Use of SWA armour as protective bonding conductor?

[SW1970]

Hi folks, attempting to fix a screw up by previous installer. TN-C-S domestic property with garage outbuilding. MET is in consumer unit in main house; garage distribution circuit protected by RCBO. Garage sub board populated with MCBs. Cable to garage is 6mm2 SWA 3 core. Garage has extraneous conductive parts. Equipotential bonding back to the relevant board exists in main house and in garage.

Here's the problem. The armour is not terminated to the MET in the main house but is terminated to the earth marshalling terminal in the garage sub board. The third core is used as the CPC for the distribution circuit to the garage. Table 54.8 requires a 10mm2 for the main protective bonding conductor. It's a posh property and adding in a parallel 10mm2 earth cable is going to be tricky. (I know I could add a rod but trying to avoid that just now). What I'd love to do is terminate the SWA armour properly and use it as the protective bonding conductor. Assuming that the SWA is BS3646, the equivalent CSA of the armour is 15mm2 (data from earthingnuts website) so it is of adequate size.

Does the above approach look ok? Do the regs restrict use of armour as protective bonding conductor? Many thanks.

 

[Deleted member 26818]

The CSA of the armour does not have a conductivity equivalent to 15mm² copper.
What you are looking at in the earthing nuts website, is the thermal withstand equivalence based on the k2/k1 values.
The conductivity equivalence between copper and steel is about 9:1.
So you would have to divide the CSA of the armour by 9 to establish the conductivity equivalence.
Depending on what cable you have that would give an equivalence of 4mm²(PVC) or 2.5mm²(XLPE).

 

[Engineer54]

If i've read this post correctly, then you have said that the main equipotential bond in the garage is already in place, therefore you don't need the 3rd core in your SWA, to perform as your main bonding conductor, just your distribution cables CPC!!


Doesn't matter if the SWA is connected to the EMT or the MET , as they should both be connected together via the 3rd core of your 6mm SWA cable....


QUOTE: Garage has extraneous conductive parts. Equipotential bonding back to the relevant board exists in main house and in garage.

 

[Deleted member 26818]

If i've read this post correctly, then you have said that the main equipotential bond in the garage is already in place, therefore you don't need the 3rd core in your SWA, to perform as your main bonding conductor, just your distribution cables CPC!!


Doesn't matter if the SWA is connected to the EMT or the MET , as they should both be connected together via the 3rd core of your 6mm SWA cable....

Whilst bonding is in place in the garage, the conductor used to connect back to the MET in the house is only 6mm².

 

[Engineer54]

Whilst bonding is in place in the garage, the conductor used to connect back to the MET in the house is only 6mm².

Not sure what he means Spin, if he is talking about the SWA providing the bonding, or the bonding was in place originally, ....ie part of the original house install??

 

[SW1970]

Many thanks guys. Yep, bonding from services in garage back to sub board in garage is ok, but I'm missing the all important 10mm2 from garage back to the MET. It's last chance saloon I think then, rod or parallel cable, unless any other offers on the table?

 

[Engineer54]

Many thanks guys. Yep, bonding from services in garage back to sub board in garage is ok, but I'm missing the all important 10mm2 from garage back to the MET. It's last chance saloon I think then, rod or parallel cable, unless any other offers on the table?

Ah ...sorry i must have misread your OP then!! ... In that case you really will need to get a 10mm from the EMT back to the MET, and there is really no way around that i'm afraid!!!

 

[Engineer54]

By the way, is this garage totally detached from the main house or is it an attached garage??

 

[Deleted member 26818]

How old is the installation, is it pre 16th edition?
Is the SWA PVC or XPLE?
If I remember correctly, the minimum CSA required in the 15th for a main bonding conductor was only 6mm² for PME, as such I would apply a code 4 if there are no signs of heat damage.
If the SWA is PVC, then the combined CSA equivalence of the armour and the 6mm² CPC will be 10mm².
I must admit, that I don't like the fact that the armour is not earthed at the supply end.
I would check the Zs reading for the armour at the supply end.

 

[SW1970]

Not sure of original install date - did the requirements for the 10mm2 bonding conductor we see now in table 54.8 exist in the 16th as well? If they did, client might be able to claim from a scheme operator. Many thanks.

 

[SW1970]

Thanks - it's a completely detached garage. It's PVC SWA. Spin - can I use the armour and CPC combined in some way?

 

[Deleted member 26818]

I'm not 100% sure when the requirement for 10mm² bonding came into force.
Definately there in 2004.
It may have been when ESQCR 2002 was introduced.

 

[Deleted member 26818]

Thanks - it's a completely detached garage. It's PVC SWA. Spin - can I use the armour and CPC combined in some way?

Not ideal, but the CSA of the armour and the CPC would meet the requirement for 10mm².
You'd have to make a note of it on the Certificate.
Something along the lines of 'utilising the SWA armour and the 6mm² CPC to meet the requirements of 544.1.1 to provide a 10mm² CSA Protective Bonding Conductor'.

 

[SW1970]

So do you mean:

- combine the 6mm2 third core with the equiv 4mm2 armour to give a 10mm2 protective bonding conductor
- the other 2 cores remain as L and N
- terminate SWA properly at boths ends and extend to MET (main house) and EMT (garage)
- in the garage, the equipotential bonding from the services should run to the EMT (are in fact already in place)

This is what I had in mind with the original post (yes, really), but wasn't sure if it's a goer or not. Not sure how the distribution circuit CPC is catered for then?

 

[Guest123]

can I use the armour and CPC combined in some way?

I don't believe this arrangement would be permissible as the conductor needs to be large enough to carry the fault current on it's own. This is why when installing an additional earthing conductor along with a 4 core SWA for example, we don't size one to use in addition to the armour but size it to carry out the function itself.

 

[Deleted member 26818]

Yes that's exactly what I'm suggesting.
To my mind, a conductor can be both a bonding and a CPC at the same time.
Just waiting for someone to disagree and state that the 10mm² has to be a single condutor, or that both conductors have to have the same CSA or something.

 

[Guest123]

Yes that's exactly what I'm suggesting.
To my mind, a conductor can be both a bonding and a CPC at the same time.
Just waiting for someone to disagree and state that the 10mm² has to be a single condutor, or that both conductors have to have the same CSA or something.

Lets have your reasoning then.

 

[SW1970]

Thinking that the protective bonding conductor may have to carry large fault current in the event of failed incoming neutral or neutral of the distribution circuit. In this case, the distribution circuit is protected by RCBO so it should trip anyway?

 

[Deleted member 26818]

Lets have your reasoning then.

I was assuming that the 6mm² CPC was adequately sized.
With PME, the CSA is chosen so the conductor can withstand diverted neutral currents for long periods of time, rather than earth fault currents for short periods of time.

 

[netblindpaul]

There has been research and testing provided to the "industry" that goes along with the idea that the fault current will be shared acceptably by an internal core & the SWA, & / or an external cpc, the internal core & the SWA.
So you can do any of the above.
Also if you check GN1 you will get armour csa's for compliance with table 54.7, from this you should be able to assess your suitability.

Also you can use the adiabatic to size bonding conductors.