Depends how they were connected - the benefit of an SPD will decline with longer connecting leads (i.e. away from the direct path from surge source to equipment to be protected) - usually 0.5m total is recommended, so if the total (of both L & N) from the SPD to the point where the other CU is connected (e.g. > 200mm to the Henley blocks) I'd say probably not adequately.

   - Andy.

Depends how they were connected - the benefit of an SPD will decline with longer connecting leads (i.e. away from the direct path from surge source to equipment to be protected) - usually 0.5m total is recommended, so if the total (of both L & N) from the SPD to the point where the other CU is connected (e.g. > 200mm to the Henley blocks) I'd say probably not adequately.

   - Andy.

Thanks Andy. Is that in the scenario where the SPD is in the CU further away than the CU without an SPD? If the SPD is in a CU nearest the origin and installed correctly less than 0.5m then this SPD will redirect the surge before it gets to the CU further away?

Agreed.

If you had a double-decker CU, a single SPD could be used. So if instead, the CUs are side-by-side, there is no reason in principle why a single SPD should not suffice, but the cables must still be short enough.

Jimi's second posting suggests that the second CU is supplied by a distribution circuit. My understanding is that an SPD at CU1 protects downstream circuits, but if the distribution circuit is too long (> 20 m?) another SPD may be required.