Summary    The leakage switch is an important electrical component in electrical safety protection. By analyzing the performance characteristics of the leakage switch, it is safer and more reliable to use in actual equipment leakage protection.
Key words   Leakage switch ( RCD ) ground fault to prevent personal indirect electric shock  Electrical safety leakage current
1. Foreword
Leakage switches are used more and more widely in the protection of electrical equipment due to their high sensitivity, especially in hand-held and mobile equipment, socket circuit protection. When the equipment has a ground fault, the faulty loop power supply can be cut off quickly and in time to ensure personal safety. In the following, through the characteristic analysis of the leakage switch on the canteen equipment circuit, we can further deepen the understanding of the leakage switch, make the use of the leakage switch more flexible, and fully exert the protection function of the leakage switch. At the same time, we also need to understand the limitations of the use of leakage switches, its role is not foolproof, to do grounding fault protection of electrical equipment, but also need to assist other measures.
2. Characteristics of the leakage switch
Leakage switch overcurrent protection function is the same as ordinary circuit breaker, which can provide overload and short circuit protection. The leakage protection function passes the current transformer installed in the switch to make the phase line and the neutral line pass through the transformer. When the equipment is in normal operation, the phase line and the neutral current in the current transformer are balanced, the current is equal, and the direction is opposite. The generated magnetic fields cancel each other out, and no induced current is generated in the transformer, and the switch does not operate. When the switch protection circuit has a ground fault, the fault current returns from the PE line to the power supply. The current through the transformer phase line and the neutral line is unbalanced, generating a changing electromagnetic field, inducing the trip current, causing the leakage switch to operate, cutting off the faulty loop power supply, and protecting Personal safety.
3. Application of leakage switch in canteen equipment circuit
The canteen equipment mainly includes steamer, oven, electric baking pan, frying pan, mixer, water heater, etc. Their common feature is mobile equipment, not fixed installation. Generally, the socket box is used for power supply, and the circuit breaker is only used when the equipment capacity is large. In addition, the canteen equipment is generally in a humid environment, or it is treated with water-containing foods, which is extremely prone to electric shock accidents. Therefore, it is very important to do a good job in the safety protection of canteen electrical equipment.
The TN-S system is used in the renovation of the canteen of Daqing Petrochemical Engineering Co., Ltd. The wiring method of RCD in the system is as shown in the following figure:
In the canteen of the main distribution box, the 5- core cable is used for the incoming line. The PE line is grounded at the entrance, and the equipotential bonding is set in the building. All socket PE wires and device housings are grounded through the total equipotential bonding box MBE . When the equipment has a ground fault, the cable sheath is generally damaged, the phase line is connected to the equipment, and the fault current will be returned to the power supply along the PE line connected to the equipment casing. The current in the RCD transformer is unbalanced, and the induced current acts as a trip unit. The RCD quickly cuts off the power supply to achieve the risk of preventing electrical contact.
According to the requirements of the specification, the time of the faulty circuit at the end of the distribution line of the mobile equipment is not more than 0.4s . Therefore, in view of the particularity of the canteen equipment, to meet the above requirements, in the canteen renovation, all equipment is protected by leakage switch, mainly to prevent indirect personal electric shock, switch trip current In = 30mA .
The protection function of RCD is mainly manifested in the failure of the protected line itself. When the power supply system outside the building fails, the fault voltage can be transmitted along the PE line to the canteen equipment. When the fault voltage exceeds 50V , an electric shock accident can also occur. Therefore, the canteen building is provided with a total equipotential bonding, which reduces the fault voltage value introduced outdoors and protects personal safety.
RCD uses two ways to distribute power in the canteen equipment distribution system. One is that the main switch of the socket box is selected as the leakage switch, and the branch circuit is directly connected to the socket for power supply. See the following figure.
The advantage of this type of distribution line system is that the socket box has lower cost and better economy. The disadvantage is that when there are many outlet circuit power distribution devices, or the leakage current is large, the cumulative total value of the leakage current is large, which may cause the switch to malfunction. In addition, when the branch circuit fails, the main switch action trips, the socket box is powered off, and all loop devices cannot be used, and the influence range is large.
The other is that the socket box total incoming line switch is an ordinary switch, and each branch circuit adopts a leakage switch as shown in the following figure:
The advantage of the power distribution system of the socket box is that when the branch circuit equipment is faulty, the fault circuit can be cut off, and other circuits are not affected, and the work can continue. In addition, when the equipment is in a humid environment or the long-term use of insulation and aging leakage current is large, the branch circuit leakage current is not accumulated, and the increased current value will not cause the switch to malfunction. This kind of power distribution system has high reliability and is very suitable for practical use on site. . The disadvantage is that the number of leakage switches used in the socket box is high, the cost is high, and the economy is poor.
In practical applications, most of the canteen equipment is in a humid environment, and the leakage current is larger than the dry environment. When the equipment is used over the years, the insulation level is reduced and the leakage current is also increased. Only one total leakage switch is set in the socket box. The accumulated leakage current value is easy to cause malfunction. In particular, a branch circuit device is faulty, other devices cannot work, and the reliability is not high. Therefore, the socket box power distribution system is used in the canteen equipment, and it is better to choose the latter.
4. Conclusion
The leakage switch described in this paper mainly refers to the leakage circuit breaker, which has the functions of overload, short circuit protection and leakage protection. In the actual application process, the following points should also be noted:
( 1 ) When the secondary leakage switch is installed in the distribution system, the action current and time should be matched between the upper and lower switches. According to the design manual, the operating current is generally 3 times lower than the lower switching current, and the operating time is 0.4. s above.
( 2 ) Install a leakage switch at the main distribution box of the building. In order to reduce the fire caused by the grounding arc, the operating current requirement is not more than 0.5A.
( 3 ) The exposed conductive part of the electrical device protected by the leakage switch ( RCD ) shall be grounded via the PE wire, and the PE wire shall not pass through the magnetic circuit of the current transformer in the RCD .
( 4 ) It is strictly forbidden to reverse the neutral line and the PE line passing through the RCD , or connect the neutral line on the RCD load side to the ground intentionally or unintentionally to cause the RCD to malfunction.
( 5 ) The RCD leakage switch does not operate at 0.5In , and the total line leakage current value is not more than 0.3In .Shenzhen Xcool Vapor Technology Co.,Ltd , https://www.szxcoolvape.com