In a three-phase four-wire low-voltage power supply system, the function of the neutral line is: when the three-phase load is unbalanced, it can ensure that the impedance on the neutral line is zero, thereby eliminating the neutral point displacement and keeping the voltages of each phase balanced. If one phase in the three phases has an open circuit, it only affects that phase, while the voltages of the other two phases remain unchanged, and electrical equipment connected to these two phases can still operate normally. However, if the neutral line in the three-phase four-wire system is disconnected due to some reason, it will cause unbalanced three-phase voltages, which may burn out electrical equipment or make it impossible for electrical equipment to operate normally. There are several situations of neutral line disconnection:
1. Single-phase power supply: When a neutral line disconnection fault occurs within the scope of single-phase power supply, the vacuum oil filter, since its introduction to the market, has been well received by power plants, metallurgy and other industries. In cases of severe oil emulsification, it can complete three times of dehydration and degassing under the same machine model comparison, with efficiency 3-4 times that of domestic machines of the same type. It can cause the lights within the power supply range to not turn on, and other electrical appliances to be unusable. At this time, using a neon test pen to test, both the phase wire and the neutral wire light up; using a digital test pen, both the phase wire and the neutral wire show phase voltage, but measuring with a voltmeter shows no voltage indication. Based on the above situation, it can be determined that the neutral line in that power supply range is disconnected.
2. The specific manifestation of a neutral line disconnection fault in a certain branch of a three-phase four-wire line is: within the power supply range of this branch, some users' lights are not bright enough, fluorescent lamps cannot start, TV brightness decreases, images shrink, and electrical appliances with voltage protection cannot start or automatically shut down, while some users' voltage significantly increases, lights are extremely bright, fan speeds increase, and in severe cases, lights or other electrical appliances burn out quickly. If the above situations occur, it can be determined that the neutral line of this branch is disconnected.
3. When a neutral line disconnection fault occurs within the power supply range of a certain three-phase distribution transformer, that is, the neutral busbar is disconnected, the specific manifestations are the same as those of a neutral line disconnection fault in a three-phase four-wire branch, except that the scope is larger, the harm is more severe, and the losses are more serious.
Main preventive measures: In three-phase four-wire power supply, single-phase loads should be distributed as evenly as possible, maintain three-phase load balance, strengthen monitoring of three-phase currents, and adjust in a timely manner when imbalance is found; in three-phase four-wire power supply, the neutral line current should not be greater than 1/4 of the phase line current, and the neutral line cross-section should not be less than 1/2 of the phase line cross-section. For single-phase power supply, the neutral line cross-section should be the same as the phase line; the connection of the neutral line should be firm and reliable. The incoming and outgoing lines of distribution transformers and distribution panels should use copper-aluminum transition wire clamps, and strengthen inspection and maintenance, especially night inspections. If joint sparking is found, handle it in a timely manner; the neutral line of a three-phase four-wire line should not be equipped with a fuse or a separate switch; when disconnecting a three-phase four-wire circuit, the phase lines should be disconnected first, then the neutral line, and the sequence should be reversed when connecting.
