What is the difference between electronic and electromagnetic residual current devices?

　　Residual current devices are classified into two types according to their tripping mechanisms: electronic and electromagnetic.

　　① Electromagnetic trip-type residual current devices use an electromagnetic trip mechanism as an intermediate component. When a leakage current occurs, the mechanism trips and disconnects the power supply. The drawbacks of this type of protector are: high cost and complex manufacturing processes. Its advantages include: strong immunity to interference and excellent resistance to shocks (such as overcurrent and overvoltage surges) provided by its electromagnetic components; no need for an auxiliary power supply; and unchanged leakage characteristics even under zero-voltage conditions or after phase failure.

　　② Electronic residual-current devices use a transistor amplifier as an intermediate mechanism. When a leakage current occurs, the amplifier boosts the signal and transmits it to a relay, which then controls the switch to disconnect the power supply. The advantages of this type of protector include: high sensitivity (down to 5 mA); small setting errors; simple manufacturing process; and low cost. However, its disadvantages are: transistors have relatively weak shock resistance and poor immunity to environmental interference; they require an auxiliary power supply (electronic amplifiers typically need a DC power supply of around ten volts), making the leakage characteristics susceptible to fluctuations in the operating voltage; and when one phase is missing in the main circuit, the protector will lose its protective function.

　　An electromagnetic residual current circuit breaker does not require a auxiliary power supply, contains no circuit board components, and is independent of the operating voltage of the switch. As long as the leakage current in the circuit reaches the tripping current threshold of the electromagnetic mechanism, the residual current circuit breaker will trip, cutting off the power supply downstream and thereby protecting personal safety.

　　An electronic residual-current circuit breaker requires a supplementary power supply, which is derived from the outgoing terminal (load side) of the RCD. This supplementary power supply is dependent on the operating voltage of the circuit. If the voltage drops below a certain threshold—specifically, the exact voltage at which the device fails to trip must be determined by the manufacturer—then even if a leakage current has been generated in the circuit and has reached the switch’s tripping threshold for leakage current, the switch still won’t trip.

　　A residual-current device, also known as a leakage circuit breaker or leakage switch, is primarily used to protect against electric shock hazards that could be fatal to humans in the event of a leakage fault in electrical equipment. It features overload and short-circuit protection functions, making it suitable for safeguarding circuits or motors against both overloads and short circuits. Under normal operating conditions, it can also be used for infrequent switching and starting of circuits.

　　When the power grid is grounded, the residual-current device can operate normally.

　　According to their protective functions and applications, they can be categorized into three types: leakage current protection relays, leakage current protection switches, and leakage current protection sockets.

　　The operating principles of electromagnetic and electronic residual current devices (RCDs) differ:

　　In electromagnetic residual current protective devices, the induced voltage in the secondary circuit is not amplified; instead, it directly actuates the trip mechanism. Therefore, no auxiliary power supply is required (the operating function is independent of the supply voltage).

　　The secondary circuit of an electronic residual-current device (RCD) requires the induced voltage to be amplified by an electronic amplification circuit before it can actuate the trip mechanism. Therefore, the electronic RCD needs to be supplied with power for its electronic amplification circuit and trip mechanism to function properly (its operating performance is dependent on the supply voltage).

　　What protective functions does an RCD (residual current device) have?

　　A residual-current device (RCD) is primarily a protective device designed to provide safety in the event of a leakage current fault in electrical equipment. When installing an RCD, it is advisable to also install an overcurrent protection device. If a fuse is used for short-circuit protection, its specifications should be compatible with the RCD’s breaking capacity.

　　Currently, residual-current circuit breakers—integrated devices that combine leakage protection with a power switch (automatic air circuit breaker)—are widely adopted. This new type of power switch offers protection against short circuits, overloads, leakage currents, and undervoltage. Its installation simplifies wiring, reduces the size of the electrical panel, and makes management more convenient.

　　The model designation on the leakage circuit breaker’s nameplate has the following meanings: When using the device, please pay attention—since leakage circuit breakers offer multiple protective functions, you should carefully identify the specific cause of a tripping event. If the leakage circuit breaker trips due to a short circuit, you must open the cover and inspect the contacts for severe burn damage or pitting. If the breaker trips because of an overloaded circuit, do not immediately attempt to reset it.

　　Since the circuit breaker is equipped with a thermal relay for overload protection, when the current exceeds the rated value, the bimetallic strip bends and causes the contacts to open. The contacts can only be closed again after the bimetallic strip has naturally cooled down and returned to its original state.

　　When a trip occurs due to an earth leakage fault, the cause must be identified and the fault eliminated before re-closing the circuit breaker. It is strictly prohibited to force the circuit breaker to close. When an earth leakage circuit breaker trips, its handle typically remains in the middle position. To re-close it, first push the operating handle downward (to the tripped position) to reset the operating mechanism, then lift the handle upward to close the circuit breaker. Earth leakage circuit breakers can be used as switching devices for power circuits with relatively large capacities (greater than 4.5 kW) that are not frequently operated. (Reprinted from Yunhan Xincheng)