I am more than happy to respect the exclusion zones at the ends and in the middle of joists (Electrician's Guide to the Building Regulations); but it seems odd that cables which are passed through joists must go through notches in the zone between 0.07 - 0.25 times the span, and holes in the 0.25 - 0.40 zone.
In essence, despite the removal of a substantial amount of material, the upper joist in my diagram is acceptable, but the 16 mm hole in the lower one is not.
It seems to me that any risk to the cable is much lower in holes than notches; and of course, if plumbers have already occupied the notch zone, there isn't much choice.
In order to remove and replace cablies descending to sockets below, I need to lift the second board in from the wall; but it would be a real PITA to have to lift another board (especially if tongue and groove) closer to the centre of the room particularly because there isn't much working space unless the furniture is removed.
I have no intention of placing holes below notches, but will the house fall down if I don't follow the Guide? ?
