Types of RCDs: Sensitivity and Construction Explained

What are the different types of RCDs? A residual current device is classified along two independent axes: sensitivity (the rated residual operating current IΔn, set at 10, 30, 100, 300 or 500 mA) and construction type (AC, A, F or B under IEC 62423), which determines what shape of fault current the device can physically detect. Specify the wrong sensitivity and the device trips too often or too late; specify the wrong type and it may never trip on a real fault at all. This guide covers the IΔn sensitivity bands, the AC/A/F/B waveform classes, super-immunized (SI) and time-delayed (S) variants, and how rated current and pole count complete a full RCD specification.

Sensitivity (IΔn): 10 mA, 30 mA, 100 mA and 300 mA

IΔn is the trip threshold, not a fixed number stamped on every device — manufacturers offer a family of sensitivities on the same body. 10 mA and 30 mA are personal-protection values: they cover additional protection against direct contact, and 30 mA is the standard human-safety threshold written into most wiring codes for socket outlets and bathroom circuits. 100 mA, 300 mA and 500 mA are fire-protection or equipment-protection values — leakage current at those levels can ignite insulation or arc across dust well before it reaches a level that stops a heart.

Lower is not automatically safer for a whole board. A single 30 mA RCD covering an entire distribution board will trip on the cumulative leakage of everything downstream — filters, long cable runs, several SMPS power supplies — long before any one circuit actually faults. What we see in the field: boards fitted with one whole-board 30 mA RCD start nuisance-tripping within weeks once a few VFDs or LED drivers get added, and the fix is splitting the board into RCBO-protected final circuits, not chasing a "faulty" RCD.

Construction Types AC, A, F and B: What Each One Detects

The sensitivity number tells you when it trips; the type letter tells you whether it can see the fault at all. An RCD detects leakage by sensing an imbalance induced in a toroidal core, but pulsating or smooth DC components in the residual current can partially or fully saturate that core, "blinding" a device that wasn't built to handle them.

• Type AC — detects pure sinusoidal AC residual current only. No pulsating or smooth DC component.
• Type A — Type AC plus pulsating DC residual current. Covers most modern electronic and SMPS loads; the practical default for new installations.
• Type F — Type A plus mixed-frequency residual currents, relevant to single-phase variable frequency drives and frequency-controlled loads.
• Type B — Type F plus smooth (pure) DC residual current. Required for three-phase VFDs, EV charge points and transformerless PV inverters.

Why Type AC Is Losing Ground

Type AC was the original RCD classification, and it still turns up on economy ranges for loads with no electronics whatsoever. It won't detect anything more than that. Any switch-mode power supply, variable frequency drive, or LED driver can inject a pulsating or smooth DC residual component into the earth path, and a Type AC device simply cannot register it: the fault current keeps flowing, the fault stays live, and the RCD stays closed. Several national wiring codes now restrict or ban Type AC for socket circuits and any circuit known to feed electronic equipment, making Type A the practical minimum.

Super-Immunized (SI) and Time-Delayed (S) Variants

SI (super-immunized) is an immunity rating layered on top of a type classification, not a separate waveform class. An SI-rated RCD resists nuisance tripping from transient leakage such as switching surges, high-frequency noise, and lightning-induced transients, and most ranges also add resistance to corrosion and dust ingress. Fit SI where an unwanted trip is expensive: server rooms, cold stores, process lines with a real shutdown cost per minute.

Time class is a separate axis again. General-purpose (instantaneous) RCDs trip as soon as the fault crosses IΔn. S-type (selective) devices add a deliberate time delay and a higher effective threshold, roughly 2x IΔn, so they sit upstream of a faster instantaneous device and let the downstream unit clear the fault first. G-type adds a shorter delay for intermediate discrimination. Manufacturers combine these freely with AC/A/F/B — an "A-SI" or "B-S" designation on a nameplate is normal, not a typo.

Rated Current, Poles and Backup Protection

Type and sensitivity describe what the device detects; rated current (In — typically 25, 40, 63, 80 or 100 A) and pole count (2P for single-phase, 4P for three-phase) describe what it can carry. An RCCB has no overcurrent or short-circuit protection of its own: it must sit behind an MCB or fuse sized to clear a fault before the RCCB's rated conditional short-circuit current (Inc, commonly 6 or 10 kA with the correct backup device) gets exceeded. An RCBO folds the MCB into the same module, so this backup coordination is built in rather than specified as a separate device.

Matching Type and Sensitivity to the Load

Selection is a stack of three independent decisions: pick the sensitivity band for the protection goal (personal vs fire/equipment), pick the type for the load's residual waveform, then pick current rating and poles for the circuit itself. Get the order right and the rest follows the load's data sheet rather than guesswork — see our RCD selection checklist for sensitivity, type and poles for the full decision sequence.

Where an upstream RCD backs several downstream RCBOs or RCCBs, selectivity keeps a single circuit fault from dropping the whole board.

This is why a 300 mA S-type RCD commonly sits at the board incomer above a row of 30 mA instantaneous RCBOs: the upstream device is both less sensitive and slower, so the downstream device clears the fault first and only the faulty circuit drops — this depends on the cumulative leakage already present on the board, so measure it before assuming a new circuit will fit under the existing budget.

Series worth knowing by name: Schneider's Acti9 iID RCCB and Vigi add-on block, ABB's F200 RCCB and DS201 RCBO, and Siemens' 5SV RCCB and 5SU1 RCBO all span AC/A/F/B and SI variants within the same physical module width, which is what actually varies between brands more than the underlying electrical principle. Browse the range at our residual current devices collection, or go straight to RCBOs for the combined MCB+RCD modules. For the full picture across all RCD subtopics, see the RCD protection guide; for the module-format question specifically, see RCCB vs RCBO differences. For VFD, EV and solar type selection, see Type A vs Type B for VFDs, EV chargers and solar, and for how RCD sits alongside RCCB, RCBO and older ELCB terminology, our RCD, RCCB and RCBO terminology comparison.

Conclusion

Two axes, not one: sensitivity (IΔn) sets the trip threshold for personal or fire protection, and type (AC/A/F/B, plus SI and S/G time classes) sets whether the device can detect the fault waveform at all. Confusing the two is how boards end up with the wrong device on the wrong circuit — a 30 mA Type AC RCD on a rack of switch-mode power supplies looks correct on paper and does nothing in practice. Specify both independently, check rated current and poles against the circuit, and confirm selectivity between board-level and final-circuit devices before commissioning.