Circuit protection
RCCB & RCBO Selector (Type & IΔn)
Turn the load waveform, protection purpose, and expected standing leakage into a preliminary residual-current requirement: RCD type (AC, A, F or B), rated sensitivity IΔn, and whether an RCCB with a separate MCB or an all-in-one RCBO fits the circuit. A design starting point — the mandatory type and sensitivity follow the national wiring rules.
Type: match the waveform, not just the load
An RCD only detects the residual-current shapes it is built for. Fit a type that cannot see the earth fault and the protection is defeated, so the waveform the load can produce governs the choice.
| Type | Detects | Typical circuits |
|---|---|---|
| AC | Sinusoidal AC residual current | Legacy resistive loads. Not permitted for general use in many regions. |
| A | AC + pulsating DC | The common default — sockets, lighting, single-phase electronics. |
| F | Type A + mixed frequencies | Single-phase VFDs, washing machines, some IT loads. |
| B | Type F + smooth DC | Three-phase VFDs, EV chargers, PV inverters. |
Sensitivity: what IΔn is for
- 30 mA — additional protection for people. Required for most socket circuits and wet locations.
- 100–300 mA — fire protection on feeders and installations.
- ≤ 500 mA — fault protection / disconnection where personal-shock protection is provided elsewhere.
Standing (healthy) leakage from filters and long cables must stay well below the trip threshold. IEC 60364 guidance keeps the total steady leakage under about one third of IΔn, so the working rule is:
If several circuits share one RCD, their leakage adds up — split the load across more RCDs, or use a lower-leakage arrangement, rather than raising IΔn past the level required for protection.
RCCB or RCBO?
An RCCB provides residual-current protection only and needs an upstream MCB for overload and short-circuit protection. An RCBO combines both in one module, isolates the affected circuit alone, and suits per-circuit protection in a modern board. Choose the RCBO when board space and per-circuit selectivity matter; choose an RCCB + MCB to protect a group of final circuits together.
Frequently asked questions
Can I still use a Type AC RCD?
Many wiring standards no longer allow Type AC for general circuits because modern electronic loads produce pulsating DC residual currents it cannot detect. Type A is the usual minimum. Check the current edition adopted for the project.
Why does my RCD trip with nothing obviously wrong?
Cumulative standing leakage from electronics and long cables can approach the threshold. Keep IΔn ≥ 3 × the total leakage and split circuits across more RCDs instead of raising the sensitivity.
Do EV chargers need a Type B RCD?
EV and three-phase drive circuits can produce smooth DC residual current that blinds Type A/F. A Type B RCD, or a charger with built-in DC fault detection plus Type A, is used — follow the equipment instructions and local rules.
Select the RCD
With the type, sensitivity, and pole count set, match a residual-current device to the circuit and market.
Preliminary guidance only. The mandatory RCD type, sensitivity, and arrangement must be confirmed by a qualified person against the adopted wiring standard, the equipment, and the site leakage.