(1) When the zero line is disconnected, due to load asymmetry, the neutral point drift is caused, and the voltage of one phase of the light load will increase. When it exceeds the rated voltage of the electrical equipment, it will shorten the service life
of the equipment. If it exceeds the rated voltage too much, it may even damage the equipment and cause a "group explosion" of household appliances and lighting lights.
(2) When the zero line is disconnected, the voltage of the heavy load phase decreases, the three-phase voltage is unbalanced, and the electrical equipment cannot reach normal output. The starting current of the motor increases, making it difficult to start. The motor heats up severely, the incandescent lamp is dim, and the fluorescent lamp cannot start.
(3) When the neutral line is broken, due to the equipment not working, it is easy to misjudge that the equipment is not live and cause electric shock accidents.
When a phase to ground fault occurs, the voltage between the neutral line and the ground increases. For equipment that uses protective grounding, its casing will also carry dangerous voltage. In addition, due to the fact that the transformer casing is
connected to the neutral point of the transformer and then grounded, electric shock accidents can occur when people accidentally touch the equipment, transformer casing, or grounding down conductor. At the same time, step voltage electric shock and contact electric shock may occur near the fault grounding point or the working grounding point of the transformer.
(4) Distribution transformers are easily damaged by lightning strikes. The lightning arrester of distribution transformers generally adopts a "three in one" grounding method, that is, the grounding lead of the lightning arrester, the metal shell of the
distribution transformer, and the neutral point on the low-voltage side are connected together, and then connected to the grounding device together.
Under normal circumstances, the zero wire has the same potential as the ground and is generally not charged. Therefore, zero wire electric shock accidents should not occur. However, in the following situations, it can also cause zero wire electrification and may lead to zero wire electric shock accidents:
(a) The three-phase load is severely asymmetric. Due to severe load asymmetry, a large current flows through the neutral line (i.e. the neutral line), which has a certain impedance. Therefore, there is a voltage drop on the neutral line. Although
there is no voltage on the working grounding wire of the transformer neutral point, the neutral line carries electricity. The voltage on the zero line closer to the load side is higher, the impedance of the zero line is greater, and the voltage is higher.
(b) Zero wire breakage or poor contact. When the neutral line is broken or poorlyconnected, if the load is severely uneven, it will cause potential drift at the neutral point on the load side, resulting in dangerous voltage on the neutral line.
In addition to load asymmetry causing an increase in the zero line potential, three-phase power supply asymmetry is also an important reason. For example, poor contact of the three-phase switch of the power supply, even with only two-phase or single-phase power supply; The melting of a fuse on the high-voltage or low-voltage side of a transformer may cause dangerous voltage to be generated on the neutral line.
(c) Poor grounding of the neutral wire. Poor grounding of the neutral line (i.e. grounding of the transformer neutral point) or broken grounding wire can significantly increase the potential of the neutral line under certain conditions. For example, if the ground wire of the neutral line is broken and another phase experiences a ground fault, the voltage between the neutral line and the ground will increase to the phase voltage (220V), or even higher. At this point, it would be dangerous for a person to stand on the ground and come into contact with the neutral line.
(d) Capacitive transfer. In some cases, even if the low-voltage line has been disconnected from the power supply, due to the fact that the neutral line and transformer are generally not connected, the high-voltage power supply may be transmitted to the neutral line through the capacitance between the high and low voltage windings of the transformer. If the neutral line is poorly grounded or the grounding wire is broken, thousands of volts of high voltage may appear on the neutral line.
(e) Phase grounding. When the insulation of the equipment or circuit is damaged, a phase to ground short circuit fault will occur.
