 box for the second CPC, but I guess it could happen.

Thanks. I will ask how they recorded the results in this case.

 

[haptism]

If the circuit was singles in steel conduit with a separate cpcs you would not be considering recording the conduit resistance, this is no different.

Yes thats a good point, didn't think of it like that.

 

[westward10]

Yes thats a good point, didn't think of it like that.

If the design intends the separate core to be the cpc then record that size and it's R2. Check continuity of the armour and forget it, don't over think this.

 

[Deleted member 26818]

You have to remember that the purpose of a CPC is to connect exposed-conductive-parts of equipment to the main earthing terminal.
The armour of an SWA cable, and also earthed conduit or trunking concealed in walls is intended to provide an earth path if the cable or conductors are penetrated by a metallic object.
Sometimes the armour, conduit or trunking is used as a CPC.

 

[Deleted member 26818]

Depending on armour length/csa it should be earthed both ends to prevent any potential difference.

Unless of course it is supplying a TT'd outbuilding.

 

[sparksburnout]

Depending on armour length/csa it should be earthed both ends to prevent any potential difference.

Could you explain that? Potential difference between what??

 

[westward10]

Unless of course it is supplying a TT'd outbuilding.

True but you would have to contain the armour at the TT end.

 

[haptism]

Thanks for the responses it really helpfull and bear in mind that I have not done much testing so not too familiar with the test sheet yet !

 

[westward10]

Could you explain that? Potential difference between what??

A few years back I tested a circuit consisting of a 1.5 4-core SWA using a separate core as the cpc. Earthed at dist board end, from here it went to an external light a considerable distance from the board, terminated at an all insulated enclosure. Zs between the separate cpc and armour was greatly different.

 

[westward10]

True but you would have to contain the armour at the TT end.

No it isn't, record the R2 of the separate core.

 

[telectrix]

A few years back I tested a circuit consisting of a 1.5 4-core SWA using a separate core as the cpc. Earthed at dist board end, from here it went to an external light a considerable distance from the board, terminated at an all insulated enclosure. Zs between the separate cpc and armour was greatly different.

it would as the resistance of the armour will be > the copper core.

 

[sparksburnout]

A few years back I tested a circuit consisting of a 1.5 4-core SWA using a separate core as the cpc. Earthed at dist board end, from here it went to an external light a considerable distance from the board, terminated at an all insulated enclosure. Zs between the separate cpc and armour was greatly different.

Yes it quite possibly was but why is this a problem?

 

[westward10]

it would as the resistance of the armour will be > the copper core.

I know.

 

[westward10]

Yes it quite possibly was but why is this a problem?

The armour and cpc should be bonded together to prevent potential differences under fault conditions

 

[telectrix]

The armour and cpc should be bonded together to prevent potential differences under fault conditions

but they are at the supply end if the armour is not connected at the load end. where would this PD come from?

 

[sparksburnout]

I am a bit confused here westie! Now, I just want to get this straight as I am bit concerned. Taking your example, a remote accessory (or DB) powered from an armoured cable using one of the cores as the CPC. I have to say I have also terminated the armour at both ends but then thought "why the hell am i doing this?" If you just leave the remote end un-terminated then the cable is protected against damage by virtue of the armour being earthed at the origin. Under fault conditions then yes the armour and CPC could be at a different PD but there is no possibility of the armour being able to introduce this? Surely it is better to use the Zs of the CPC provided by the core alone rather than with the armour providing a parallel path, as this might degrade over time?

 

[westward10]

Everything is possible.