Electric locomotive/multiple unit trains run within the range of power supply will produce higher harmonic current, excessive phase process will produce excitation inrush current, etc., the current in the primary loop creates magnetic field changes, affect the control loop of the control voltage fluctuation, trigger circuit breaker operate incorrectly. Aiming at the problem of research, combined with the research for the electrified railway traction power supply system is a typical load current features, car breaker electromagnetic field calculation model is established, the control circuit voltage fluctuation in load change. Studies have shown that compared with the higher harmonic current, excitation inrush current caused by the magnetic field change is bigger, the closer distance high-voltage main channel and control circuit, the bigger the induced over-voltage, circuit breaker control circuit of voltage fluctuation is higher, more than 0.6 V, voltage fluctuation, 5% study for high frequency overvoltage conditions the emu provides the basis for circuit breaker operation stability.