A Fault Where The Neutral Wire In A Three-phase,four-wire System Has Broken.

A fault where the neutral wire in a three-phase, four-wire system has broken.

Jun 13,2026

A factory operates on a three-phase, four-wire power supply system. Recently, due to a break in the external neutral wire and the absence of protective circuits such as zero-sequence current protection in the distribution system, multiple PLCs and instruments were damaged, resulting in production and financial losses. Below, we discuss how to diagnose and prevent neutral wire breakage faults in this configuration.
The neutral line serves as the grounding point for power supply equipment, a phenomenon commonly referred to as zero-point drift. When three-phase loads are unbalanced, the neutral line balances phase voltages by maintaining zero impedance, thereby eliminating neutral point displacement and ensuring symmetrical voltage distribution across all phases—where each phase's load voltage consistently equals the supply phase voltage regardless of load variations. If one phase experiences an open circuit, only that phase is affected while the other two maintain stable voltages, allowing connected devices to operate normally. However, when the neutral line in a three-phase four-wire system breaks due to fault conditions during load asymmetry, transformer neutral point displacement occurs. As shown in the equivalent three-phase load circuit, the single-phase load bypasses the neutral line and connects in series with another phase load across both phase lines. According to voltage division principles in series circuits, the less loaded phase receives higher voltage levels (up to approximately 380V line voltage), causing voltage imbalance: some phases become excessively high, potentially damaging electrical equipment, while others experience insufficient voltage for proper operation.

Diagnosis of a neutral line open circuit:

(1) When a neutral line open circuit occurs within a single-phase power supply area, the lights within the affected zone fail to illuminate and other electrical appliances become inoperable. Using a neon light voltage tester, both the phase wire and neutral wire will light up; a digital voltage tester will display phase voltage on both wires; however, measuring with a voltmeter shows no voltage indication. Based on these findings, a neutral line open circuit within that single-phase power supply area can be confirmed.
(2) When a zero-line open-circuit fault occurs in a branch of a three-phase four-wire circuit, the following symptoms may manifest: within the power supply area of that branch, some users experience insufficient light intensity, fluorescent lamps fail to start, instruments display abnormal readings, and appliances with under-voltage protection cannot be powered on or shut down automatically; meanwhile, other users exhibit significantly elevated voltage levels, exceptionally bright lighting, increased speed of single-phase motors, and in severe cases, rapid burning out of lights or other electrical devices. The occurrence of these symptoms confirms an open circuit in the zero line of that branch.
(3) A neutral line open-circuit fault occurs within the power supply range of a three-phase distribution transformer, specifically when the neutral busbar experiences an open circuit. The manifestations are identical to those of a neutral line open-circuit fault in a three-phase four-wire branch, but the affected area is larger, the consequences more severe, and the losses more substantial.

The main causes of neutral wire circuit failure include:

(1) severe imbalance in three-phase loads, excessive neutral current, or insufficient cross-sectional area of the neutral conductor, leading to insulation burnout.
(2) Poor contact at the neutral wire connection point causes sparking, which over time may lead to a neutral wire short circuit.
(3) Poor contact between the neutral terminal of the distribution transformer and the conductor, coupled with inadequate maintenance, has caused a neutral line open circuit.
(4) The neutral lead-out wire inside the distribution transformer has a circuit fault.
(5) In a three-phase four-wire circuit, a fuse or separate switch is installed on the neutral line; when the fuse blows or the switch opens, it causes an open circuit in the neutral line.
(6) When disconnecting a three-phase four-wire circuit, first disconnect the neutral wire.
(7) Zero-line circuit breaks caused by other faults, such as zero-line damage from strong winds, or breakage due to vehicle collisions with utility poles or guy wires.

Preventive Measures:

(1) Use a three-phase four-wire power supply; single-phase loads should be distributed as evenly as possible to maintain balanced three-phase loads. Strengthen monitoring of three-phase currents and make timely adjustments upon detecting imbalance.
(2) The neutral line current shall not exceed one-fourth of the phase line current, and the cross-sectional area of the neutral conductor shall not be less than half that of the phase conductor.
(3) The neutral wire connection must be secure and reliable. For incoming and outgoing lines of distribution transformers and distribution panels using aluminum conductors, copper-aluminum transition clamps shall be employed, with enhanced inspection and maintenance efforts—particularly nighttime inspections—and immediate action taken upon detecting any sparks at connections.
(4) It is strictly prohibited to install fuses or separate switch devices on the neutral wire of a three-phase four-wire circuit.
(5) When disconnecting a three-phase four-wire circuit, first disconnect the phase wires, then disconnect the neutral wire; reverse this order when reconnecting.
(6) In the event of a neutral line circuit failure, immediately disconnect the three-phase power supply to mitigate potential hazards.
(7) It is prohibited to connect the zero and ground wires together to the equipment housing or similar components.
Due to the arbitrary operation of electrical equipment, three-phase load imbalance is inevitable. Incidents of equipment damage caused by neutral wire faults, which disrupt production and daily life and necessitate compensation, occur frequently. Therefore, only by strengthening inspection and maintenance of the neutral wire and preventing potential issues can its safe operation be ensured, disputes and economic losses minimized, and normal human activities maintained.