I've also remembered, the 48V on the master section switch also feeds the meter distribution board (6way BICC), keyboard / Total recall bus driver (6 way BICC), and the producer panel (12 way BICC).
Though not used at any of those points, it could be shorting on the receptacle pins on each board. Long shot, but worth checking.
Unplug all of those and see if the problem remains.
Check continuity whilst unplugged and switched off, between +48V and ground at any of the plugs.
Anthony,