Relays used in the circuits of relay protection devices and electrical automatic devices are generally made of electromagnetic, inductive, electrodynamic and magnetoelectric principles. In recent years, there have been magnetoelectric devices using rectified current, magnetic relays with saturated inductance coils and electronic relays, and relays using semiconductors are being studied. The purpose of studying and adopting these new principles of relays is to simplify the structure of relays, improve their reliability and improve their parameters. Use indirect methods to indicate that there is a fault or improper working state in the power system. For example, this kind of relay has the ability to react with the generation of gas, the generation of heat, the increase of pressure, etc. The relay protection device must have the function of correctly distinguishing whether the protected element is in normal operation state or has a fault, whether it is a fault in the protection area or a fault outside the protection area. In order to realize this function, the protection device needs to be composed based on the characteristics of changes in electrical physical quantities before and after power system failure. The three-phase / six phase relay protection tester is developed by using Zui new digital technology, high-precision electronic devices, microcomputer system and new structure of line and cable fault tester. It can independently complete the device test in professional fields such as microcomputer protection, relay protection, excitation, measurement, cable fault tester and fault recording. It is widely used in scientific research, production and electrical test sites in electric power, petrochemical, metallurgy, railway, aviation, military and other industries. After power system failure, the main characteristics of power frequency electrical quantity change are: (1) current increase. In case of short circuit, the current on the electrical equipment and transmission line between the fault point and the power supply will increase from the load current to greatly exceed the load current. (2) Voltage drop. In case of phase to phase short circuit and grounding short circuit faults, the phase to phase voltage or phase voltage value of each point of the system decreases, and the closer to the short circuit point, the lower the voltage. (3) The phase angle between current and voltage changes. During normal operation, the phase angle between current and voltage is the power factor angle of load, generally about 20. In case of three-phase short circuit, the phase angle between current and voltage of cable fault tester is determined by the impedance angle of the line, generally 60 85. In case of three-phase short circuit in the opposite direction of protection, the phase angle between current and voltage is 180 (60 85). (4) The measured impedance changes. Measuring impedance is the ratio of voltage and current at the measuring point (protection installation). During normal operation, the measured impedance is load impedance; In case of metal short circuit, the measured impedance changes to line impedance. After fault, the measured impedance decreases significantly and the impedance angle increases. Negative sequence current and negative sequence voltage components appear; In case of single-phase grounding, negative sequence and zero sequence current and voltage components appear. These components do not appear during normal operation. The relay protection of various principles can be formed by using the change of electrical quantity during short-circuit fault.