Column Earthing Arrangements, and Lightning.
Posted: Wed Jul 13, 2016 11:03 pm
I have recently started wondering about earthing arrangements for two of my lighting columns outside.
I would appreciate some guidance please, as the wiring regulations are blissfully unhelpful here. Even the On Site Guide only suggests the use of lampposts as a temporary earthing point for earthing portable generators.
We are on a TNS lead sheathed supply cable here. This is known to have a high impedance, as the sheathing is failing and the DNO is converting most of us over to TNCS.
An independent supply is taken underground from the incoming consumer side with 10mm split concentric to supply a Distribution Board for several outbuildings with their own consumer units, from these:
A - There is a 2.5mm RCD protected ring final socket circuit, with a fused spur taken off to feed a galvanised column sunk in moist earth. This column and lantern are connected to the circuit PE.
B - There is a 1.5mm RCD protected single radial circuit, with a fused spur taken off to feed a 5m high column sunk entirely in concrete. This column and lantern are connected to the circuit PE.
I have not been able to test it, but could it be possible that column A would act as a supplementary earthing electrode to the system, with it potentially having a lower impedance to ground from the ring final than a route through three DBs to a poor earthing point anyway?
If so, then would I need to separate the column from the installation to effectively have it its own TT supply by removing the bond between the cutout and the column body?
Column B is now a very real target for lightning strikes, with the removal of some nearby higher trees. If it got struck, then due to the resistance of the entirely concreted base (which has been tested and has a high impedance to ground) is it possible that the 1.5mm radial and supply circuits would get fried?
If so, then is there a more correct way of earthing this?
Your thoughts on the matter would be appreciated, Thanks.
I would appreciate some guidance please, as the wiring regulations are blissfully unhelpful here. Even the On Site Guide only suggests the use of lampposts as a temporary earthing point for earthing portable generators.
We are on a TNS lead sheathed supply cable here. This is known to have a high impedance, as the sheathing is failing and the DNO is converting most of us over to TNCS.
An independent supply is taken underground from the incoming consumer side with 10mm split concentric to supply a Distribution Board for several outbuildings with their own consumer units, from these:
A - There is a 2.5mm RCD protected ring final socket circuit, with a fused spur taken off to feed a galvanised column sunk in moist earth. This column and lantern are connected to the circuit PE.
B - There is a 1.5mm RCD protected single radial circuit, with a fused spur taken off to feed a 5m high column sunk entirely in concrete. This column and lantern are connected to the circuit PE.
I have not been able to test it, but could it be possible that column A would act as a supplementary earthing electrode to the system, with it potentially having a lower impedance to ground from the ring final than a route through three DBs to a poor earthing point anyway?
If so, then would I need to separate the column from the installation to effectively have it its own TT supply by removing the bond between the cutout and the column body?
Column B is now a very real target for lightning strikes, with the removal of some nearby higher trees. If it got struck, then due to the resistance of the entirely concreted base (which has been tested and has a high impedance to ground) is it possible that the 1.5mm radial and supply circuits would get fried?
If so, then is there a more correct way of earthing this?
Your thoughts on the matter would be appreciated, Thanks.