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Residual current device RCDs have become widely used and are recognized by everyone, but there are still some misconceptions in their use. Changhui Instruments lists a few of these misconceptions here for your reference. http://yunrun.com.cn/upload/202208/11/202208110131454430.png 1. Residual current device RCDs are not a panacea. It’s important to understand that RCDs are not infallible; installing one does not guarantee complete safety. For example, in the case of a fault occurring elsewhere that is transmitted along the PE wires of the same system to non-faulty devices, an RCD is ineffective and cannot provide protection. One night in July 2021, a girl on the third floor of an apartment building in Longgang District, Shenzhen, died from an electric shock while taking a bath; her mother was also shocked in an attempt to save her, but was rescued after receiving medical treatment. The cause of the electric shock was leakage from the ceiling in the living room on the second floor, along with a malfunctioning RCD in that room and improper installation of the PE wire, all of which led to this tragedy. m.yunrun.com.cn 2. The type of residual current device used is incorrect. There are 4 types of RCDs, and each type has its own appropriate application areas. In practice, many users opt for cheaper AC-type RCDs with inferior performance in order to reduce costs. Table 1 Classification of RCDs. From a developmental perspective, the application scope of type AC will further shrink and it will eventually be phased out. The cost of type B needs to be significantly reduced in order to expand its use. Additionally, end protection should use RCDs without delay in tripping, while those at the front end can use delayed-type products. 3. Regarding the setting values of residual current device protection devices, the RCD settings mainly include the rated operating current value and the delay time; errors are more likely to occur with the former. To ensure personal safety, for RCDs used for electric shock protection, the rated operating current value of such residual current devices must not exceed 30mA. Relevant provisions are set out in China’s mandatory standard GB 55024-2022 on General Specifications for Building Electrical Systems and Intelligentization, as well as in relevant IEC standards. Figure 1: Human body affected by current and exposure time. Figure 1 is from IEC 60479-1:2018 “Effects of current on human beings and livestock – Part 1: General aspects”, which deals with the effects of current on humans and livestock, Part 1: General aspects. Theoretically, zones AC-1, AC-2, and AC-3 are relatively safe. However, this chart is based on tests conducted on humans, animals, and corpses, and it reflects probabilities; moreover, the tests were mainly carried out on people from Europe and America, so it cannot be determined with certainty whether they are entirely applicable to people from East Asia (as we lack similar research studies). Therefore, using a 30mA upper limit is relatively safe. There are still many misconceptions regarding the use of Residual Current Devices (RCDs). After all, these devices have been in use for over 40 years, so we can discuss this further at some other time.