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Separate main bonding to outbuilding

Former Community Member
Former Community Member
Hi, I just wanted people’s thoughts on this please.

I have been to a job with a TN-S supply. A sub main feeds a separate building with an SWA, the armour being used as the CPC. I have calculated the armour has not the additional cross sectional area to used as the required main bonding. I understand from GN8 that the main bonding needs to be extended to the building as specified in size by BS7671. I realise that this main bond will run from the MET to the Earth marshalling terminal in the second building. My question is this:

The SWA is buried and cannot be dug up (well easily). There is a second duct that runs between the buildings which is empty, but it follows a different route to the SWA. I seem to remember reading that it is best to run a separate bonding cable in close proximity to the existing SWA, am I correct or did I dream this? Is it advisable not to run the additional cable via a different route? If so, my only option will be to TT the second building.


Thanks in advance.


KS
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  • 0
    Ok, you need bonding. Now what are your extraneous parts and could they contribute diverted neutral current? Are they external metal service pipes, water or gas or iron sewers, or are they odd items with a significant resistance to Earth? The size of the main bonding required needs some consideration, as if all these have a resistance of say 100 Ohms to Earth, the diverted current is limited to a low value. Main bonding conductor size is directly related to the potential current. You say the supply is TN-S, this is no longer a reasonable assumption due to DNO policies, repairs to the distribution network may change this at the drop of a hat. What size is your submain cable? You may like to refer to table 54.8, in all cases the SWA armour is large enough for main bonding up to 95mm2. Remember the main bonding conductor(s) never need to be larger than the supply neutral even with PME, and for 100A supplies 10mm2 is normally considered adequate. I am afraid that somehow you have gained the wrong information here, many people do not understand main bonding.
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  • 0
    Ok, you need bonding. Now what are your extraneous parts and could they contribute diverted neutral current? Are they external metal service pipes, water or gas or iron sewers, or are they odd items with a significant resistance to Earth? The size of the main bonding required needs some consideration, as if all these have a resistance of say 100 Ohms to Earth, the diverted current is limited to a low value. Main bonding conductor size is directly related to the potential current. You say the supply is TN-S, this is no longer a reasonable assumption due to DNO policies, repairs to the distribution network may change this at the drop of a hat. What size is your submain cable? You may like to refer to table 54.8, in all cases the SWA armour is large enough for main bonding up to 95mm2. Remember the main bonding conductor(s) never need to be larger than the supply neutral even with PME, and for 100A supplies 10mm2 is normally considered adequate. I am afraid that somehow you have gained the wrong information here, many people do not understand main bonding.
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