RCD types AC, A, F and B explained

Two RCDs marked 40 A and 30 mA can provide different protection. AC, A, F or B identifies the residual-current waveforms a device is designed for. Explore the shapes and everyday examples.

One waveform, different RCD capabilities

Choose an RCD type

A adds pulsating DC residual-current detection to AC capability.

Choose a leakage waveform

Pulsating DC

Current pulses in one direction and may fall to zero. Rectifiers can produce this shape; A, F and B cover it.

A: This waveform is covered by the type

The graph shows residual leakage current, not appliance supply current. Shapes are illustrative; amplitudes, frequencies and trip times are not simulated. Coverage alone does not mean a trip: current magnitude and test conditions also matter.

Where it can occur · A

Electronic appliances containing rectifiers. A is often a starting point for household circuits.

What to check · A

A does not detect standalone smooth DC as a protective function. Withstanding a limited superimposed DC component is a different property.

Contents

Three markings, three different questions

40 A

40 A · load current

Rated current under manufacturer conditions, not leakage sensitivity. An RCCB needs coordinated overcurrent protection; it does not protect itself against overload.

30 mA

30 mA · sensitivity

Rated residual operating current IΔn. A 30 mA and B 30 mA devices cover different waveforms; trip thresholds also depend on waveform.

A · waveform type

A letter or associated symbol identifies the type. The enlarged letter here is illustrative; actual symbols and positions vary by model.

Connect the RCD type to the equipment

Household electronics

Check type A or the type specified for the equipment. Appliance wattage and socket count alone cannot determine the letter.

Washing machines and air conditioning

A single-phase variable-speed drive may require F. A particular model may require something else, including B: consult its RCD instructions.

Three-phase drives

Rectifiers and converters can create smooth DC leakage. Check whether B is needed and which frequencies apply, rather than deciding from phase count alone.

Solar inverters

If an RCD is required on the AC circuit, its type depends on AC/DC separation, design and manufacturer instructions. The EV 6 mA exception is not a general PV rule. Reverse power flow also needs a suitable directional rating.

EV charging: why 6 mA DC?

Does the charger documentation confirm a 6 mA DC RDC-DD?

Not sure

“DC protection” or “leakage monitoring” alone is insufficient. Find the model, standard and disconnection arrangement or ask the manufacturer. Do not assume the A + RDC-DD option is satisfied.

6 mA DC detection limits DC exposure for protection compatibility. It does not replace required 30 mA shock protection. Multiple chargers and other DC sources require an assessment of cumulative leakage through shared RCDs.

Common mix-ups

RCD type B ≠ MCB B16

RCD type B concerns residual-current waveforms. In B16, B is the MCB instantaneous trip characteristic and 16 is its rated current. An RCBO can have a C16 curve and type A residual protection together.

S means time delay

S denotes a selective delayed device, not another waveform after B. A-S and B-S are examples. Coordinate delay and sensitivity downstream; S is not an ordinary substitute for fast additional protection.

DC can blind upstream protection

DC can saturate a sensing core and impair detection. A downstream B does not automatically make every upstream A suitable: DC passes through both before disconnection. Check the complete chain using manufacturer data.

How to check the selection

1
Find equipment instructions for RCD type, IΔn, frequencies, built-in DC protection and wiring arrangement. Ask the manufacturer if information is missing.
2
Check local rules, earthing, rated current and overload protection. Waveform type does not resolve these separate requirements.
3
Coordinate all RCDs in series and other leakage sources. Required installation tests still apply; TEST does not establish correct type selection.

Frequently asked questions

Is B always better and safer?

It covers more waveforms, but current rating, sensitivity, frequency range, directionality and coordination still matter. Buying B does not correct design errors.

Can an existing AC RCD stay?

The letter alone cannot assess an installation. Equipment, possible leakage shapes, device condition and national rules need review by an electrician.

Does every inverter need F?

No. F suits specified single-phase drives. Smooth DC or other design features may require B; consult the exact model instructions and installation rules.

Does a successful TEST prove the type is correct?

No. TEST exercises an internal test circuit. It does not reproduce every leakage waveform from your equipment or prove coordination.

Which RCD type does a heat pump or induction hob need?

There is no single letter for these appliances. Check the installation manual: many models specify type A, but some inverter heat pumps require F or B. If the manual is silent, ask the manufacturer; an electrician makes the final choice under national rules.

RCD types AC, A, F and B: which leakage can they detect?