Causes and preventive measures for malfunction of leakage protector at construction site

1. Working principle of leakage protector
At present, the most widely used leakage protector in construction sites is the current type leakage protector, which consists of five parts: zero-sequence current (voltage) transformer, leakage amplifier, tripping mechanism, main switch, test button, etc. Take the three-phase four-wire leakage protector as an example. In the three-phase four-wire power grid, the relationship of the three-phase four-wire composite current is: IU+IV+IW+IN=0. The four-wire passes through the zero-sequence current transformer, the composite current is zero, and there is no current flowing on the secondary side of the transformer, so the magnetic flux is zero, and the residual current action protection device does not work. When someone is electrocuted, there should be a current IR flowing from the phase line through the human body into the earth and back to the neutral point of the transformer, forming a closed circuit. In addition, there is the residual current generated by the leakage of the three-phase low-voltage power grid in normal operation. At this time, the current passing through the primary side of the zero-sequence current transformer is IU+IV+IW+IN=I∑Z+IR. Under the action of the current I∑Z+IR, the core of the zero-sequence current transformer has magnetic flux, and its secondary side induces current, that is, there is a signal. This signal is amplified and returned to the actuator, which can cut off the power supply circuit and protect the user.
2. Reasons for malfunction of leakage protectors at construction sites
(I) External interference
One of the reasons for malfunction and refusal to operate leakage protectors used for temporary power supply at construction sites is that they are subject to external interference. External interference is divided into voltage interference, load fault current interference, and surrounding climate and environmental influences.
1. Voltage interference
(1) Lightning overvoltage
The overvoltage caused by the forward and reverse conversion process during a lightning strike generates ground leakage current through the ground capacitance of overhead lines, insulated wires, cables and electrical equipment, which is sufficient to cause malfunction of the residual current protector or even direct damage.
(2) Neutral point displacement overvoltage

When the neutral point overvoltage is too high, it will cause damage to the power supply and electronic circuit of the protector; when it is too low, it will cause the electromagnetic switch to fail to operate due to insufficient absorption and beating rate.
2. Interference of lines and electrical equipment
(1) Some lighting lines on the construction site are randomly pulled and connected, the wires are aging, the insulation resistance of the lines and electrical equipment is low, the leakage is large, and even grounding occurs, causing the protector to frequently operate or fail to operate.
(2) Due to poor insulation of the neutral line at the output end of the leakage switch, the neutral point on the power supply side is not grounded when the grounding protection is installed. When an electric shock occurs, the protector is bypassed and the sensitivity decreases or fails to operate.
(3) The line arrangement is chaotic. When large equipment is started, the instantaneous large current will cause a shunt capacitor between the line and the ground. When the current returns to normal, the capacitor discharges and causes the leakage switch to malfunction.
(4) When a leakage protector is used to control multiple circuits in outdoor construction, multiple small leakage currents accumulate together, which may cause the residual current protector to operate.
3. Interference of environmental conditions
The residual current protector is affected by changes in environmental conditions, mainly referring to the deterioration of the use environment conditions, such as high temperature in summer, humidity in rainy season, or electrical machinery with strong vibration and impact installed near the protector, or erosion by corrosive gases, which causes the electronic components of the protector, such as the electromagnetic coil or mechanism, to rust and mold, resulting in malfunction or refusal of the protector.
(II) Incorrect wiring of leakage switch installation
During the installation of leakage protector, malfunction, refusal or failure to achieve the best effect are often caused due to incorrect wiring or installation method not suitable for line structure:

1. Use a single-phase load, but the neutral line does not pass through the leakage protector. When the single-phase load is connected, the leakage switch will operate;
2. After the neutral line passes through the leakage protector, it is directly grounded or grounded through the electrical equipment, and the leakage protector will trip;
3. After the neutral line passes through the leakage protector, it is connected with the neutral lines of other leakage protectors or other neutral lines without leakage protectors installed.
4. Three-phase loads such as motors are generally not connected to the neutral line. Four-core cables are used, one of which should be connected to the PE protection line and the motor housing. However, in some cases, this PE protection line is connected to the neutral line, which actually grounds the neutral line through the motor housing. If the neutral line resistance is large, it may cause the leakage protection to trip irregularly;
5. The load is not evenly distributed after the leakage protector. When leakage faults occur in electrical equipment and lines or leakage current increases, the upper leakage protection will trip before the final leakage protection of the welding machine or both leakage protections will trip at the same time;
6. There are many mobile devices on the construction site, such as vibrators, hand drills, small cutting machines, tamping machines, small welding machines, etc., which are used randomly. Some equipment is not even connected to the switch box (two-level protection), but is directly connected to the sub-box. When the machine leaks electricity, this also increases the probability of frequent tripping of the total leakage protector.
(III) Poor quality of leakage protector and improper parameter configuration
The leakage protector is not purchased and installed on site according to the program parameter requirements formulated by relevant specifications and standards, and due to poor product quality, the actual internal setting parameters and nameplate parameters do not meet the "General Requirements for Residual Current Operated Protectors" (GB6829-1995), and the products shipped from the factory will also have false operation and refusal to operate.

1. The total capacity exceeds 50kW, but the construction organization design is not prepared according to the specifications, and the leakage protector is not configured according to the designed specification parameters. The final stage does not comply with the requirements of the "Technical Specifications for Temporary Power Safety at Construction Sites", and the rated leakage action current used in the switch box exceeds the 30mA leakage protector, or a delayed leakage protector is selected. When a leakage fault occurs, the switch box leakage protector is slow to act and cannot provide protection. The rated leakage action current of the final leakage protection is too small, and it is not considered that there may be a relatively large normal leakage current on the distribution line after the leakage protector. This causes the leakage protector to be too sensitive and act.
2. The main distribution box does not implement three-level power distribution as required by the "Technical Specifications for Temporary Power Safety at Construction Sites", and the leakage protectors of the main distribution box, distribution box and switch box act at the same time, causing the entire construction site to shut down.
3. A sampling survey and test of the leakage protectors used at the construction site showed that some leakage protectors were of poor quality, and the setting value of the internal electrical appliances of the protectors did not match the nominal value on the electrical nameplate. For example, according to the specification, a leakage protector with a rated leakage action time of 150mA and 0.2s was selected in the main distribution box. When we used the leakage switch tester to test the leakage time, some of its parameters were only 68mA and 0.1s. The misoperation of the total leakage protector often causes a complete power outage at the construction site, which has adverse consequences for the construction quality and construction period.
IV. Preventive measures
The main reason for the above-mentioned faults is that some construction site electricians do not understand the principle and use of leakage switches, are not familiar with the technical specifications for temporary electricity safety in buildings, and do not have the ability to handle and cope with the safety electricity problems caused by the harsh environment and special production conditions of the construction site. The management of the construction site and the training of electricians on safe electricity use should be strengthened, and corresponding preventive measures should be formulated from a technical perspective.

(I) Avoid external interference
1. For the cause of false operation caused by lightning overvoltage interference, in addition to installing lightning arresters or breakdown gaps on overhead lines, and installing 150mA, 0.2s delay type leakage circuit breakers at the main distribution box.
2. In order to prevent neutral point displacement overvoltage damage or reduce the sensitivity of leakage circuit breakers, the load should be adjusted to be distributed as evenly as possible on the three-phase line, the phase sequence of the branch line should be changed, the unbalanced current of the three-phase insulation resistance should be reduced, and the neutral line should be exchanged so that the conductor cross-section is not less than the conductor cross-section of each phase line.
3. The insulation resistance of motors and other electrical equipment in normal operation should not be less than 0.5MΩ.
4. For equipment with large starting current such as welding machines, electromagnetic leakage protectors that are not very sensitive to surge overvoltage and overcurrent should generally be selected; or electronic leakage protectors that are twice the rated current of the welding machine should be selected. However, as the final leakage protection, the rated leakage action current should not be greater than 30mA, and a special protector with secondary voltage reduction protection function should be installed.
5. For on-site mechanical equipment, the "one machine, one switch, one leakage, one box" system shall be strictly implemented.
6. If the on-site environmental conditions are poor, the leakage switch is often damp due to high temperature and rainy seasons, or may be corroded by harmful corrosive gases, an electromagnetic leakage protector with good moisture-proof and shock-proof performance, strong anti-interference ability, and not affected by voltage fluctuations should be selected.
(II) Correct installation and wiring
1. Strictly distinguish between the working neutral line and the protective neutral line, and connect them correctly. When the leakage protector is marked with the load side and the power side, the wiring should be installed and connected according to regulations.
2. The neutral line of the three-pole four-wire or four-pole leakage protector should be connected to the leakage protector. The working neutral line passing through the leakage protector shall not be used as a protective neutral line, cannot be used as a repeated grounding, or connected to the exposed conductive parts of the equipment. The working neutral line on the load side shall not be shared with other circuits.

3. 220V electrical equipment shall not be connected to the protection neutral line, otherwise the normal operation of the leakage protector will be destroyed.
4. When the capacity of a leakage protector is insufficient, two or more leakage protectors cannot be used in parallel.
5. The wiring principle of leakage protectors at construction sites is: the power supply side input line (including the working neutral line) of the lower leakage protector must all be connected to the load side of the upper leakage protector.
(III) Reasonable configuration and selection of high-quality leakage protectors
1. Reasonable arrangement of leakage protectors according to the actual situation of the construction site. Form a secondary leakage protection within each protection range, and form a tertiary leakage protection when necessary. This can improve the protection sensitivity of the secondary or tertiary leakage protection within each protection range, improve the action rate of the leakage protector when there is a fault leakage within the protection range, and reduce the total leakage protector tripping. Its selection should follow the following basic principles:
(1) The rated voltage, rated current, short-circuit breaking capacity, rated leakage action current, and breaking time of the leakage protector meet the requirements of the protected power supply line and electrical equipment.
(2) The technical conditions of the leakage protector shall comply with the relevant provisions of GB6829 and have the national 3C certification mark. Its technical rating shall be consistent with the technical parameters of the protected line or equipment.
(3) Select the leakage protector according to the power transmission method of the electrical equipment
a. For electrical equipment powered by a single-phase 220V power supply, a two-pole two-wire or single-pole two-wire leakage protector shall be selected;
b. For electrical equipment powered by a three-phase three-wire 380V power supply, a three-pole leakage protector shall be selected;
c. For electrical equipment powered by a three-phase four-wire 380V power supply, or electrical appliances shared by single-phase equipment and three-phase equipment, a three-pole four-wire or four-pole four-wire leakage protector shall be selected;

(4) Environmental requirements for electrical equipment: Selection of leakage protectors.
a. The protection level of the leakage protector should be adapted to the environmental conditions of use;
b. For electrical equipment with large electrical voltage deviation, electromagnetic leakage protectors should be preferred;
c. For electrical equipment in high or low temperature environments, electromagnetic leakage protectors should be preferred;
d. For electrical equipment in areas with frequent lightning activities, electromagnetic leakage protectors with no impulse voltage action should be used;
e. For leakage protectors installed in harsh environments such as flammable, explosive, humid or corrosive gases, special protection leakage protectors should be selected according to relevant standards, otherwise corresponding protective measures should be taken.
2. After the leakage protector is put into operation, it is necessary to press the test button and use a leakage switch detector to check whether the leakage protector is sensitive and reliable every month under the power-on state. Check whether the parameters have changed, analyze the operation of the leakage protector, and replace the faulty leakage protector in time.
5. Conclusion
In short, the misoperation of the leakage protector is the result of the combined effect of multiple factors on the construction site. Correctly configure and select the leakage protector and wiring; set up power lines according to the specifications and check the electrical equipment regularly; strengthen the temporary power management at the construction site and improve the quality of electrical operators through training, so as to reduce the misoperation of the leakage protector as much as possible, ensure the safety of electricity use at the construction site, and create a good power supply environment for normal construction.