SPD(s) for an outdoor TT install

the BS7671 advice on SPDs assumes wiring of the mains supply coming in as the source of the surge voltages.

It is not really the correct advice for the external antenna situation, of which your external lamp post is a milder example. The problem is always to see which voltage is moving relative to what, and all the SPD can do is  become conductive enough to connect two things that were not connected before, at one point for the duration of the over-current.

Unless the lamp post is taller than the surrounding building or on a very long feed, so it's 'earth' is a long way off the one for the house supplying it, I'd not worry. Yes some L-N-PE either at the post to protect the light fitting, or at the point the cable enters the house to protect the house may catch certain unlikely cases, but they are that, unlikley. 


Where you really do have things you do need to protect, like tall antennas outside and an expensive listening station inside, then the incoming cables are shielded and all have metallic glanding to a common entry panel, and the idea is that high currents may flow in the armours and braids,  and may go in and out on the mains, or the telecomm cables or the antenna feeders, and all of these cables are kept at the same voltage so the equipment in the bunker is never 'doing the splits' between different voltages whatever happens outside.

However, you are not working to those sort of Defstan or STANAG specifications, and do not need your installation to survive a small nuclear strike,  the requirements are a lot less onerous, and the cost/risk balance is very much in favour of cheaper.


However, an outside light with a SWA feed back to the board and with the SWA armour earthed promptly to the MET is less likely to give trouble than one where the SWA is fed from another circuit indoors that also supplies a load of delicate stuff.

In the same way if long external telcomms cables  call by the MET on their way in , rather than casually connect direct to some double insulated box of digital magic in the living room, the chances for that box surviving a thunderstorm are increased, as is the TV if the antenna braid is actually earthed to something.


In the UK the protection of telephone lines at the customer end is not normally done, as the kit that would be damaged does not belong to the likes of BT Openreach anyway, so replacing it is free, to them, and lightning is quite rare here compared to many places.


In places like Germany you will see things like  This little  box from Dehn  to catch the worst on the phone line as it comes in. Not common in the UK, and not normally an issue.


Mike.

the BS7671 advice on SPDs assumes wiring of the mains supply coming in as the source of the surge voltages.

It is not really the correct advice for the external antenna situation, of which your external lamp post is a milder example. The problem is always to see which voltage is moving relative to what, and all the SPD can do is  become conductive enough to connect two things that were not connected before, at one point for the duration of the over-current.

Unless the lamp post is taller than the surrounding building or on a very long feed, so it's 'earth' is a long way off the one for the house supplying it, I'd not worry. Yes some L-N-PE either at the post to protect the light fitting, or at the point the cable enters the house to protect the house may catch certain unlikely cases, but they are that, unlikley. 


Where you really do have things you do need to protect, like tall antennas outside and an expensive listening station inside, then the incoming cables are shielded and all have metallic glanding to a common entry panel, and the idea is that high currents may flow in the armours and braids,  and may go in and out on the mains, or the telecomm cables or the antenna feeders, and all of these cables are kept at the same voltage so the equipment in the bunker is never 'doing the splits' between different voltages whatever happens outside.

However, you are not working to those sort of Defstan or STANAG specifications, and do not need your installation to survive a small nuclear strike,  the requirements are a lot less onerous, and the cost/risk balance is very much in favour of cheaper.


However, an outside light with a SWA feed back to the board and with the SWA armour earthed promptly to the MET is less likely to give trouble than one where the SWA is fed from another circuit indoors that also supplies a load of delicate stuff.

In the same way if long external telcomms cables  call by the MET on their way in , rather than casually connect direct to some double insulated box of digital magic in the living room, the chances for that box surviving a thunderstorm are increased, as is the TV if the antenna braid is actually earthed to something.


In the UK the protection of telephone lines at the customer end is not normally done, as the kit that would be damaged does not belong to the likes of BT Openreach anyway, so replacing it is free, to them, and lightning is quite rare here compared to many places.


In places like Germany you will see things like  This little  box from Dehn  to catch the worst on the phone line as it comes in. Not common in the UK, and not normally an issue.


Mike.