RCD Types AC, A, F, B and SI Explained

What are the different RCD types AC, A, F, B and SI? They classify a residual current device by the shape of fault current its toroidal sensing core can detect and trip on, per IEC 62423 and IEC 61008-1. Fit the wrong type and a real earth fault can pass straight through the core undetected, because the sense winding never sees enough flux change to trip the mechanism. This article covers what waveform each type senses, why Type AC is now restricted for electronic loads, where Type F and Type B apply, what "SI" (super-immunized) actually changes, and how the type marking interacts with sensitivity and pole count when specifying a device.

Why the Type Marking Matters More Than It Looks

An RCD's IΔn number tells you how much leakage current trips it. The letter after that number tells you whether it detects the fault at all. Two devices both rated 30 mA can behave completely differently on the same circuit if one is Type AC and the other Type A, because the ability of the sensing transformer to induce a trip current depends on the shape of the residual waveform, not just its magnitude. A sinusoidal 30 mA leakage from a resistive heater looks nothing like a rectified 30 mA leakage from a drive's input bridge, electrically. IEC 62423 formalizes the classification into Type AC, A, F and B, each tested against progressively more complex waveforms: pure sinusoidal, pulsating DC, mixed frequency, and smooth DC.

Type AC: Pure Sinusoidal Residual Only

Type AC detects only a pure sinusoidal AC residual current, tripping reliably on faults like insulation breakdown to earth in resistive or purely inductive loads with no electronic front end. It is the baseline design covered directly in IEC 61008-1 and remains legal for simple circuits — lighting, resistive heating, motors without electronic speed control. The limitation: any DC component in the fault current, even a modest pulsating one from a half-wave rectifier or a phase-control dimmer, can saturate or bias the toroidal core enough to blind Type AC to a genuine fault. That is why wiring codes increasingly restrict or ban Type AC on circuits feeding anything with a rectifier, switch-mode supply, or electronic ballast, which today means most socket circuits, IT equipment, and LED lighting.

Type A: The Modern Default

Type A adds detection of pulsating DC residual current superimposed on the AC component, covering the leakage signature of single-phase rectifiers, switch-mode power supplies, and most consumer or IT electronics. This is what socket circuits need today, because nearly everything plugged into them rectifies AC somewhere in its front end. Type A is now the default minimum in most wiring regulations for socket outlets and has effectively replaced Type AC as the baseline spec on new installations. What we see in the field: panel builders sometimes still spec Type AC on older drawings out of habit, and it usually takes one nuisance non-trip during commissioning to force the change to Type A.

Type F: Mixed Frequencies From Single-Phase Drives

Type F extends Type A's detection range to residual currents with mixed frequency content, which shows up downstream of single-phase variable-frequency drives and some single-phase inverters. A single-phase pump drive or washing machine can generate a residual current waveform that a plain Type A device does not fully resolve at the frequencies its switching produces. Type F is a narrower niche than A or B — it applies specifically where the load is single-phase and frequency-controlled, not three-phase. Three-phase drives need Type B, not Type F. The two are not interchangeable upgrades of each other; they cover different fault signatures entirely.

Type B: Smooth DC Residual for Drives, EV and PV

Type B covers everything Type A and F do, plus smooth (pure) DC residual current — the signature fault current from three-phase VFD rectifier bridges, transformerless PV string inverters, and EV chargers without onboard DC fault detection. A smooth DC fault current can saturate a standard toroidal core in one direction, permanently biasing it and preventing it from ever generating enough differential flux to trip on a subsequent AC fault, on that circuit or even upstream. That failure mode is why Type B is mandatory, not optional, wherever three-phase VFDs, EV charge points without integrated 6 mA DC detection, or transformerless PV inverters sit on the circuit. Type B units cost more, and some ranges exist only as RCCBs rather than RCBOs at higher current ratings — check the specific series before assuming a Type B RCBO exists at the amperage you need. For EV-specific sizing, see our note on RCD protection for EV charger circuits.

SI / Super-Immunized: A Separate Axis From Type

SI (super-immunized) is not a fifth waveform class. It is an added immunity rating layered onto a Type A, F or B device, describing resistance to nuisance tripping from transient leakage — switching surges, high-frequency noise, lightning-induced transients — plus better tolerance of dust and corrosion in the mechanism. A site with long cable runs, many switch-mode supplies, or frequent lightning activity accumulates enough transient common-mode leakage that a standard Type A device trips on events that are not a real earth fault. Swapping to the SI variant of the same type and sensitivity does not change what fault current the device can detect; it changes how much of a transient spike it tolerates before tripping. This depends on the cumulative leakage on the board — a single nuisance-prone device is often a sign the whole distribution board's background leakage needs measuring, not just a swap to SI on one circuit.

Choosing the Right Type for the Load

Start from the load, not the panel schedule template. A resistive or simple inductive load with no electronics downstream can use Type AC, though most specifiers default to Type A anyway because it costs little more and covers whatever electronics get plugged in later. Any single-phase electronic load with a rectifier — chargers, LED drivers, most IT equipment — needs Type A as a minimum. Single-phase VFDs and inverters push the requirement to Type F. Three-phase VFDs, EV chargers without integrated RDC-DD, and transformerless PV inverters require Type B, full stop, with no downgrade path. Layer SI on top of whichever type fits if the site has a documented nuisance-tripping history or a heavy switch-mode electronic load.

Brand ranges do not all cover the same types in the same form factor. Schneider's Acti9 iID and ABB's F200 both offer Type A, F and B variants as compact RCCBs from the same residual current devices family, while Siemens 5SV covers AC, A and F and reserves true Type B for its 5SM3 residual current relay rather than a compact RCCB. When the RCD needs to fit in the same module width as an MCB, check whether the type you need exists as an RCBO in that brand's range before specifying it on the drawing. Terminology around RCCB, RCBO and RCD is covered in our MCB vs RCBO vs RCD vs RCCB comparison, and sensitivity selection — the IΔn number alongside the type letter — is covered separately in our guide to RCD sensitivity classes.

For a full checklist that combines type with sensitivity, poles and rated current, see how to select RCD sensitivity, type and poles. For the specific case of drives and renewables, our comparison of Type A vs Type B for VFD, EV and solar loads goes deeper on sizing. Both build on the fundamentals in our RCD protection guide.

Conclusion

The type letter on an RCD is a waveform-detection spec, not a marketing tier — confusing it with sensitivity is the most common misspecification we see on incoming panel drawings. Match the type to what the load can actually put through the core: AC for genuinely resistive circuits, A as the practical default for anything electronic, F for single-phase drives, and B, without exception, for three-phase VFDs, uncovered EV chargers and transformerless PV. Add SI only when nuisance tripping is a documented problem, not as a default upgrade. Cross-check the final selection against sensitivity and pole count in the RCD protection guide before it goes on the panel schedule.