Type B RCD

Standard Type AC and Type A RCDs rely on the magnetic coupling of residual current in their toroidal core — they work well for sinusoidal AC and pulsating DC residual currents but cannot detect smooth DC components. Modern power electronic equipment (three-phase inverters, VFDs, certain EV chargers, and PV systems with transformerless inverters) can produce smooth DC fault currents that saturate the toroidal core of Type A RCDs, potentially preventing them from detecting even AC residual currents — a dangerous desensitisation effect. Type B RCDs incorporate additional detection circuitry, often using Hall-effect sensors or flux-gate magnetometers, that responds to DC components as well as AC and pulsating DC. IEC 62423 requires Type B RCDs to operate at DC residual currents from 0.4 × IΔn to 2 × IΔn. Type B+ (enhanced) RCDs additionally detect high-frequency residual currents above 50 Hz, which some inverter topologies can produce. While significantly more expensive than Type A devices, Type B RCDs are increasingly required by standards — BS 7671 Section 722 requires them for Mode 4 DC EV charging, and many VFD manufacturers mandate Type B protection in their installation guides to prevent the DC desensitisation risk.