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.
Yes, good question Andy. It seems to me that CT1 provides protection against OVs to earth, probably the most common OV resulting from lightning activity and CT2 gives protection between live conductors and also N/E. Neither connection method appears to give protection to cover all modes. It is not unreasonable for clients, or indeed electrical contractors, to assume that when they install SPDs, mostly in CT1 arrangement, OV protection is in place without further consideration.
I think CT1 still effectively provides protection against common-mode transients. For example, if there is 4kV across L1-L2, then there is a total of 4kV across L1-E-L2, so a minimum of 2kV across at least one of L1-E and L2-E (and in fact most likely 4kV L1-E and 0V L2-E or vice versa). So at least one of the SPDs will trigger.
