RCD Selection in 2026: Why Type AC is Becoming Obsolete

For decades, Type AC RCDs were the standard default choice in UK domestic distribution boards. In modern installations packed with inverter drives, heat pumps, LED power supplies, and EV chargers, Type AC devices are no longer suitable and can fail to trip under genuine fault conditions.

The Problem: DC Residual Current & Core Saturation

Traditional Type AC RCDs are designed strictly for sinusoidal alternating currents. When modern power electronics (rectifiers, switch-mode power supplies, variable speed drives) develop an earth fault, they frequently generate pulsating DC or smooth DC residual currents.

When DC residual current passes through the magnetic toroid of a Type AC RCD:

• The DC component magnetically saturates (or "blinds") the toroid core.
• The secondary sensing coil can no longer induce the voltage needed to trigger the trip relay.
• Result: The RCD remains stuck closed, providing zero protection to the user even during a direct AC fault.

The RCD Hierarchy: Types Explained

1. Type AC

Detects sinusoidal AC residual currents only. Prohibited by BS 7671 on circuits feeding equipment capable of generating DC residual components. In practice, almost all modern electronic circuits fall into this category.

2. Type A

Detects sinusoidal AC and pulsating DC residual currents, with smooth DC residual current tolerance up to 6 mA.The current UK standard minimum for general socket-outlets, lighting, induction hobs, and domestic appliances.

3. Type F

Designed specifically for single-phase variable frequency inverter-driven equipment (e.g. modern inverter washing machines, dishwashers, heat pump circulation units). Detects mixed-frequency residual currents and has higher surge immunity against nuisance tripping.

4. Type B

Detects sinusoidal AC, pulsating DC, high-frequency AC, and smooth DC residual currents up to hundreds of mA. Mandatory on commercial PV inverters without galvanic isolation, three-phase EV charging infrastructure, and large industrial VFD drives.

RCD Type Selection Matrix