Use of SWA armour as protective bonding conductor?

[netblindpaul]

I will in a day or 2 I guess!!!!

 

[SW1970]

Thanks for very interesting posts. So do you think there's enough here for me to take the approach proposed by Spin?

 

[Chr!s]

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.

I think there are several issues here.

Firstly, the ERA report is based on fault current sharing between the armour(cpc) and a parallel cpc. If the third core is used as a cpc then i dont see the ERA report being to much of a help.

The system is TNC-S, so if there are extraneous conductive parts with to be connected we need to consider the use of not just the armour as a bonding conductor but also the use of a core. This is due to the possible heating effect of the diverted neutral current which could cause the insulation to prematurely fail. One way to look at it is if the loading of the cable is low then the heating effects of any neural current will be limited.

If you decide that this is not an issue then we need to decide as Spin has mentioned whether we can use the combined CSA to satisfy the bonding requirements. I'm not aware of a regulation which says this is not an option. If we run two 6mms or a 10mm, i see no issue, the ECP'S are effectively connected to the met via a CSA required by the regs.

Where the Main Protective Bonding conductor is made up of the armour and a core, then in effect the ECP'S are connected to the MET with A CSA required by the regs.

I think this comes down to engineering judgement and what effect the diverted neutral currents may have on the cable, i think the issue is will it comply with Chapter 52?

Regards Chris

 

[SW1970]

I elected to run new 10mm2 bonding cables and have spent the whole day doing it. My decision was based around the semi rural nature of the property, fed underground but overhead to pole transformer not far away. Risk of neutral loss is higher than an urban scenario and that swung it for me. If I was to come up with an analogy, the requirements for these large bonding conductors in TNC-S are like the provision of a protective barrier underneath a bridge in case a section of the bridge was to fall down: it feels like overkill but if the regs (and a strong body of opinion) agree with it, I would be challenging the status quo with my signature on a lesser solution. I have really enjoyed this thread; thanks to all who participated.