RCD Operating Time

Residual current devices (RCDs) provide protection against electric shock and fire by detecting imbalance between phase and neutral currents — indicating current is leaking to earth through a fault or a person. The operating time determines how quickly the supply is disconnected after the residual current threshold (IΔn) is exceeded. For 30mA RCDs used for personal protection, the maximum operating time is 300ms at IΔn and 40ms at 5IΔn — fast enough to prevent ventricular fibrillation in most circumstances. Type S (selective or time-delayed) RCDs have longer operating times (150–500ms at IΔn) to achieve discrimination with downstream RCDs. RCD types differ in their sensitivity to fault waveform: Type AC detects sinusoidal residual currents only, Type A also detects pulsating DC, Type F adds detection of mixed-frequency residual currents, and Type B detects smooth DC residual currents. The choice of RCD type is critical for EV charging circuits (Type A minimum per BS 7671 Section 722), variable frequency drives (Type B required for DC fault detection), and medical locations (Type A or B with 30mA or 10mA rating). Regular testing — both by test button and instrument — verifies that operating times remain within limits throughout the device's service life.