Over-current tripping (OCT) device is usually required in air circuit breakers (ACBs) in DC traction power supply system

which can trigger the separation of contacts in ACB

achieving rapid interruptions of severe fault currents under nearby faults or failures of secondary control and protection system. The operating performance of OCT device can be evaluated in three aspects: setting range

setting resolution

and response time. In this paper

based on typical operating characteristics

the working principle and problems of the existing OCT device are discussed first. Combined with theoretical analysis and finite element simulation

to deal with the setting range and setting resolution

a novel magnetic-coupling tripping scheme is proposed based on the regulation of internal magnetic field

which can remarkably improve the setting range and setting resolution of OCT device; to deal with the response time

a fast-tripping scheme is proposed based on the optimization of magnetic-circuit structure

which can reduce the response time of OCT device. The proposed scheme has been verified by simulations and experiments.