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This article is a slightly modified version of a write-up by Prakash Tendulkar.
The dual current (AC/DC) locomotives (WCAM and WCAG series) of Indian Railways can run either under the 25kV AC traction that is common in most of India, or on the 1.5kV DC traction power that is currently restricted to the Mumbai suburban area. These locos have several features that help protect the locomotive traction circuits from damage when the wrong catenary is connected to - 25kV AC when 1.5kV DC is expected or vice versa.
The Pantograph Selector Switch switch has five positions with the center position being the 'off' position. The remaining four positions are 'E', 'DC', '(O)', 'AC' and 'E'. The extreme end 'E' positions are for emergency operation, e.g., DC pantograph under AC wires and vice versa. 'DC' and 'AC' are used under normal circumstances to connect to the DC or AC catenaries. The locos usually have two different pantographs, a dual-blade DC pantograph capable of dealing with the higher current drawn in DC mode, and a single-blade AC pantograph.
Under normal conditions, if the wrong pantograph is raised by mistake, the pantograph will touch the catenary and retract immediately, and a buzzer will sound in the locomotive cab. Note that under normal circumstances there is a gap of a few seconds between when the pantograph is successfully connected to the OHE contact wire and when the current starts flowing to the locomotive circuits. Initially only the Voltage Sensing Devices (VSD) are connected in the circuit to the live catenary. The pantograph is retracted when the VSD are fed with wrong power (DC when expecting AC or vice versa). Other circuits are not connected at this time, which helps protect them from the effects of the wrong traction power being connected to.
This consists of a potential transformer and a resistor connected in series to the primary winding of the potential transformer; the other end of the resistor is grounded. The junction of the potential transformer's primary and the resistor is grounded though a capacitor. Two relays, QAC (for AC operation) and QDC (for DC operation) are used. The QDC relay is energized if the DC pantograph is raised and DC voltage is sensed. Similarly, the QAC relay is energized if the AC pantograph is raised and AC voltage is sensed. There are two slave relays used for interlocking. All these relays operate on 110V DC fed by batteries.
This is an electropneumatic switch used under 1500V DC traction. Slave relays provide protection before the DS is energized. The switch is insulated for 25kV AC. The coil for this switch is energized when the DC pantograph is raised and remains raised after VSD senses the correct DC voltage.
This is similar to the Disconnecting Switch DS, except that it is used for 25kV AC operations.
In the WCAM-1 loco, there was a restriction when DC emergency pantograph was raised under DC wires. The loco could be operated in series only due to limited current collecting capability of the AC pantograph. This restriction has been removed in the WCAM-2 and WCAM-3 locos. I am not certain, but this restriction may still be in effect for the WCAG-1.