1 Problem Statement In the power system, the secondary circuit plays a very important role in ensuring the safe operation of the system. Under normal system operation, in order to ensure the safety of personnel and equipment, the "Safety Regulations for Power Operation Sites" stipulates that an electrical connection of the secondary circuit of the current transformer must have a reliable grounding point. At the same time, in order to ensure the correct operation of relay protection and automatic devices, the current circuit is required to be grounded at one point. However, the current secondary circuit of the substation has a large number of connected devices and a wide range of extensions. Due to human wiring errors or some unavoidable natural laws, such as insulation aging, multiple grounding points often occur in the secondary circuit of an electrical connection. Moreover, most of the secondary circuits of the system are outdoors, and the probability of insulation damage is high. Multiple grounding points lead to incorrect operation of protection, causing large-scale power outages to occur frequently in the system. The following are some reasons for two-point grounding: (1) The control panel and protection panel in the main control room are grounded separately, causing two-point grounding. (2) When the 10kV switchgear leaves the factory, the grounding point of the current secondary circuit is already connected to the circuit breaker shell. During installation, the switchgear is connected to the grounding grid, which already has one grounding point. It is also grounded in the control room, causing two-point grounding of the current circuit. (3) During the renovation of the substation, the grounding point was reconnected to the control room and the grounding point of the outdoor terminal box was disconnected. However, due to human negligence, the current loop was grounded at two points. (4) The insulation of the secondary current loop was damaged and grounded, resulting in the current loop being grounded at two points. 2 Hazards of Two-Point Grounding (1) The secondary current loop is connected by a cable. When a short circuit current or lightning current appears on the grounding network, the potential difference between the two points of the cable shield layer causes current to flow through the shield layer, which may burn the shield layer. When current flows through the shield layer, it will generate interference signals for each core wire. (2) In the secondary current loop, if there are grounding points on both sides of the relay current coil, on the one hand, the parallel loop formed by the two grounding points and the ground will short-circuit the current coil, greatly reducing the current passing through the current coil. In addition, when a grounding fault occurs, a ground potential difference will be generated between the two grounding points due to the zero-sequence current passing through the ground network, which will generate a large additional current in the current coil. These two reasons will cause a significant difference between the current passing through the relay current coil and the fault current supplied to the secondary winding of the current transformer, leading to incorrect protection operation and inaccurate metering. 3. Solution To eliminate the potential hazard of two-point grounding, AC current is supplied to phase A on the primary side of the current transformer. A clamp-on ammeter is used to monitor phase A and the neutral (N) line in the secondary current circuit. If the currents in phase A and N are equal, the current circuit is grounded at one point, which also verifies the correctness of the wiring. If the N line current is approximately half that of phase A, the current circuit is grounded at two points, specifically in the main control room and the switchyard. If the N line current is slightly less than half that of phase A, the current circuit is also grounded at two points, specifically in the main control room (the protection panel and control panel are grounded separately). For example, if the current transformer ratio is 300/5, and the current flowing through phase A is 60A, while the secondary current circuit at phase A shows 1A, and the N-point current of the secondary circuit also shows 1A, then it is a single-point grounding. If the secondary current circuit at phase A is approximately 1A, and the N-point current is approximately 0.5A, then it is a two-point grounding in both the main control room and the switchyard. If the secondary current circuit at phase A is approximately 1.23A, and the N-point current is approximately 0.77A, then the control panel and protection panel in the main control room are grounded respectively, causing a two-point grounding. This method applies equally to neutral-point non-directly grounded power grids, directly grounded power grids, and in both electromagnetic and microprocessor-based protection systems. It can solve the long-standing problem of multiple grounding points in AC secondary circuits in power systems, enabling timely detection of such grounding and preventing protection maloperation and failure to operate due to multiple grounding points, thus ensuring the safe operation of the power grid. 4. Conclusion In the commissioning of new protection systems in substations and the upgrading of existing protection systems, the above method ensures that the secondary current circuit must have one and only one grounding point. Practice has proven that this method can not only check for multi-point grounding issues but also verify the correctness of the current circuit wiring.