STA & ST Contactor

The ST (Starting) and STA (Starting Auxiliary) contactors work in tandem during the engine cranking sequence. When the start button is pressed, these contactors close to connect the locomotive's storage batteries to the main generator, temporarily treating the generator as a motor to turn the diesel engine. The STA acts as the pilot or auxiliary logic switch to ensure the sequence is safe, while the ST handles the massive surge of current required to physically rotate the engine. Once the engine reaches firing speed and runs on fuel, these contactors drop out to isolate the battery circuit from the high-voltage system.