Hi Guys,
ive come across this same problem at least 3 times. All caused by 9.5kW showers on 6mm sq pvc/pvc.
The first two occasions, the problem lay in the 45Amp shower isolators, where the neutral conductor at the supply side, had melted the insulation completely, yet the conductor itself, remained intact.
Now the estimated lengths of these circuits(all separate addresses):
one 15m run, one 20m run (wired to 3036 30a fuse).
Now the third problem is the shower which is not yet turned on, as the plasterer has to finish, prior to commissioning of the circuit. The run is less than 5m and ive got the same problem, all for the sake of 1A.
Im going to comission the circuit, which is protected by a 40A breaker, housed with 30mA RCD (shower mcb unit) and ill monitor how it goes by checking the isolator, every so often. An l would suggest everyone else with this issue does the same.