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An air switch refers to a circuit breaker with air insulation; its symbol is MCB, which stands for miniature circuit breaker. Let’s take a look at the image below: http://yunrun.com.cn/upload/201909/30/201909300200471648.png In this image, the component in an MCB that is used for short-circuit protection is called a magnetic release, while the component used for overload protection is called a thermal release. The difference between air switches and residual current devices: yunrun.com.cn/tech/2722.html. The core of a magnetic release device is the coil and its iron core. When a short-circuit current flows through the coil, the magnetic field and flux generated by the electromagnetic coil act on the armature at the end face of the iron core, causing the MCB to trip. As for the thermal trip device, its core is a bimetallic strip. We imagine that one side of the bimetallic strip is made of iron, and the other side is made of copper. Iron sheets and copper sheets have different coefficients of thermal expansion; the better the electrical conductivity, the higher the coefficient of thermal expansion. Thus, when current flows through the bimetallic strip heater, it heats up and bends. After continuing for a while, when the bending force and degree of the bimetallic strip are sufficient, it pushes the air switch to trigger tripping. An air switch is actually a type of low-voltage thermal-magnetic circuit breaker. Let’s take a look at the diagram below, which shows the structural layout of a thermomagnetic circuit breaker: http://yunrun.com.cn/upload/201909/30/201909300227117647.png. Suppose the rated current of the circuit breaker is I; then the relationship between the circuit breaker’s operating time t and the current I is given by t=f×(K/I) or t=f×(K/I2). This relationship in the formula is referred to as an inverse-time relationship. We can see that the greater the current, the shorter the operating time of the circuit breaker. Let’s take a look at the image below: http://yunrun.com.cn/upload/201910/01/201910011740236722.png. The image shows 4 different MCB tripping characteristics, namely Type B, Type C, Type D, and Type K. Let’s take characteristic B as an example for discussion. We can see that when the short-circuit current is less than 3 times the rated current In (3In), the short-circuit protection of the air switch does not activate ； The air switch operates when the short-circuit current exceeds 3In. Let’s take a look at the following image: http://yunrun.com.cn/upload/201909/30/201909300238396454.png This image shows the B characteristic curve mentioned earlier; note that the current value for overload protection starts at 1.13In. Let’s take a look at the working principle of the leakage protector. The symbol for a residual current device is RCD. Let’s take a look at the image below: http://yunrun.com.cn/upload/201909/30/201909300249323742.png In this image, we can see the core of the zero-sequence current transformer, with a phase wire L and a neutral wire N passing through it. Since the phase current IL and the neutral current IN are equal in magnitude but opposite in direction, that is, IL + IN = 0, no flux is generated in the core of the zero-sequence current transformer under normal conditions, and therefore the RCD will not operate. When a leakage current occurs, the leakage current Id flows out through the ground wire PE. At this time, the current relationship is: IL + Id + IN = (IL + IN) + Id = Id. A magnetic flux is generated in the core of the RCD, and its secondary winding produces an operating voltage that drives the control mechanism within the RCD to activate. The action of the control mechanism drives the upstream MCB connected to the RCD to perform the circuit disconnection operation. It can be seen that an RCD cannot operate independently; it must work together with an MCB to form a system that performs leakage protection functions. In the right diagram of the above image, if the enclosure of the electrical appliance is not grounded, the RCD will not provide leakage protection. In the grounded system TN-C with a neutral wire, it has only a live wire and a neutral wire, without a ground wire. When an electrical appliance leaks current, the zero-sequence current transformer of the RCD does not generate flux, and therefore it does not activate. Therefore, RCD is not suitable for TN-C grounding systems. However, when a person comes into contact with live wires, the current generated by the electric shock flows back to the power source through the ground, causing an equivalent leakage current Id to flow through the zero-sequence current transformer of the RCD; as a result, the leakage protection device RCD activates. From this perspective, RCDs can still be used in TN-C grounding systems. Of course, in TN-C-S and TN-S grounding systems, the protection provided by RCDs takes precedence, preventing electric shock accidents to humans. In a TN-C grounding system, electric shock to the human body occurs first, with the protection provided by the RCD coming afterward. It can be seen that for a TN-C grounding system, it is best to create a separate ground wire to form a TT system within the TN-C grounding system, thereby ensuring electrical safety. Generally, for personal safety protection, the leakage operating current is 30 milliamps ； To prevent electrical fires, the leakage current should be greater than 100 milliamps ； It provides both personal safety protection and fire protection, with a leakage current ranging from 30 to 100 milliamps. Let’s take a look at the image below: http://yunrun.com.cn/upload/201909/30/201909300257191409.png. It shows the operating current range of RCDs, which ranges from 30 milliamps to 500 milliamps. These RCDs with different specifications can be used for various leakage protection scenarios. Finally, in a typical home electrical panel, circuit breakers and residual current devices can be installed together, and this is the case in almost every household. Author: Zhang Baifan