Abstract: This paper analyzes the causes of circuit malfunction due to improper selection of DC contactors and proposes preventive measures. Keywords: DC contactor selection, malfunction, cause analysis 1 Introduction DC contactors are one of the most commonly used low-voltage electrical appliances in power automatic control systems and electric drives. As the starting and actuating elements of switches, they can remotely control the starting and stopping of electrical circuits. Structurally, DC contactors are characterized by high reliability, safe use, and convenient maintenance; economically, they are inexpensive. However, if the DC contactor is improperly selected during maintenance, it is very likely to cause the electrical circuit to fail to operate or malfunction, or even trigger a major power grid accident, especially posing a huge threat to the safe operation of substations supplying power to coal mines. 2 Accident Example Our 35kV substation is responsible for supplying power to three mines in the old area of ​​Jincheng Anthracite Mining Group. In August 2002, a low-oil switch in the substation, which was in hot standby mode, suddenly closed on its own. The operator manually tripped the circuit breaker remotely via the control switch. After tripping, the switch automatically closed again, and this repeated tripping and closing occurred. The line connected to this switch was empty, with no load on the opposite side, and no accident occurred. 3. Cause Analysis The switch in our plant is a SN10-35Ⅰ type, and its protection device uses conventional electromagnetic relay protection. Regarding the above control circuit, the control switch KK and the anti-pumping intermediate relay TBJ were first inspected and tested. The inspection revealed no problems with the closing circuit and the tripping indicator circuit. The cause was estimated to be in the closing contactor HC. Upon inquiry, it was learned that before the accident, the plant's relay protection personnel had replaced the closing contactor due to a fault. The replacement closing contactor was a CZO-40/20 type DC contactor. Its rated parameters are: rated voltage 220V, coil turns 24000 turns, resistance 2200Ω. After replacement, multiple closing and opening tests were performed on the switch, and everything was normal; the switch operated normally. The original closing contactor was a CZO-40/C type DC contactor with rated parameters of 220V, 4200 coil turns, and 288Ω resistance. Because the parameters of the replaced contactor differed from the original, a difference in operating voltage was suspected, and the performance of the two contactors was tested. The results were: CZO-40/C had a minimum operating voltage of 90V and a minimum operating current of 0.3A; CZO-40/20 had a minimum operating voltage of 117V and a minimum operating current of 0.05A. Regardless of the type of contactor, their operating principle is the same: a coil wound around a yoke or iron core generates a magnetomotive force (MF) when current flows through it, attracting a moving armature. This directly or through a lever transmission causes the moving and stationary contacts to make contact, thus energizing the circuit. Whether the contactor operates depends on the magnitude of the MF generated by the electromagnetic coil. The MF F generated by the electromagnetic coil is proportional to the number of turns N of the electromagnetic coil and the current I passing through it, with the relationship: F ​​= NI. Based on this formula, the minimum operating MF of the two DC contactors can be calculated. For CZO-40/C: F=NI=4200×0.3=1260 (Amperes) For CZO-40/20: F=NI=24000×0.05=1200 (Amperes) Under normal circumstances, when the switch is closed, contacts 5 to 8 of the control switch KK close. When the control voltage is 220V, the magnetomotive forces generated by the two DC contactors are as follows: For CZO-40/C: F_closed=NI_closed=4200×220/288=3208 Amperes>1260 Amperes, the contactor can operate reliably; For CZO-40/20: F_closed=NI_closed=42000×220/2200=2400 Amperes>1200 Amperes, the contactor can also operate reliably. A resistor R with a resistance of 2500Ω is connected in series in the indicator light circuit. Through this circuit, the excitation current flowing through the contactor coil decreases, and the magnetomotive force generated by the DC contactor coil also decreases. When the control voltage is 220V, the magnetomotive forces generated by the two DC contactors are as follows: For CZO-40/C: F=NI=4200×220/(2500+288)=331 amp-turns < 1260 amp-turns, the contactor does not operate; For CZO-40/20: F=NI=42000×220/(2500+2200)=1123 amp-turns < 1200 amp-turns, the contactor does not operate. It can be seen that under normal circumstances, when the switch is closed, the magnetomotive forces generated by the two DC contactors are much greater than their respective minimum operating magnetomotive forces, and both can operate reliably. In the trip indicator circuit, the magnetomotive force generated by the CZO-40/C DC contactor is much smaller than its minimum operating magnetomotive force, so the contactor does not operate. However, the magnetomotive force generated by the CZO-40/20 contactor is very close to the minimum operating magnetomotive force, and is in a critical state. If the control circuit voltage is higher than 220V, the CZO-40/20 contactor may malfunction, causing the switch to close incorrectly. It was found that when the switch closes incorrectly, the control circuit connected to the battery has an adjusting voltage working, making the control voltage reach 250V. Calculated at 250V: The magnetomotive force of the CZO-40/20 is: F=NI=4200×250/(2500+2200)=1276 (Ampere-turns)>1200 (Ampere-turns), so the contactor operates. The magnetomotive force of CZO-40/C is: F=NI=4200×250/(2500+288)=377 (Amperes-turns)<1260 (Amperes-turns), therefore the contactor does not operate. It can be seen that when the control circuit voltage fluctuates between 220 and 250V, the CZO-40/20 DC contactor sometimes operates and sometimes does not. When the control voltage is at the normal 220V, the contactor can open and close normally, thus masking the problem. Conclusion Whether the resistance of the replaced DC contactor increases or decreases, it may create potential safety hazards. Therefore, caution must be exercised when repairing and replacing contactors, and the model should not be changed lightly. Even when replacing with the original model, appropriate tests must be performed. Considering the issue of control voltage fluctuations, and as a preventative measure, when replacing indicator lights and resistors, they must be matched to the entire circuit to avoid accidents and ensure the reliability of power supply to the coal mine.