RCD Selection for Australia and New Zealand: Type A, B, F Explained

A Residual Current Device (RCD) detects earth leakage current and disconnects the circuit before it can harm a person or start a fire. AS/NZS 3000:2018 mandates 30 mA RCDs on socket outlet and lighting circuits in every Australian installation. But not all RCDs are the same. Type AC, A, B, and F each detect different fault waveforms, and choosing the wrong type means the RCD may not trip on a real fault. This guide explains each type, when to use it, and how the RCD Selection Calculator helps you get it right.

Why RCDs matter

An RCD protects against three hazards. First, direct contact: a person touches a live conductor and current flows through their body to earth. Second, indirect contact: a fault energises exposed metalwork and a person touches it. Third, fire: a small earth leakage current (below the overcurrent trip threshold) persists long enough to ignite combustible material around the cable.

A 30 mA RCD trips in under 300 milliseconds at rated sensitivity, and under 40 milliseconds at 5 times rated sensitivity (150 mA). This is fast enough to prevent ventricular fibrillation in most adults. Without an RCD, a fault current below the MCB trip threshold could flow indefinitely.

How an RCD detects a fault

An RCD works by comparing the current flowing out on the active conductor with the current returning on the neutral. In a healthy circuit, these are equal. If some current is leaking to earth (through a person, through damaged insulation, or through a fault), the outgoing and returning currents are no longer balanced. The RCD detects this imbalance and trips.

The key limitation: a Type AC RCD only detects sinusoidal (AC) residual currents. If the fault current contains a DC component (from a rectifier, inverter, or VFD), the DC saturates the RCD core and it may fail to trip. This is why different RCD types exist.

RCD sensitivity ratings

• 30 mA: personal protection. Required for socket outlets, lighting, and circuits supplying portable equipment per AS/NZS 3000.
• 100 mA: additional fire protection. Sometimes used on fixed appliance circuits or as an upstream backup.
• 300 mA: fire protection for distribution circuits. Protects the cable rather than the person.
• 10 mA: enhanced personal protection for sensitive environments (medical, wet areas). Not mandated by AS/NZS 3000 but specified in some project briefs.

Type AC RCDs

Type AC detects sinusoidal AC residual currents only. It is the simplest and cheapest RCD type. Suitable for purely resistive loads (incandescent lighting, heating elements) where no rectification occurs. Not suitable for modern electronic loads.

In New Zealand, Type AC is no longer compliant for new socket outlet installations. Australia has not mandated this nationally, but Type A is recommended for all new work.

Type A RCDs

Type A detects everything Type AC does, plus pulsating DC residual currents. Pulsating DC occurs when a single-phase rectifier (e.g. in an LED driver, a switchmode power supply, or an induction cooktop) is present on the circuit.

Type A is the New Zealand minimum for socket outlet protection. It is also required for circuits supplying single-phase solar inverters (Type A at minimum per AS/NZS 4777.2) and for circuits where LED drivers or electronic transformers are the dominant load.

Type B RCDs

Type B detects everything Type A does, plus smooth DC residual currents above 10 mA. Smooth DC fault currents can occur when a three-phase rectifier (as in a VFD, a three-phase EV charger, or a non-isolated solar inverter) develops an earth fault on the DC side.

Type B is required when the load can produce smooth DC residual current that would saturate a Type A core. The most common cases are EV chargers without internal DC fault detection, grid-connected inverters with non-isolated topology, and some VFD installations. Always check the manufacturer datasheet.

Type F RCDs

Type F is a newer variant that detects everything Type A does, plus composite waveforms with a superimposed frequency up to 1 kHz. It is designed for single-phase VFD-fed loads (washing machines with inverter motors, heat pump compressors) where the fault waveform contains high-frequency components that a Type A may not detect reliably.

Type F sits between Type A and Type B in terms of detection capability and cost. It is not yet widely mandated in Australia but is appearing in European-influenced specifications.

AS/NZS 3000 requirements by circuit type

• Socket outlets: 30 mA RCD mandatory.
• Lighting circuits: 30 mA RCD mandatory.
• Circuits supplying portable equipment: 30 mA mandatory.
• Fixed appliance circuits: generally exempt unless in a wet area.
• Water heater circuits: exempt if permanently connected and not in a wet area.
• EV charger circuits: 30 mA RCD mandatory, Type A minimum (Type B if no internal DC detection).
• Solar PV circuits (AC side): Type A minimum per AS/NZS 4777.2.

Worked example: residential with solar and EV charger

A residential installation has: general socket outlets, LED lighting, a 5 kW single-phase solar inverter, and a 32 A Mode 3 EV charger without internal DC fault detection.

• Socket outlets: 30 mA Type A (pulsating DC from phone chargers, LEDs).
• LED lighting: 30 mA Type A (LED drivers contain rectifiers).
• Solar inverter AC circuit: 30 mA Type A per AS/NZS 4777.2. If the inverter is non-isolated, Type B.
• EV charger: 30 mA Type B (charger has no internal DC detection).

Common mistakes

• Using Type AC where Type A or B is required. Type AC cannot detect DC fault components from modern electronic loads.
• Oversizing the RCD rated current. The rated current must match the circuit protection, not the total switchboard capacity.
• No selectivity between upstream and downstream RCDs. If a 30 mA and a 300 mA RCD are in series without time delay, both may trip on the same fault.
• Assuming the EV charger has internal Type B detection. Always check the manufacturer datasheet. If in doubt, install a Type B RCD externally.

Where the calculator fits in

The ElecCalc RCD Selection Calculator recommends the correct RCD type and sensitivity based on the circuit type, load characteristics, and installation context. For the earth conductor side of fault protection, use the Earth Conductor Sizing Calculator.