CareHomeDeskCareHomeDeskTypes of RCD, where they are required, and what the 18th Edition of the wiring regulations changed.
A Residual Current Device (RCD) detects imbalances between live and neutral conductors -- caused by current flowing to earth through a fault or through a person -- and disconnects the circuit in milliseconds, preventing electrocution or fire. RCDs do not replace fuses or MCBs (which protect against overcurrent) -- they provide additional protection against earth faults.
The 18th Edition of BS 7671 (the Wiring Regulations, published 2018 and amended 2022) significantly extended the requirement for RCD protection. All final circuits in new and rewired installations require RCD protection, including lighting circuits. Socket outlets require 30mA RCD protection wherever they might be used to supply portable equipment outdoors or in bathrooms, kitchens, laundries and other high-risk locations.
Type AC RCDs respond to sinusoidal AC fault currents. Type A RCDs additionally respond to DC pulsating residual currents -- required where electronic equipment (inverters, variable-speed drives, EV chargers) is connected. Type F RCDs provide additional protection for certain high-frequency currents. The EICR should identify where incorrect RCD types are installed.
RCDs must be tested using the integral test button at least quarterly (monthly for high-risk premises). The test button test confirms the mechanism operates -- it does not confirm the trip time or sensitivity. A full functional test using a calibrated RCD tester is required as part of the EICR. Log all test button tests in the electrical maintenance log.
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