RCD Selection in 2026: Why Type AC is Becoming Obsolete

Under BS 7671:2018+A2:2022 Regulation 531.3.3, the traditional Type AC Residual Current Device (RCD) is restricted to narrow applications. Modern homes and industrial installations contain switch-mode power supplies, inverter-driven heat pumps, EV chargers, and solar inverters. When these electronic loads develop earth leakage, DC residual currents can magnetically saturate Type AC toroids, rendering the device blind and unable to trip even during a fatal AC shock.

The Restrictive Scope of Type AC (Regulation 531.3.3)

Regulation 531.3.3 states the rule explicitly:

Note 1 to Regulation 531.3.3 gives examples of fixed equipment without DC components: simple electric immersion heaters and basic filament lighting without dimmers or control electronics. Because virtually all other modern circuits, including general-purpose socket-outlets, LED drivers, heat pumps, induction hobs, and IT equipment, contain rectifiers and power electronics, Type AC cannot be selected for general installation circuits.

The Physics of DC Core Saturation ("Toroid Blinding")

A standard Type AC RCD relies on a high-permeability magnetic core (toroid). Under normal operation, the sum of outgoing line and return neutral currents is zero. When an earth fault occurs, the imbalance produces a magnetic flux in the core that induces a trip voltage in the secondary winding.

However, if the earth fault contains a smooth or pulsating DC component:

• The unidirectional DC current generates a permanent magnetic flux offset within the toroid core.
• This flux pushes the core material into magnetic saturation.
• Once saturated, subsequent AC fault currents cannot produce alternating flux changes in the core.
• Result: No voltage is induced in the secondary coil, the trip actuator never fires, and the RCD remains closed during a hazardous earth fault.

The Modern RCD Hierarchy: Types Explained

1. Type AC (BS EN 61008 / BS EN 61009)

• Detection Capability: Residual sinusoidal alternating currents only (50/60 Hz).
• DC Immunity: Zero. Even small DC currents (≥ 2 to 4 mA) can saturate the toroid.
• Permitted Use: Only fixed resistive equipment known to contain zero electronics.

2. Type A (BS EN 61008 / BS EN 61009)

• Detection Capability: Residual sinusoidal alternating currents AND residual pulsating direct currents.
• DC Tolerance / Immunity: Remains fully functional with up to 6 mA smooth DC superimposed on the fault current.
• Vital Distinction: Type A does not detect smooth DC earth faults; it simply tolerates up to 6 mA of smooth DC without blinding. If smooth DC exceeds 6 mA, a Type A RCD also becomes saturated and blinded.
• Standard Application: The minimum standard specification for domestic socket-outlets, lighting, induction hobs, and everyday electronics.

3. Type F (BS EN 62423)

• Detection Capability: All Type A capabilities, plus residual currents composed of mixed/composite frequencies (up to 1 kHz) generated by single-phase variable-speed inverters.
• DC Tolerance: Superimposed smooth DC immunity increased to 10 mA (compared to 6 mA for Type A).
• Enhanced Surge Immunity: Incorporates a short time-delay (typically 10 ms) to prevent unwanted tripping from capacitor charging surges.
• Standard Application: Single-phase inverter-driven washing machines, dishwashers, ventilation units, and heat pump circulation systems where specified by manufacturer instructions (Regulation 134.1.1).

4. Type B (BS EN 62423 / BS EN 60947-2)

• Detection Capability: Sinusoidal AC, pulsating DC, high-frequency AC (up to 100 kHz), AND smooth DC residual currents.
• Operating Principle: Uses active Hall-effect fluxgate sensors or dual flux-sensing electronics to measure smooth DC magnetic fields directly.
• Standard Application: Transformerless three-phase solar PV inverters, three-phase variable frequency drives (VFDs), and EV charging stations without an integral 6 mA RDC-DD.

The Dangerous Blind-Spot Cascade: Upstream vs Downstream RCDs

A critical installation safety rule frequently overlooked on site is the interaction between upstream and downstream RCD types:

If a smooth DC earth fault occurs on a Type B circuit (e.g. 15 mA DC leakage from a 3-phase drive), the downstream Type B RCD may have a 30 mA threshold and not trip immediately. However, that 15 mA smooth DC flows through the upstream distribution board. When it passes through an upstream Type A RCD (which only tolerates up to 6 mA DC), the upstream core saturates.

The upstream Type A device is now completely blinded. If another circuit in the installation subsequently develops a lethal 230 V AC line-to-earth fault, the upstream main switch / RCD will fail to trip. Type B and Type F circuits must always be fed from dedicated front-end supplies or downstream of upstream Type B devices.

Preventing Unwanted Tripping: Regulation 531.3.2

Modern electronic equipment generates continuous protective conductor current (standing leakage) through internal EMC filter capacitors (Y-capacitors). Under Regulation 531.3.2:

• The total standing protective conductor current of all equipment on the load side of an RCD should not exceed 30% of its rated residual operating current (I_Δn). For a standard 30 mA device, maximum standing leakage must be ≤ 9 mA.
• Where multiple electronic circuits share a single front-end RCD, standing leakage easily exceeds 9 mA, causing chronic nuisance tripping. Subdividing circuits using individual RCBOs (Regulation 531.3.2(ii)) resolves this issue and limits outages to the single faulty circuit.