A mechanical switching device (or a combination device) in low-voltage distribution systems, capable of automatically disconnecting the circuit when the residual current reaches or exceeds a set value, is known as a Residual Current Operated Protective Device (RCD), also referred to as a Ground Fault Circuit Interrupter (GFCI). Installing RCDs in low-voltage distribution systems is one of the effective measures to prevent electric shock accidents, as well as to prevent electrical fires and equipment damage caused by residual currents. To achieve the function of residual current protection, the installation of RCDs must be coordinated with other technical measures for electric shock prevention, such as automatic circuit disconnection, grounding, equipotential bonding, etc.


  1. Principle of Residual Current Protection The main characteristic of residual current protection is as follows:

Its core is wrapped around all current-carrying conductors of an electrical circuit. The magnetic flux generated within the core is instantaneously related to the arithmetic sum of these conductor currents; assuming the current flowing in one direction is positive (I1), then the current flowing in the opposite direction is negative (I2). In a fault-free normal circuit, I1 + I2 = 0, producing no magnetic flux in the core, and the electromotive force in the coil is zero. Ground fault current (Id) passes through the core towards the fault point but returns to the power source through the ground or the TN system’s protective conductor. Thus, the currents through the core are no longer balanced, and the current difference creates magnetic flux in the core. This current is referred to as the “residual” current, and this principle is known as the “residual current” principle.

  1. Classification and Features of Residual Current Protective Devices Residual Current Protective Devices are classified into the following two types based on their operation mode and characteristics: (1) Electromagnetic Type RCD The secondary circuit of a zero-sequence current transformer outputs a voltage that directly triggers the residual current trip device, known as an electromagnetic type RCD. Its characteristic is that its operation is independent of line voltage, has strong anti-interference ability, and high reliability, but is expensive to manufacture and thus more costly. (2) Electronic Type RCD An electronic amplification circuit is inserted between the secondary circuit of the zero-sequence current transformer and the trip device. The output voltage of the transformer’s secondary circuit is amplified by the electronic circuit before exciting the residual current trip device, known as an electronic type RCD. Its characteristic is that its operation depends on line voltage, and its reliability is not as high as the electromagnetic type RCD. However, it is cheaper to manufacture and thus more affordable, making it a more commonly used type of RCD.
  2. Selection of Residual Current Protective Devices (1) For protection against direct contact electric shocks, RCDs should only be used as additional protection to basic protective measures, not as the sole direct contact protection. In this case, RCDs with high sensitivity and quick action should be selected, with an operating current not exceeding 30mA. (2) For protection against indirect contact electric shocks, it should be appropriately combined with the system’s grounding form of the power grid. When the PEN line passes through the RCD’s current transformer, the opposing magnetic fields of the fault currents cancel each other out, making the RCD ineffective in TN-C systems. It is necessary to switch to TN-S systems, TN-C-S systems, or local TT systems to use RCDs. For TT systems, priority should be given to installing RCDs as a protective measure against electric shock. (3) For protection against electrical fires, the rated operating current of the protective device should be much higher than the operating current for electric shock protection. The minimum power to cause an electrical fire is 60-100W, requiring the RCD’s rated operating current to be no more than 300mA for reliable fire prevention. (4) When selecting the type and technical parameters of a Residual Current Operated Protective Device, consider the system capacity, normal residual current leakage size of the circuit, application site, environmental conditions, system wiring, and operating mode. Emphasize the correct use of end-load protection (such as the installation of RCDs within a user’s home) to ensure personal and property safety. Additionally, in low-voltage networks with a single outgoing line, protection can be installed at the source end. For multiple feed lines, protection can also be installed at each feed end. For centralized loads with larger capacities, protection should be installed at the starting point of the branch line, ultimately forming two-level or three-level protection. When determining the use of graded protection, attention should be paid to the coordination of the operation of each level of protection, appropriately selecting the operating current, operating time, delay, and additional protection functions of each level of Residual Current Operated Protective Device.