Type A vs Type B RCD: Which for VFD, EV and Solar
Which RCD type actually protects a VFD, an EV charger, or a solar inverter? A residual current device only trips if its detection circuit can resolve the waveform of the fault current it sees: Type A resolves sinusoidal AC and pulsating DC, Type B additionally resolves smooth DC and higher-frequency components per IEC 62423. Fit a standard Type A device downstream of a three-phase drive, a DC-side PV fault, or an EV charger with no supplementary DC monitoring, and a real leakage current can pass through undetected while the RCD sits closed. This article covers why the core saturates, which equipment classes force a Type B upgrade, and where a Type A plus a 6 mA DC detector is still a compliant, cheaper option.
What Actually Separates Type A from Type B
Every RCD sums the instantaneous current in the line and neutral conductors through a toroidal core. In a balanced circuit that sum is zero; a fault path to earth unbalances it, and the resulting flux induces a trip signal in the sense winding. Type AC devices resolve only pure sinusoidal residual current. Type A adds pulsating DC components superimposed on the AC waveform, which covers rectified loads like variable speed motors, UPS input stages, and most switch-mode power supplies. Type B goes one step further: it resolves smooth, unidirectional DC residual current with no AC ripple at all, plus mixed and higher frequencies up to the range IEC 62423 defines.
The distinction is not marketing. It is a hard physical limit of the core.
Why a Standard Core Stops Tripping on Smooth DC
A toroidal core has a finite permeability range. Feed it a sustained, unidirectional DC current and the core's magnetic operating point shifts toward saturation. Once saturated, the core's ability to couple a superimposed AC signal into the sense winding collapses. A Type A device built on a plain iron core can go effectively blind to further leakage the moment a genuine DC fault current establishes itself, whether or not that DC current alone would have tripped anything. Type B devices use electronic detection (a separate DC sensor plus an AC toroid, or a flux-gate arrangement) specifically to avoid this failure mode.
Formula: Residual Current Detection Condition — Source: IEC 60755, Clause 6.2
IΔ = |IL + IN|
| Symbol | Description | Unit |
|---|---|---|
| IΔ | Instantaneous residual current summed by the toroidal core | A |
| IL | Line conductor instantaneous current | A |
| IN | Neutral conductor instantaneous current | A |
| IΔn | Rated residual operating current, the threshold that must trip the device | A/mA |
The formula itself does not change between types. What changes is whether the sense winding can still resolve that sum once a DC offset is present. IEC 60755 is the classification standard that defines AC, A, F and B; IEC 61008 and IEC 61009 cover the RCCB and RCBO product requirements built on top of it.
Variable Frequency Drives: Where the DC Component Comes From
A three-phase VFD rectifies the incoming AC to a DC bus before the inverter stage re-synthesizes variable-frequency output for the motor. An insulation fault on the DC bus, or leakage through the EMC input filter under fault conditions, can present as smooth DC current to earth with no AC component riding on it. A Type A RCD downstream of that drive will not clear that fault reliably. Single-phase VFDs are a partial exception: they can produce mixed-frequency leakage that Type F resolves, but a genuine smooth-DC fault path still needs Type B. What we see in the field is drive manufacturers specifying Type B (or Type A plus a DC-sensitive residual current relay) on the drive's own installation manual, and installers missing it because the panel spec sheet only says "RCD required."
EV Chargers: Type B, or Type A Plus 6 mA RDC-DD
An EV on-board charger's power electronics can produce a smooth DC fault current under certain failure modes, the same blind-spot risk as a VFD. IEC 62955 gives installers a second compliant path: a Type A RCD combined with a residual DC current detection device (RDC-DD) rated at 6 mA, wired to disconnect the circuit if smooth DC exceeds that threshold. Many charge point manufacturers build the RDC-DD into the unit itself, which lets the upstream protection stay Type A. Check the charge point's datasheet for "integrated 6 mA DC detection" or equivalent wording before assuming Type B is mandatory on that circuit; the requirement moves depending on where the DC sensor actually sits. For circuit sizing and pole count once the type question is settled, see our EV charger RCD requirements guide.
Solar PV Inverters: Why Transformerless Designs Push Toward Type B
Transformerless string inverters, now the majority of the market above a few kW, hold the DC array close to earth potential through internal capacitive coupling. A ground fault on the DC side can inject a residual current into the AC side that is substantially DC in character rather than pure AC. Some inverters include internal residual current monitoring (RCMU) tuned to the inverter's own DC behavior, which can satisfy the protection requirement without an external device, but this depends on the specific inverter's certification and the local wiring regulation. Where an external RCD is specified on the inverter's AC output and the installation isn't relying on an already-certified internal RCMU, Type B is the default answer, not Type A.
Types AC, A, F and B Compared
| Criteria | Type AC | Type A | Type F | Type B | SI (super-immunized) |
|---|---|---|---|---|---|
| Waveform detected | Pure sinusoidal AC only | AC + pulsating DC | Type A range + mixed frequencies | Type A range + smooth DC + mixed frequencies | Same as base type (AC/A/F/B), plus immunity to nuisance trips |
| Typical load | Simple resistive/lighting circuits | SMPS, LED drivers, single-phase electronics | Single-phase VFDs, some induction hobs | Three-phase VFDs, EV chargers, transformerless PV | Sites with surges, high-frequency noise, corrosion or dust |
| Standard | IEC 61008 base type | IEC 62423 | IEC 62423 | IEC 62423 | Layered on IEC 62423 type designation |
| Common status today | Restricted or banned for electronic loads in many codes | Modern default | Application-specific | Mandatory on smooth-DC fault paths | Optional upgrade where unwanted trips are costly |
SI is an immunity option layered on top of a waveform type, not a fifth waveform category on its own; an "SI" device is still fundamentally a Type A, F or B underneath. For the full type breakdown, see Type AC, A, F, B and SI classifications.
Selecting Type When the Load List Doesn't Say
Start from the load, not the panel schedule template. If a circuit feeds a three-phase drive, an EV charger, or a transformerless PV inverter and there is no documented internal DC detection, specify Type B. If the circuit feeds general single-phase electronics with no drive or charger downstream, Type A covers it. Some panel builders standardize on Type B across an entire board to avoid the analysis, which is defensible on a board with mixed and future-unknown loads, but it costs more per pole and is not required by any code as a blanket rule; this depends on how much future-proofing the project actually needs. For sensitivity (IΔn), pole count and the rest of the selection sequence, see our RCD selection checklist. For general background on the residual current devices family this decision sits inside, see the RCD protection guide, and browse current stock in our residual current devices collection or the combined RCBOs range where the DC-sensitive types are stocked alongside standard Type A.
Frequently Asked Questions
Can a Type A RCD protect a three-phase VFD?
Not reliably. A three-phase drive's DC bus can produce smooth DC fault current with no AC component, and a Type A core can saturate and stop detecting further leakage once that DC current is present. Use Type B unless the drive manufacturer documents an alternative.
Does every EV charger need a Type B RCD?
No. IEC 62955 allows a Type A RCD combined with an integrated or external 6 mA RDC-DD as a compliant alternative. Check the charge point's datasheet for built-in DC detection before assuming Type B is required.
Is Type B always required for solar PV?
It is the default for transformerless string inverters unless the inverter has certified internal residual current monitoring that already covers DC fault detection. Check the inverter's own installation manual and local wiring regulation.
What does super-immunized (SI) add on top of Type A or B?
SI improves resistance to nuisance tripping from transient surges, high-frequency leakage, corrosion and dust. It does not change which waveform the device can detect; that is set by the AC/A/F/B classification.
Can I just fit Type B everywhere to avoid the analysis?
You can, and some panel builders do on boards with mixed or unknown future loads. It costs more per pole than Type A and is not mandated as a blanket rule by any major code, so it is a cost-versus-certainty decision, not a compliance requirement.
Will a Type A RCD trip falsely or fail silently on a DC fault?
It fails silently. The core saturates and loses the ability to couple the superimposed AC residual signal into the sense winding, so the device does not trip at all rather than tripping incorrectly.
Conclusion
The Type A versus Type B decision comes down to one question: can the fault path on this circuit produce smooth DC residual current with no AC component? Three-phase VFDs, EV chargers without integrated DC detection, and transformerless PV inverters answer yes, and belong on Type B or a documented Type A plus RDC-DD alternative. Everything else running on single-phase electronics, lighting or general socket circuits is covered by Type A. Confirm against the equipment manufacturer's own installation documentation before finalizing the panel schedule, since the DC detection responsibility sometimes sits inside the load device rather than the upstream RCD.
