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 circuit the two currents are equal and opposite, so their magnetic fields cancel. Any imbalance — current returning via the earth path instead of the neutral — leaves a net flux that induces a current in a sense winding and releases the trip mechanism.
ΔI = | I(line) − I(neutral) |Ratings: what 30 mA actually buys you
Only a 30 mA (or lower) device provides 'additional protection' against electric shock. BS 7671 requires it for socket outlets rated up to 32 A intended for general use, and for circuits in bathrooms and similar.
| Test current | Maximum disconnection time (30 mA RCD) |
|---|---|
| IΔn (30 mA) | 300 ms |
| 2 × IΔn (60 mA) | 150 ms |
| 5 × IΔn (150 mA) | 40 ms |
Choosing the type by waveform
| Type | Detects | Typical use |
|---|---|---|
| AC | Sinusoidal a.c. residual only | Legacy resistive loads. No longer the default choice. |
| A | a.c. + pulsating d.c. residual | Modern general use — anything with electronics/SMPS. The new baseline. |
| F | a.c. + pulsating d.c. + mixed frequencies | Single-phase inverter loads, some washing machines, class-1 VSD equipment. |
| B | All of the above + smooth d.c. residual | Three-phase EV chargers, PV inverters without isolation, three-phase VSDs. |
RCD, RCBO or main-switch RCD?
A stand-alone RCD only does earth-fault protection; it must be paired with overcurrent protection. An RCBO combines a 30 mA RCD and an MCB in one module, so a fault on one circuit trips only that circuit — far better than a shared RCD taking out half the board. For new work, per-circuit RCBOs are usually the right answer for both safety and nuisance-trip resilience.