Earthing/Bonding advise please

Main supply to house is overhead assumed to be TNS due to the supply cable being Split Concentric....

Assume nothing. To be TN-S the cables would have to have separate N and PE all the way back to the transformer - and even if that's the case now there's little guarantee it won't get changed in the future as the DNOs have a nasty habit of linking N and PE when making changes. Even in new systems they sometimes run split-con to individual customers (especially in multi-occupancy buildings) from the TN-C-S main - in an attempt to reduce diverted N currents. Unless there are specific guarantees, these days the normal policy is to treat any supplier's earthing facility as if it were PME.

 

I have a supply cable to an out building, currently in 16mm T&E

OK so only a 6mm² protective conductor - probably not enough for a main bonding conductor, certainly not for PME.

 

The pipe currently has no bonding.

OK, a few options there - the most common approach is probably to "TT" the outbuilding - i.e. earth via a local electrode (itself usually only around £5 to buy) instead of the cable's c.p.c. and make sure everything is covered by RCDs (usually would be anyway these days) then just bond the pipe to the outbuilding's CU. The TT approach also has advantages if the nature of the outbuilding makes it less than ideal for PME earthing - e.g. a non-insulating floor (concrete without a d.p.c. for example) or steel cladding or plans to supply and outdoor an EV charge point.


Or replace the pipe with a plastic one - if only for a metre or two where it enters the building - so designing out the bonding problem altogether (presuming there are no other extraneous-conductive-parts). The protective conductor in the supply cable then only need to fulfil the requirements of a c.p.c. and so the existing 6mm² would be fine.


Replacing the cable is likely to be the most expensive and (if buried) the most effort - but technically can be a good solution. One with a 16mm² Cu protective conductor should be fine for both a c.p.c. and a main protective bonding conductor - the requirement is normally only for 10mm². There is a cautionary note to using a conductor within a cable as a protective bonding conductor, especially on PME systems, in that it can carry some current (diverted N) in normal circumstances - which like any current will warm the cable and so potentially reduce its overall current carrying capacity. But provided you're not loading the cable to within an fraction of its maximum capacity there's usually not a problem in practice - if the (single phase) load is less than the 3-phase rating of the cable you'll likely to have a generous safety margin.


Another option, if the existing cable is still suitable (not buried for example) is to run an additional green/yellow conductor alongside - a single 10mm² (jointed to the existing c.p.c. at both ends) would be more than adequate.


  - Andy.

Thanks for you time Andy, much appreciated.


The old T&E is unsightly as it runs around the middle of the house, I am clipping the SWA around the bottom of the house and a short underground run (about 1.5 meters) under some flags to the out building.  Also the cable was selected with a good safety margin.

I'll stick with this idea as I have the materials and not a fan of TT.

Not an option to replace pipe for plastic as it comes in under a concrete floor.

Is it common to find unbonded extraneous conductors? 


Another question, what size should the links be from the SWA glands to the earth terminal?


Thanks again.

Is it common to find unbonded extraneous conductors?

Maybe not common these days, but certainly far from unknown.

 

Another question, what size should the links be from the SWA glands to the earth terminal?

Slightly tricky that - if you want to avoid any nasty calculations about how any fault current is going to divide between the PE core and the armour, the simplest thing is to assume the same size as the whole of the minimum required for its various functions - as a c.p.c. you can either calculate it (based on worst combination of Zs, the protective device and  material) or size it the same as the line conductor, as bonding conductor it would have to be at least 10mm². In that it's a 16mm² cable and crimping on a 16mm² lug isn't any harder than a 10mm² one and you'll probably have a 16mm² offcut I'd probably just do it in 16mm² and have done with it.


  - Andy.

Simples ??