The formation and coupling principle of electrical fast transient/burst（EFT/B） is studied to effectively protect microprocessor protection devices from the EFT/B disturbance. The forming process of EFT/B caused by switch operation is dealt with. By analyzing the EFT/B disturbance formed by operating control relay in the secondary circuit， it is observed that the coupled disturbance voltage of the output relay coil can badly interfere with the DC power source voltage of the microprocessor protection devices. Such initiative countermeasures as fly-wheel diode， electromagnetic shielding and RC protection are used to restrain the formation and coupling of EFT/B disturbance. The results indicate that the EFT/B disturbance from the interference source and coupling path can be effectively restrained.