Short answer
For a disconnector rated 27.5kV for the OCS (rated voltage corresponding to the 25kV industrial-frequency traction supply system) four items need checking in selection: rated current (400A to 3150A, chosen by feeder current, commonly 630A, 1250A, 1600A, 2000A and 2500A); earthing switch (whether an earthing switch is fitted, for earthing during maintenance); operating mechanism (manual CS series or motor-operated, motor drive being preferable for remote control and automation); and mounting arrangement (outdoor pole-mounted, vertical or horizontal break, double-pole).
Standards and acceptance
The products are built to GB 1985-2014 "High-voltage alternating-current disconnectors and earthing switches" and to the common technical requirements of GB/T 11022 (current 2020 edition); CRCC-certified types cover the GN2 and GW1 series. DC disconnectors for urban rail (DC 750 V / 1500 V, 3150 A / 6000 A class) are selected separately to the DC system.
Related products and reading
HV disconnector series | 27.5kV outdoor disconnectors | DC disconnectors for urban rail | 27.5kV disconnector maintenance guide | Disconnector designations and symbols