I wasn't able to examine the shaver TX it had been installed under a floor that had then been laminated. There was no FCU it had been tapped into the RFC.
Apparently the Unit has its own internal fuse but of course if that did blow you need to rip the floor up to replace.
Anyway upshot was low IR readings with it in circuit. Disconnected RFC from both sides of it and the rest of the ring read fine. Had to "assume" the TX has some sort of filtering to earth. Had been working fine for years and worked fine on new RCBO for that RFC and has done since.