Residual Current Devices (RCDs)

Purpose and Working Principle of RCDs

The primary purpose of a Residual Current Device is to prevent electric shock to people. A secondary, related goal is to prevent current leakage to conductive casings of household appliances such as water pumps or washing machines. The risk of electric shock or fire increases when the building's electrical network includes wiring with worn-out (old or damaged) insulation, or unreliable and improperly made connections (terminals, wire twists, etc.).

In recent years, combined RCDs have appeared on the market, offering protection not only against differential (residual) current but also against so-called overcurrent.

The operating principle of RCDs is simple and based on measuring the difference (balance) in current. The amount of current passing through the RCD must be equal on two conductors in a single-phase circuit and on four monitored conductors in a three-phase system. The measurement of the algebraic sum of currents and triggering of the device is performed via a current transformer.

If the RCD detects that different amounts of electricity have passed through different conductors within the same time interval, it immediately breaks the circuit, fully blocking further electricity supply. This happens because unequal current flow through monitored and measured conductors indicates a leakage further along the wiring run beyond the RCD. It is most effective to install the RCD directly at the main electrical entry point into the home.

Operating Parameters of RCDs

The strictest requirements for RCD operation are in the USA. There, the device’s accuracy is 5 mA within 25 ms if intended for human protection (residential buildings), and 30 mA for equipment protection. European standards are somewhat less strict: 10 mA for human protection and 500 mA for equipment and machinery.

RCDs are significantly more effective than standard circuit breakers (overcurrent protection devices), as their trip threshold is many times lower than that of fuses. The best fuses respond to leakage currents starting from 2 amperes and above. But 2 amperes is far above the lethal threshold for the human body. Moreover, RCDs must trip within 40 milliseconds—this time is much shorter than the duration required for electric current passing through the human body to cause ventricular fibrillation (irregular heart muscle contractions). Indeed, ventricular fibrillation is the main cause of death in cases of electric shock.