Perhaps a rather rudimentary question but what is “full mode” protection? I assume that it is a combination of common and differential modes but it seems to me that it is generally common mode that is provided.
And with the TT installation you don’t want a SPD that will fail leaving a connection between neutral and earth leaving it as TN-C-S.
That risk is there if the N-PE the SPD fails to short - as there's no guarantee that any OPD will operate to isolate it with the small voltage difference between N and PE.
With CT2 we connect L-N and N-PE so at least a N-PE short won't be as immediately dangerous as a L-PE one.
I have a feeling that many manufacturers pick a SPD technology for the N-PE SPD that is less inclined to fail short-circuit (gas dischage types?) but I've not found that written anywhere yet.
The other option is to put the SPD after the first tier RCD - but then the surge current has to go through the RCD - not only not protecting the RCD from damage but risking more nuisance tripping too. Rock and hard place.
And with the TT installation you don’t want a SPD that will fail leaving a connection between neutral and earth leaving it as TN-C-S.
That risk is there if the N-PE the SPD fails to short - as there's no guarantee that any OPD will operate to isolate it with the small voltage difference between N and PE.
With CT2 we connect L-N and N-PE so at least a N-PE short won't be as immediately dangerous as a L-PE one.
I have a feeling that many manufacturers pick a SPD technology for the N-PE SPD that is less inclined to fail short-circuit (gas dischage types?) but I've not found that written anywhere yet.
The other option is to put the SPD after the first tier RCD - but then the surge current has to go through the RCD - not only not protecting the RCD from damage but risking more nuisance tripping too. Rock and hard place.
