RCD Types: AC, A, F and B Explained
Which residual current device to fit, why it matters for EV chargers, solar and VSDs, and what the ratings mean.
How an RCD works
Line and neutral both pass through a toroidal core. In a healthy single-phase circuit their instantaneous currents are equal and opposite, so the vector sum is approximately zero. Current returning by a protective conductor, Earth, another circuit's neutral, or another unintended path creates residual flux that can operate the trip mechanism according to the device characteristic.
ΔI = | I(line) − I(neutral) |Ratings: what 30 mA actually buys you
Under BS 7671, additional protection is provided by an RCD with rated residual operating current not exceeding 30 mA where the relevant regulation requires it. The requirement depends on circuit, location, intended use, exceptions, and the current edition; an RCD supplements rather than replaces basic protection and fault protection.
| Current field check | General non-delay expectation |
|---|---|
| AC test at IΔn | Operate within 300 ms under the stated BS 7671 verification conditions |
| Integral test button | Operate functionally; follow the device and installation instructions |
| Other tester sequences | Use for diagnosis or where another applicable requirement calls for them; do not present legacy 2×/5× routines as universal current tests |
Choosing the type by waveform
| Type | Detects | Typical use |
|---|---|---|
| AC | Sinusoidal a.c. residual current | Only where the known load and current rules make this type suitable. |
| A | a.c. and specified pulsating d.c. residual current | Many electronic loads, subject to equipment instructions and waveform assessment. |
| F | Type A capability plus specified composite/mixed-frequency residual currents | Certain single-phase inverter loads where the equipment/design calls for Type F. |
| B | Broad waveform capability including specified smooth d.c. residual current | Applications such as drives, PV, storage, or EV equipment only where required and fully coordinated. |
RCD, RCBO or main-switch RCD?
An RCCB provides residual-current protection but not overload or short-circuit protection, so coordinated overcurrent protection is still required. An RCBO combines residual-current and overcurrent functions for one circuit. Individual RCBOs can improve circuit division and reduce cumulative leakage compared with a shared RCCB, but the final arrangement still depends on selectivity, fault rating, neutral switching, RCD type, continuity-of-service risks, and manufacturer compatibility.