Hazards and prevention of zero line disconnection in low voltage power supply system

With the development of society and the popularization of power facilities, power supply lines and electrical equipment are everywhere. Safe use of electricity has also become a focus of national production and life safety. This article briefly introduces the hazards and prevention of neutral line break in low-voltage power supply systems.

1. Hazards of Zero Line Break

During the operation of the power supply system, the zero line may be broken due to factors such as thermal effect, mechanical force, joint oxidation or external force. Once the zero line is broken, the load neutral point will be seriously displaced due to the lack of zero line to conduct unbalanced current, resulting in serious imbalance of the three-phase power supply voltage. In the three-phase four-wire unbalanced power supply system, the zero line is interrupted, and the load neutral point will shift to the phase with large load, and the voltage of the phase with large load will be reduced; while the phase voltage with small load will increase. The more serious the imbalance of the three-phase load, the greater the displacement of the load neutral point. The symmetry of the phase voltage at the load end is destroyed, and different degrees of imbalance appear. In the zero line circuit breaker accident we encountered, the voltage of the phase with large load can be reduced by 30~60V, making the bulb red, and the fluorescent lamp and household appliances cannot be started; while the phase voltage of the phase with small load can be increased to about 300V, which greatly exceeds the rated voltage of household appliances. At this time, if the fuse does not melt, the household appliances will be burned, causing unnecessary losses.

If the phase line is short-circuited to the ground when the neutral line is disconnected, the neutral point displacement will be greater. If a neutral line disconnection accident occurs in the low-voltage zero protection system, it means that the electrical equipment has lost its security measures. Once the electrical equipment leaks electricity, the human body touching the housing of the household appliance will cause electric shock, and it will not play the due protective role. It can be seen that the harm of neutral line disconnection in the low-voltage power supply system is very serious and should be paid enough attention to.

2. Technical measures to prevent neutral line disconnection

In order to prevent the neutral line from breaking and ensure the reliable operation of the neutral line, a variety of technical measures should be adopted to make the neutral line safe and reliable.

1. In a three-phase four-wire low-voltage power supply system, the three-phase power load should be adjusted to be as balanced as possible to avoid a large unbalanced current passing through the neutral line during normal operation.

2. The cross-sectional area of ​​the neutral line in the three-phase four-wire low-voltage power supply system should be more than half of the cross-sectional area of ​​the phase line; for a single-phase power supply circuit, the cross-sectional area of ​​the neutral line should be the same as that of the phase line. The current carrying capacity of the neutral line in the low-voltage power supply system shall not be less than the rated current of the largest single-phase power-consuming equipment in the power supply circuit. The aluminum neutral line of the outdoor overhead line shall not be less than 16 square millimeters; the copper wire shall not be less than 10 square millimeters: the neutral line shall use multiple strands of wire.

3. Notes on zero line connection

(1) It is strictly prohibited to connect conductors of different metals, different specifications, and different lay directions within the span.

(2) Within one span, each neutral wire should not have more than one joint. Aluminum stranded wires should be crimped.

(3) When connecting the aluminum stranded wire bow wire or the bow wire to the main wire, a parallel groove wire clamp should be used. The specifications of the wire clamp should correspond to the cross-section of the conductor.

(4) Copper and aluminum wire clamps should be used to connect copper and aluminum wires. The specifications of the wire clamps should be consistent with the cross-section of the wire.

(5) When the main line entering the distribution box needs to be branched, it is not advisable to bundle multiple branch neutral wires together. Instead, a neutral wire terminal block should be used for branching. The neutral wire terminal block should be made of the same material as the neutral conductor. If they are different, a copper-aluminum transition measure should be used.

4. In order to ensure the safety and reliability of zero connection protection and reduce the displacement of the load neutral point after the zero line is broken, the zero line of the low-voltage distribution line should be repeatedly grounded at the trunk or branch terminal and every one kilometer along the line. The zero line of the low-voltage household line should be repeatedly grounded at the entrance of a large building, and the grounding resistance of each repeated grounding device should not be greater than 10 ohms.

5. Strengthen the inspection and maintenance of the low-voltage power supply end. During each inspection, pay attention to whether there are trees, antennas, construction machinery, etc. that endanger the safe operation of the line; whether there are fallen branches, kites, wires, etc. on the line. Check whether there are broken strands or burns on the wires; whether the joints are discolored. Check whether the zero-line joints of the household line and the distribution box are overheated or broken. If the above conditions are found, effective measures should be taken to repair them. When the power supply line is shut down for maintenance, the focus should be on the joints as far as the zero line is concerned.

6. It is strictly forbidden to install a fuse or switch on the neutral line in a three-phase four-wire low-voltage power supply system to prevent the neutral line from breaking. Repeated grounding of the neutral line is not optional, but a very important technical measure to ensure safe operation. It must be paid attention to in design and construction.