WARNING: The bases are high voltage devices and should only be handled by competent people who can exercise common sense.
The maximum voltage for both the PMTs and dynode chain is 3,000 V. In actual use, however, there should be no need to exceed the 1,800-2,100 V operating parameters, since both PMTs and dynode chain have high gain. Nevertheless, the bases are high voltage devices and care should be exercised during handling and set up. The external aluminum parts, the front and rear housing, and the back plate (17), are all grounded via the ground of the BNC (18) and SHV (19) connectors. Since the back plate is connected to the coupling nut via the three steel posts, the front plate is also grounded via the coupling nut and the back plate. Common sense, however, dictates that the bases are not to be handled while under high voltage, even when multiple grounding connections are provided.
The mu-metal shield is also under high voltage, since it is connected to the cathode. Electrical isolation between the mu-metal shield and the front tubular housing is assured by the high dielectric retainer ring (12) and the plastic insulator (09) at the free end of the mu-metal shield. The air gap between the mu-metal shield and the front tubular housing is 6 mm, thus the breakdown value (18,000 V) far exceeds the maximum 3,000 V of the PMT.
In the event that the mu-metal shield is inserted without the plastic insulator ring, or some oaf decides to operate the base without the outside housings, the 11 MOhm resistors between the -HV and the mu-metal shield will restrict the current flow through the mu-metal shield (and the oaf's hands) to less than 0.2 mA with -2,100 V on the base.