**Summary**
The leakage switch is a crucial electrical component in ensuring electrical safety. By examining its performance characteristics, it becomes clear that it provides a safer and more reliable method for protecting equipment from leakage faults.
**Key words**: Leakage switch (RCD), ground fault, personal electric shock prevention, electrical safety, leakage current
**1. Foreword**
Leakage switches are increasingly used in the protection of electrical equipment due to their high sensitivity, especially in portable and handheld devices, as well as in socket circuits. When a grounding fault occurs, the faulty circuit can be quickly disconnected, ensuring personal safety. In this paper, we will analyze the characteristics of the leakage switch in the canteen equipment circuit to deepen our understanding of its operation. This will help us use the leakage switch more effectively and fully utilize its protective function. However, it's important to note that leakage switches are not foolproof, and additional measures may still be required for complete grounding fault protection.
**2. Characteristics of the Leakage Switch**
Like a regular circuit breaker, the leakage switch offers overcurrent protection, providing both overload and short-circuit protection. The leakage protection function works by using a current transformer installed within the switch, allowing the phase and neutral lines to pass through it. Under normal conditions, the currents in the phase and neutral lines are balanced, resulting in opposing magnetic fields that cancel each other out, and no induced current is generated. Therefore, the switch remains inactive. However, when a ground fault occurs in the protected circuit, the fault current returns through the PE line to the power supply. This causes an imbalance in the currents passing through the transformer, generating a changing electromagnetic field that induces a trip current, causing the leakage switch to activate and cut off the faulty circuit, thus preventing personal injury.
**3. Application of the Leakage Switch in Canteen Equipment Circuits**
Canteen equipment typically includes steamers, ovens, electric griddles, fryers, mixers, water heaters, and similar devices. These are usually mobile and not fixed installations, often powered via socket boxes. For larger equipment, a circuit breaker may be used. However, since canteen equipment is commonly located in humid environments or exposed to water-containing foods, the risk of electric shock is significantly increased. Therefore, proper safety protection for electrical equipment in canteens is essential.
In the renovation of Daqing Petrochemical Engineering Co., Ltd.'s canteen, the TN-S system was implemented. The wiring method for RCD in this system is illustrated in the following figure:
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In the main distribution box of the canteen, a 5-core cable is used for the incoming line. The PE line is grounded at the entrance, and equipotential bonding is established within the building. All socket PE wires and device casings are grounded through the total equipotential bonding box (MBE). When a ground fault occurs, the cable sheath is usually damaged, allowing the phase line to connect to the equipment casing. The fault current then returns to the power supply along the PE line connected to the casing. This causes an imbalance in the current through the RCD transformer, inducing a trip current that activates the RCD, cutting off the power supply and preventing electric shocks.
According to specifications, the time for a faulty circuit at the end of the distribution line should not exceed 0.4 seconds. To meet these requirements, all equipment in the canteen is protected by leakage switches with a trip current of 30 mA, primarily to prevent indirect electric shocks.
The RCD’s protection function is mainly effective when the protected line itself fails. If the external power supply system experiences a fault, the fault voltage can be transmitted along the PE line to the canteen equipment. When the fault voltage exceeds 50 V, an electric shock accident may occur. Hence, the canteen building is equipped with a total equipotential bonding, which reduces the fault voltage introduced from outside and enhances personal safety.
In the canteen equipment distribution system, RCDs are used in two ways. One approach involves selecting the socket box’s main switch as a leakage switch, with the branch circuits directly connected to the sockets. This method is cost-effective but has limitations, such as potential malfunctions when multiple outlets have large leakage currents. Additionally, if a branch circuit fails, the entire socket box is powered off, affecting all connected devices.
The second approach uses an ordinary switch for the socket box’s main incoming line, while each branch circuit is equipped with a leakage switch. This configuration ensures that only the faulty branch is disconnected, allowing other circuits to continue operating. It also prevents the accumulation of leakage currents, reducing the likelihood of false trips. Although this method is more reliable, it requires more leakage switches, increasing costs.
Given that most canteen equipment operates in a humid environment, where leakage currents are higher than in dry conditions, and considering the long-term degradation of insulation, using a single total leakage switch in the socket box may lead to frequent malfunctions. Therefore, the second method is generally preferred for better reliability and safety.
**4. Conclusion**
This paper discusses the leakage circuit breaker, which combines overload, short-circuit, and leakage protection functions. Several key points should be considered during practical applications:
(1) When installing a secondary leakage switch in the distribution system, the action current and time between the upper and lower switches must be matched. Typically, the operating current of the upper switch is about three times lower than that of the lower switch, and the operating time should be above 0.4 seconds.
(2) A leakage switch should be installed at the main distribution box of the building to reduce fire risks caused by grounding arcs. The operating current should not exceed 0.5 A.
(3) The exposed conductive parts of the electrical equipment protected by the RCD must be grounded via the PE wire, and the PE wire should not pass through the magnetic circuit of the current transformer in the RCD.
(4) It is strictly prohibited to reverse the neutral line and the PE line passing through the RCD, or to intentionally or unintentionally connect the neutral line on the RCD load side to ground, as this can cause the RCD to malfunction.
(5) The RCD leakage switch does not operate at 0.5In, and the total line leakage current should not exceed 0.3In.
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