Further developments in the design and performance of high-voltage terminal boxes

Since the publication in 1962 of a paper on the basic dielectric and safety requirements of terminal boxes, considerable successful experience has been gained in the former respect, and further developments have taken place in the latter respect. A method has been found, and proved by test, to obviate the explosion danger of compound-filled sealing boxes used in conjunction with terminal boxes, one of the few problems left over from the previous investigation. The design of phase-segregated connector-type terminal boxes with containment up to 10 kA reduces the occurrence of flame exhaust to the case of severe multiple faults for impedance-earthed systems. The ultimate complete elimination of any exhaust from a terminal box under any fault condition, as well as from the apparatus served by the terminal box, requires the use of fuses in order to limit the fault energy to a manageable value. The paper describes tests up to 750 MVA, 11 kV on phase-insulated and phase-segregated connector-type terminal boxes with pressure-relief. The same terminal boxes without pressure relief were completely successfully tested with fuse protection suitable for a 3000 hp heavy-duty squirrel-cage motor with a 250 MVA, 3.3 kV and a 750 MVA, 11 kV supply.