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The working principle of the leakage protector

Author: Site Editor     Publish Time: 2022-09-05      Origin: Site

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1. What is a leakage protector?

Answer: The leakage protector (leakage protection switch) is an electrical safety device. The leakage protector is installed in the low-voltage circuit. When leakage and electric shock occur, and the operating current value limited by the protector is reached, it will immediately act and automatically disconnect the power supply within a limited time for protection.

2. What is the structure of the leakage protector?

Answer: The leakage protector is mainly composed of three parts: the detection element, the intermediate amplification link, and the operating actuator. ①Detection element. It consists of zero-sequence transformers, which detect leakage current and send out signals. ② enlarge the link. Amplify the weak leakage signal and form an electromagnetic protector and an electronic protector according to different devices (the amplifying part can use mechanical devices or electronic devices). ③ executive body. After receiving the signal, the main switch is switched from the closed position to the open position, thereby cutting off the power supply, which is the tripping component for the protected circuit to be disconnected from the power grid.

3. What is the working principle of the leakage protector?

①When the electrical equipment leaks, there are two abnormal phenomena:
First, the balance of the three-phase current is destroyed, and zero-sequence current occurs;
The second is that there is a voltage to the ground in the uncharged metal casing under normal conditions (under normal conditions, the metal casing and the ground are both at zero potential).

②The function of the zero-sequence current transformer The leakage protector obtains an abnormal signal through the detection of the current transformer, which is converted and transmitted through the intermediate mechanism to make the actuator act, and the power supply is disconnected through the switching device. The structure of the current transformer is similar to that of the transformer, which consists of two coils that are insulated from each other and wound on the same core. When the primary coil has residual current, the secondary coil will induce current.

③The working principle of the leakage protector The leakage protector is installed in the line, the primary coil is connected with the line of the power grid, and the secondary coil is connected with the release in the leakage protector. When the electrical equipment is in normal operation, the current in the line is in a balanced state, and the sum of the current vectors in the transformer is zero (the current is a vector with a direction, such as the outflow direction is “+”, the return direction is “-”, in the The currents going back and forth in the transformer are equal in magnitude and opposite in direction, and the positive and negative offset each other). Since there is no residual current in the primary coil, the secondary coil will not be induced, and the switching device of the leakage protector operates in a closed state. When leakage occurs on the casing of the equipment and someone touches it, a shunt is generated at the fault point. This leakage current is grounded through the human body, the earth, and returns to the neutral point of the transformer (without current transformer), causing the transformer to flow in and out. The current is unbalanced (the sum of the current vectors is not zero), and the primary coil generates residual current. Therefore, the secondary coil will be induced, and when the current value reaches the operating current value limited by the leakage protector, the automatic switch will trip and the power will be cut off.

4. What are the main technical parameters of the leakage protector?

Answer: The main operating performance parameters are: rated leakage operating current, rated leakage operating time, rated leakage non-operating current. Other parameters include: power frequency, rated voltage, rated current, etc.

①Rated leakage current The current value of the leakage protector to operate under specified conditions. For example, for a 30mA protector, when the incoming current value reaches 30mA, the protector will act to disconnect the power supply.

②The rated leakage action time refers to the time from the sudden application of the rated leakage action current until the protection circuit is cut off. For example, for a protector of 30mA×0.1s, the time from the current value reaching 30mA to the separation of the main contact does not exceed 0.1s.

③The rated leakage non-operating current under the specified conditions, the current value of the non-operating leakage protector should generally be selected as half of the leakage current value. For example, a leakage protector with a leakage current of 30mA, when the current value is below 15mA, the protector should not act, otherwise it is easy to malfunction due to too high sensitivity, affecting the normal operation of electrical equipment.

④Other parameters such as: power frequency, rated voltage, rated current, etc., when choosing a leakage protector, should be compatible with the circuit and electrical equipment used. The working voltage of the leakage protector should adapt to the rated voltage of the normal fluctuation range of the power grid. If the fluctuation is too large, it will affect the normal operation of the protector, especially for electronic products. When the power supply voltage is lower than the rated working voltage of the protector, it will refuse to act. The rated working current of the leakage protector should also be consistent with the actual current in the circuit. If the actual working current is greater than the rated current of the protector, it will cause overload and cause the protector to malfunction.

5. What is the main protective function of the leakage protector?

Answer: The leakage protector mainly provides indirect contact protection. Under certain conditions, it can also be used as a supplementary protection for direct contact to protect potentially fatal electric shock accidents.

6. What is direct contact and indirect contact protection?

Answer: When the human body touches a charged body and there is current passing through the human body, it is called an electric shock to the human body. According to the cause of human body electric shock, it can be divided into direct electric shock and indirect electric shock. Direct electric shock refers to the electric shock caused by the human body directly touching the charged body (such as touching the phase line). Indirect electric shock refers to the electric shock caused by the human body touching a metal conductor that is not charged under normal conditions but is charged under fault conditions (such as touching the casing of a leakage device). According to the different reasons for electric shock, the measures to prevent electric shock are also divided into: direct contact protection and indirect contact protection. For direct contact protection, measures such as insulation, protective cover, fence, and safety distance can generally be adopted; for indirect contact protection, measures such as protective grounding (connecting to zero), protective cutoff, and leakage protector can generally be adopted.

7. What is the danger when the human body is electrocuted?

Answer: When the human body is electrocuted, the greater the current flowing into the human body, the longer the phase current lasts, the more dangerous it is. The degree of risk can be roughly divided into three stages: perception – escape – ventricular fibrillation. ① Perception stage. Because the passing current is very small, the human body can feel it (generally more than 0.5mA), and it does not pose any harm to the human body at this time; ② Get rid of the stage. Refers to the maximum current value (generally greater than 10mA) that a person can get rid of when the electrode is electrocuted by hand. Although this current is dangerous, it can get rid of it by itself, so it basically does not constitute a fatal danger. When the current increases to a certain level, the person who gets electrocuted will hold the charged body tightly due to muscle contraction and spasm, and cannot get rid of it by himself. ③ ventricular fibrillation stage. With the increase of the current and the prolonged electric shock time (generally greater than 50mA and 1s), ventricular fibrillation will occur, and if the power supply is not disconnected immediately, it will lead to death. It can be seen that ventricular fibrillation is the leading cause of death by electrocution. Therefore, the protection of people is often not caused by ventricular fibrillation, as the basis for determining the protection characteristics of electric shock.

8. What is the safety of “30mA·s”?

Answer: Through a large number of animal experiments and studies, it has been shown that ventricular fibrillation is not only related to the current (I) passing through the human body, but also related to the time (t) that the current lasts in the human body, that is, the safe electric quantity Q=I × t to determine, generally 50mA s. That is to say, when the current is not more than 50mA and the current duration is within 1s, ventricular fibrillation generally does not occur. However, if it is controlled according to 50mA·s, when the power-on time is very short and the passing current is large (for example, 500mA×0.1s), there is still a risk of causing ventricular fibrillation. Although less than 50mA·s will not cause death by electrocution, it will also cause the electrocuted person to lose consciousness or cause a secondary injury accident. Practice has proved that using 30 mA s as the action characteristic of the electric shock protection device is more suitable in terms of safety in use and manufacturing, and has a safety rate of 1.67 times compared with 50 mA s (K=50/30 =1.67). It can be seen from the safety limit of “30mA·s” that even if the current reaches 100mA, as long as the leakage protector operates within 0.3s and cuts off the power supply, the human body will not cause fatal danger. Therefore, the limit of 30mA·s has also become the basis for selection of leakage protector products.

9. Which electrical equipment needs to be installed with leakage protectors?

Answer: All electrical equipment on the construction site must be equipped with a leakage protection device at the head end of the equipment load line, in addition to being connected to zero for protection:

① All electrical equipment on the construction site shall be equipped with leakage protectors. Due to the open-air construction, humid environment, changing personnel, and weak equipment management, the electricity consumption is dangerous, and all electrical equipment is required to include power and lighting equipment, mobile and fixed equipment, etc. Certainly does not include equipment powered by safe voltage and isolation transformers.

②The original protective zeroing (grounding) measures are still unchanged as required, which is the most basic technical measure for safe electricity use and cannot be removed.

③The leakage protector is installed at the head end of the load line of the electrical equipment. The purpose of this is to protect the electrical equipment while also protecting the load lines to prevent electric shock accidents caused by line insulation damage.

10. Why is a leakage protector installed after the protection is connected to zero line(grounding)?

Answer: No matter whether the protection is connected to zero or the grounding measure, its protection range is limited. For example, “protection zero connection” is to connect the metal casing of the electrical equipment to the zero line of the power grid, and install a fuse on the power supply side. When the electrical equipment touches the shell fault (a phase touches the shell), a single-phase short circuit of the relative zero line is formed. Due to the large short-circuit current, the fuse is quickly blown and the power supply is disconnected for protection. Its working principle is to change the “shell fault” to “single-phase short-circuit fault”, so as to obtain a large short-circuit current cut-off insurance. However, the electrical faults on the construction site are not frequent, and leakage faults often occur, such as leakage caused by equipment damp, excessive load, long lines, aging insulation, etc. These leakage current values are small, and the insurance cannot be cut off quickly. Therefore, the failure will not be automatically eliminated and will exist for a long time. But this leakage current poses a serious threat to personal safety. Therefore, it is also necessary to install a leakage protector with higher sensitivity for supplementary protection.

11. What are the types of leakage protectors?

Answer: The leakage protector is classified in different ways to meet the selection of the use. For example, according to the action mode, it can be divided into voltage action type and current action type; according to the action mechanism, there are switch type and relay type; according to the number of poles and lines, there are single-pole two-wire, two-pole, two-pole three-wire and so on. The following are classified according to the action sensitivity and action time: ①According to the action sensitivity, it can be divided into: High sensitivity: the leakage current is below 30mA; Medium sensitivity: 30~1000mA; Low sensitivity: above 1000mA. ②According to the action time, it can be divided into: fast type: the leakage action time is less than 0.1s; delay type: the action time is greater than 0.1s, between 0.1-2s; inverse time type: as the leakage current increases, the leakage action time decreases Small. When the rated leakage operating current is used, the operating time is 0.2~1s; when the operating current is 1.4 times the operating current, it is 0.1, 0.5s; when the operating current is 4.4 times the operating current, it is less than 0.05s.

12. What is the difference between electronic and electromagnetic leakage protectors?

Answer: The leakage protector is divided into two types: electronic type and electromagnetic type according to different tripping methods: ①Electromagnetic tripping type leakage protector, with the electromagnetic tripping device as the intermediate mechanism, when the leakage current occurs, the mechanism is tripped and the power supply is disconnected . The disadvantages of this protector are: high cost and complicated manufacturing process requirements. The advantages are: the electromagnetic components have strong anti-interference and shock resistance (overcurrent and overvoltage shocks); no auxiliary power supply is required; the leakage characteristics after zero voltage and phase failure remain unchanged. ②The electronic leakage protector uses a transistor amplifier as an intermediate mechanism. When leakage occurs, it is amplified by the amplifier and then transmitted to the relay, and the relay controls the switch to disconnect the power supply. The advantages of this protector are: high sensitivity (up to 5mA); small setting error, simple manufacturing process and low cost. Disadvantages are: the transistor has a weak ability to withstand shocks and has poor resistance to environmental interference; it needs an auxiliary working power supply (electronic amplifiers generally need a DC power supply of more than ten volts), so that the leakage characteristics are affected by the fluctuation of the working voltage; when the main circuit is out of phase, the protector protection will be lost.

13. What are the protective functions of the leakage circuit breaker?

Answer: The leakage protector is mainly a device that provides protection when the electrical equipment has a leakage fault. When installing a leakage protector, an additional overcurrent protection device should be installed. When a fuse is used as short-circuit protection, the selection of its specifications should be compatible with the on-off capability of the leakage protector. At present, the leakage circuit breaker that integrates the leakage protection device and the power switch (automatic air circuit breaker) is widely used. This new type of power switch has the functions of short circuit protection, overload protection, leakage protection and undervoltage protection. During installation, the wiring is simplified, the volume of the electrical box is reduced and management is easy. The meaning of the nameplate model of the residual current circuit breaker is as follows: Pay attention when using it, because the residual current circuit breaker has multiple protective properties, when a trip occurs, the cause of the fault should be clearly identified: When the residual current circuit breaker is broken due to a short circuit, the cover must be opened to check whether the contacts are There are serious burns or pits; when the circuit is tripped due to overload, it cannot be reclosed immediately. Since the circuit breaker is equipped with a thermal relay as overload protection, when the rated current is greater than the rated current, the bimetallic sheet is bent to separate the contacts, and the contacts can be reclosed after the bimetallic sheet is naturally cooled and restored to its original state. When the trip is caused by leakage fault, the cause must be found out and the fault is eliminated before reclosing. Forcible closing is strictly prohibited. When the leakage circuit breaker breaks and trips, the L-like handle is in the middle position. When it is re-closed, the operating handle needs to be pulled down (breaking position) first, so that the operating mechanism is re-closed, and then closed upward. The leakage circuit breaker can be used for switching appliances with large capacity (greater than 4.5kw) that are not frequently operated in power lines.

14. How to choose a leakage protector?

Answer: The choice of leakage protector should be selected according to the purpose of use and operating conditions:
Choose according to the purpose of protection:
①For the purpose of preventing personal electric shock. Installed at the end of the line, select a high-sensitivity, fast-type leakage protector.

②For the branch lines used together with equipment grounding for the purpose of preventing electric shock, use medium-sensitivity, fast-type leakage protectors.

③ For the trunk line for the purpose of preventing fire caused by leakage and protecting lines and equipment, medium-sensitivity and time-delay leakage protectors should be selected.

Choose according to the power supply mode:
① When protecting single-phase lines (equipment), use single-pole two-wire or two-pole leakage protectors.

② When protecting three-phase lines (equipment), use three-pole products.

③ When there are both three-phase and single-phase, use three-pole four-wire or four-pole products. When selecting the number of poles of the leakage protector, it must be compatible with the number of lines of the line to be protected. The number of poles of the protector refers to the number of wires that can be disconnected by the internal switch contacts, such as a three-pole protector, which means that the switch contacts can disconnect three wires. The single-pole two-wire, two-pole three-wire and three-pole four-wire protectors all have a neutral wire that directly passes through the leakage detection element without being disconnected. Work zero line, this terminal is strictly prohibited to connect with PE line. It should be noted that the three-pole leakage protector should not be used for single-phase two-wire (or single-phase three-wire) electrical equipment. It is also not suitable to use the four-pole leakage protector for three-phase three-wire electrical equipment. It is not allowed to replace the three-phase four-pole leakage protector with a three-phase three-pole leakage protector.

15. According to the requirements of graded power distribution, how many settings should the electric box have?

Answer: The construction site is generally distributed according to three levels, so the electric boxes should also be set according to the classification, that is, under the main distribution box, there is a distribution box, and a switch box is located below the distribution box, and the electrical equipment is below the switch box. . The distribution box is the central link of power transmission and distribution between the power source and the electrical equipment in the distribution system. It is an electrical device specially used for power distribution. All levels of distribution are carried out through the distribution box. The main distribution box controls the distribution of the entire system, and the distribution box controls the distribution of each branch. The switch box is the end of the power distribution system, and further down is the electrical equipment. Each electrical equipment is controlled by its own dedicated switch box, implementing one machine and one gate. Do not use one switch box for several devices to prevent misoperation accidents; also do not combine power and lighting control in one switch box to prevent lighting from being affected by power line failures. The upper part of the switch box is connected to the power supply and the lower part is connected to the electrical equipment, which is frequently operated and dangerous, and must be paid attention to. The selection of electrical components in the electrical box must be adapted to the circuit and electrical equipment. The installation of the electric box is vertical and firm, and there is room for operation around it. There is no standing water or sundries on the ground, and there is no heat source and vibration nearby. The electric box should be rain-proof and dust-proof. The switch box should not be more than 3m away from the fixed equipment to be controlled.

16. Why use graded protection?

Answer: Because low-voltage power supply and distribution generally use graded power distribution. If the leakage protector is only installed at the end of the line (in the switch box), although the fault line can be disconnected when leakage occurs, the protection range is small; similarly, if only the branch trunk line (in the distribution box) or the trunk line (the main distribution box) is installed Install the leakage protector, although the protection range is large, if a certain electrical equipment leaks and trips, it will cause the entire system to lose power, which not only affects the normal operation of the fault-free equipment, but also makes it inconvenient to find the accident. Obviously, these protection methods are insufficient. place. Therefore, different requirements such as line and load should be connected, and protectors with different leakage action characteristics should be installed on the low-voltage main line, branch line and line end to form a graded leakage protection network. In the case of graded protection, the protection ranges selected at all levels should cooperate with each other to ensure that the leakage protector will not overstep the action when a leakage fault or personal electric shock accident occurs at the end; at the same time, it is required that when the lower-level protector fails, the upper-level protector will act to remedy the lower-level protector. Accidental failure. The implementation of graded protection enables each electrical equipment to have more than two levels of leakage protection measures, which not only creates safe operating conditions for electrical equipment at the end of all lines of the low-voltage power grid, but also provides multiple direct and indirect contact for personal safety. Moreover, it can minimize the scope of power outage when a fault occurs, and it is easy to find and find the fault point, which has a positive effect on improving the level of safe electricity consumption, reducing electric shock accidents, and ensuring operational safety.

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