RCD Protection on 16mm Submain??

[Welchyboy]

The rcd is for the walls not ceilings so cable in wall less than 50mm requires rcd
cable in ceiling lying on ceiling doesn't require rcd
as you knew already

Thats assuming that you have installed the cables properly through the ceiling joists, they should be > 50mm but if they are drilled /notched through the 1in' batten in an overboarded ceiling then this is not considered compliant or a safe cable route