RCD types AC, A, B: which one where

A residual current device works by measuring the difference between the current going out on the phase and coming back on the neutral. Any imbalance is assumed to be flowing to earth. The assumption holds for a person touching a live conductor - but it does not always hold for the current that a modern load sends to earth, and that is what the type letter is about.

The four types

The letters describe what the sensing coil can detect, not the tripping current. A 30 mA Type A and a 30 mA Type AC both claim to protect people at 30 mA - but only the Type A can see the fault when the residual current is not a clean sine wave.

Why Type AC fails silently

An electronic load - a switched-mode supply, an LED driver, a dimmer - draws current in short pulses rather than a smooth sine. When a fault occurs on the DC side of such a load, the residual current is a pulsating or smooth direct current.

A Type AC device is an AC-only current transformer. A smooth DC component saturates the core, and a saturated core stops responding to AC as well. The result is not a false trip: it is a device that sees nothing, at any current, including the 300 mA that should protect the cable.

That is why Type AC has been progressively removed from domestic scope in several European markets, and why Type A is now the sensible minimum for anything with electronics on it.

Where Type B is genuinely required

Type B is expensive - often five times a Type A - and it is needed in specific cases:

• Three-phase rectifiers without a transformer, including many drive front ends.
• PV inverters, which can inject smooth DC fault current into the installation.
• EV charging, where the on-board rectifier is on the load side.
• UPS systems with transformerless output stages.

Some of these can be handled by a Type A device plus a Type B detection module at the charge point, which is a common and cheaper arrangement for EV charging. It is a design decision with a standard behind it, not a place to improvise.

Sensitivity: 30 mA and 300 mA do different jobs

The tripping current is a separate decision from the type:

• 30 mA is the value associated with additional protection against electric shock. It is not a guarantee of safety - it is the level at which a healthy adult usually survives, most of the time.
• 300 mA is for fire protection: limiting the energy in a small earth fault so that it does not ignite surrounding material.
• 10 mA exists for wet locations and medical areas.

Selective (delayed) versions, marked S, are used upstream of 30 mA devices so that a downstream fault trips the local device rather than the whole board. The time delay is typically 40 ms at the higher current, and the S device must be rated at least three times the downstream sensitivity - 100 mA or 300 mA S above a 30 mA.

The nuisance tripping problem

A variable speed drive with a long motor cable has capacitance to earth. Every switching edge charges and discharges that capacitance, and the resulting current appears at the RCD as residual current. At 8 kHz switching frequency and 100 m of cable, a few milliamps of leakage is normal and 30 mA devices trip.

Four practical measures, in order:

1. Use a Type B or Type F device designed for the waveform - Type AC will trip and Type A may too, depending on the drive.
2. Reduce the switching frequency or fit a dv/dt filter, which cuts the leakage at source.
3. Shorten the motor cable or use a shielded cable with the shield bonded at both ends.
4. Split the RCD zones so that a drive does not share a 30 mA device with socket circuits.

Capacitive leakage also comes from long cable runs, from filters, and from banks of capacitor banks used for power factor correction. An installation with 200 m of cable and a PFC bank is not a candidate for a single 30 mA Type AC device at the origin.

Testing and failure modes

The test button on an RCD injects a calibrated imbalance through a resistor. It proves the mechanism trips, not that the sensing coil works at the rated current - a device can pass the button test and still be blind.

Two failure modes matter:

• Welded contacts. The device looks normal, the lever moves, nothing trips. Only a test with an RCD tester at the rated current finds it.
• Saturation from accumulated DC. A Type AC device in a circuit with a persistent DC component can sit permanently desensitised.

Test interval is set by local regulation; quarterly is a common minimum for 30 mA devices, and a proper instrument test annually. A device that trips instantly every time the test button is pressed has taught the user to reset it without looking - which is worse than a device that trips occasionally for a real reason.

The one-line selection rule

Type A as the default, Type B where a smooth DC fault current is possible, Type F where a single-phase inverter is involved, and Type AC only on circuits that are genuinely free of electronics. When in doubt, the cost difference between AC and A is small; the cost of a blind device is not.