Residual Current Devices (RCDs) are essential for compliance and safety under BS 7671, but not all types are the same. Many existing installs still rely on legacy Type AC devices that cannot detect the leakage currents created by modern appliances. Type A is now the baseline, but even that does not cover every scenario. In this guide we explain the differences between Type AC, A, F and B RCDs, when each should be used, and what electricians need to watch out for on site.