RCD Types Explained
Type AC, A, B, F — Which Do You Need?

An RCD (Residual Current Device) detects an imbalance between the current flowing into a circuit through the line conductor and the current returning through the neutral conductor. If some current is leaking to earth — through a person, through damaged insulation, or through a fault — the RCD detects this imbalance and trips, disconnecting the circuit.

However, not all earth leakage currents are the same. Traditional electrical loads (heaters, incandescent lights, kettles) produce pure sinusoidal AC fault currents. But modern electronic equipment — EV chargers, variable-frequency drives, heat pumps, solar PV inverters, washing machines with inverter motors — can produce fault currents with DC components, pulsating waveforms, and mixed frequencies. An RCD designed to detect only sinusoidal AC will not trip on these non-standard fault waveforms, leaving the circuit unprotected.

BS 7671 Regulation 531.3.3 requires that the RCD type is selected with consideration of the waveform of the residual current likely to occur under fault conditions. This means the electrician must consider what equipment will be connected to each circuit and select an RCD type capable of detecting the fault currents that equipment could produce. Getting this wrong is not just a compliance issue — it is a safety issue that could result in a fatal electric shock or an undetected fire.

The four main RCD types are defined in BS EN 62423, and each has a specific symbol that is printed on the device front plate. Understanding these symbols and their meaning is fundamental to specifying and inspecting RCD protection.

Type AC is the most basic type of RCD. It detects and responds to sinusoidal AC residual currents — the type of fault current produced by simple resistive loads such as heaters, incandescent lights, and immersion heaters. It was the standard RCD type for decades and is still found in many existing installations.

However, Type AC cannot detect DC components in the fault current. Modern electronic equipment with rectifiers (diodes) can produce pulsating DC fault currents or smooth DC fault currents. If these DC components flow through the toroidal core of a Type AC RCD, they can saturate the core, reducing its sensitivity to AC faults as well. This means a Type AC RCD on a circuit supplying electronic equipment could fail to trip on any type of fault — AC or DC.

If you find Type AC RCDs during an EICR inspection, they are not automatically a defect — they were compliant when installed. However, if the circuits they protect now supply electronic equipment (which is almost always the case), consider recommending an upgrade to Type A as a C3 (Improvement Recommended) observation.

Type A is now the standard RCD type for the vast majority of domestic and commercial circuits. It detects all the fault waveforms that Type AC can detect (pure sinusoidal AC) plus pulsating DC residual currents — the type of fault current produced by equipment with single-phase rectifier circuits.

Pulsating DC fault currents are produced by any equipment that converts AC to DC using a half-wave or full-wave rectifier — which includes virtually every piece of modern electronic equipment: computers, laptop chargers, phone chargers, LED drivers, washing machine and dishwasher controllers, modern boiler controls, EV chargers in Mode 2, and many more. When a fault develops in this equipment, the fault current has a DC component that Type AC cannot detect.

BS 7671 Regulation 531.3.3 states that the type of RCD shall be selected based on the waveform of residual current likely to occur. Since almost every modern circuit supplies equipment with rectifiers, Type A is the minimum appropriate type for virtually all circuits. In practice, if you are installing a new consumer unit or specifying RCBOs for a new installation, Type A should be the default choice.

Type A RCBOs are readily available from all major manufacturers (Hager, Schneider, Siemens, MK, Wylex) at prices ranging from £25 to £50 per device. The cost premium over Type AC is minimal and well worth the additional protection.

Type B RCDs can detect all the fault waveforms that Type A detects, plus smooth (pure) DC residual currents. Smooth DC fault currents are produced by equipment with three-phase rectifiers or specific power electronic circuits — most notably some EV chargers, three-phase variable-frequency drives (VFDs), and certain solar PV inverter topologies.

The most common domestic application for Type B RCDs is EV charger installations where the charger does not have built-in 6mA DC residual current monitoring. BS 7671 Chapter 72 (Electric Vehicle Charging Installations) requires that the RCD protecting an EV charging circuit must be either:

• Type A + 6mA DC detection — A Type A RCD/RCBO where the charger itself monitors for DC residual currents above 6mA and disconnects. Most modern domestic smart chargers (Zappi, Ohme, Pod Point, Easee) include this built-in DC detection, allowing a standard Type A RCBO to be used.
• Type B RCD — Required if the charger does not have built-in DC detection. The Type B RCD detects the smooth DC fault currents directly, without relying on the charger electronics.

Type B RCDs are significantly more expensive than Type A devices — typically £150 to £300 compared to £25 to £50 for a Type A RCBO. This cost difference is why most EV charger manufacturers now build 6mA DC detection into their chargers, allowing the cheaper Type A option.

In commercial and industrial installations, Type B RCDs are required for circuits supplying three-phase VFDs, some welding equipment, and certain medical devices where smooth DC fault currents are possible.

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Type F is a relatively new RCD type, defined in BS EN 62423, that fills a specific gap between Type A and Type B. It is designed for circuits supplying equipment with single-phase variable-frequency drives (VFDs) — also called inverters or variable-speed drives.

Single-phase VFDs are increasingly common in domestic and light commercial equipment:

• Heat pumps — Air source and ground source heat pumps use VFDs to control compressor speed for optimal efficiency. A fault in the VFD can produce mixed-frequency residual currents that Type A cannot detect reliably.
• Air conditioning units — Modern inverter-driven air conditioning uses the same VFD technology as heat pumps and produces the same types of fault current.
• Washing machines with inverter motors — Premium washing machines from manufacturers such as Samsung, LG, and Bosch use direct-drive inverter motors instead of traditional induction motors.
• Pool and spa pumps — Variable-speed pool pumps use VFDs for energy-efficient operation at different flow rates.

The residual currents produced by single-phase VFDs are complex — they contain DC components, pulsating components, and high-frequency components mixed together. A Type A RCD may not reliably detect these composite fault currents because the complex waveform can partially saturate the toroidal core in unpredictable ways. A Type B RCD would work but is unnecessarily expensive. Type F provides the specific detection capability needed for these fault waveforms at a cost between Type A and Type B (typically £60 to £100).

When specifying circuits for heat pump installations, always check the heat pump manufacturer's requirements — many now specify Type F or Type B RCD protection as a condition of their warranty.

There are two ways to provide RCD and overcurrent protection for a circuit: an individual RCBO (Residual Current Breaker with Overcurrent protection) for each circuit, or a shared RCD protecting a group of circuits with individual MCBs (the split-load arrangement).

For new domestic installations and consumer unit replacements, full RCBO boards are now the preferred choice among professional electricians. The additional cost is easily justified by the improved discrimination, reduced nuisance tripping, and better fault diagnosis. The time saved in call-backs alone often pays for the cost difference.

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RCDs are manufactured with different rated residual operating currents (IΔn) for different protection purposes. The three most common ratings serve fundamentally different functions and are not interchangeable.

The golden rule: where BS 7671 requires 30mA additional protection, you cannot substitute a higher-rated device. A 100mA RCD does not provide personal protection — a person receiving a 100mA shock for the 300ms trip time of the RCD is at serious risk of ventricular fibrillation.

A time-delayed RCD, marked as Type S (for selective), has a built-in delay before it trips. The purpose is to achieve discrimination (selectivity) between RCDs installed in series, ensuring that only the device closest to the fault trips.

Consider a consumer unit with a 100mA Type S RCD as the main switch and individual 30mA RCBOs on each circuit. If an earth fault occurs, the 30mA RCBO on the faulty circuit should trip first. The 100mA Type S main switch should not trip because: (a) it is less sensitive (100mA vs 30mA), and (b) its deliberate time delay means the downstream device has time to trip first.

Compare these with non-delayed RCDs: 300ms maximum at 1x IΔn and 40ms maximum at 5x IΔn. The Type S device has a wider time band that accommodates the intentional delay.

When testing Type S RCDs during an EICR, make sure your test instrument is set to the correct mode (Type S or selective). Testing a Type S device in non-delayed mode will give trip times that appear to exceed the standard limits, potentially leading you to fail a device that is actually performing correctly.

Schedule of Tests Validates RCD Trip Times

Elec-Mate's schedule of test results validates every RCD trip time you enter against the correct BS 7671 limits — automatically distinguishing between…

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