The relay protection testing instrument is divided into two circuits, the main circuit and the auxiliary circuit. The main circuit is adjusted with a large knob, and the auxiliary circuit is adjusted with a small knob. The main circuit controls the various outputs of the circuit through the "output selection" key switch on the panel, and Each time an output is switched, the digital voltage/current meter on the instrument can automatically monitor its output value. The auxiliary circuit directly adjusts the output through the output switch control, and the measurement can be measured with an external multimeter.