RCD types AC, A, B — differences and when to use each

What is an RCD

An RCD (Residual Current Device) disconnects a circuit when earth leakage current appears. It's the primary protection against electric shock and fires caused by leakage.

Operating principle: the RCD measures the difference between phase and neutral currents. In normal operation, Iphase = Ineutral, difference = 0. A leakage through a human body or damaged insulation creates a differential current — the RCD trips in 20-40 ms (at 5×IΔn).

BUT: not all RCDs are equal. The type determines what form of leakage current the device responds to.

RCD Types per IEC 60755 / GOST R 51326.1

Type AC

Responds only to sinusoidal AC leakage current.

- Cheapest and most common

• Suitable for: incandescent lamps, resistive heating, general socket circuits without electronics
• NOT suitable for: circuits with SMPS, inverters, dimmers, washing machines, dishwashers, air conditioners, computers

  Type A

  Responds to sinusoidal AC + pulsating DC leakage current.

  Pulsating DC arises in circuits with half-wave rectification — virtually all modern electronics: SMPS, dimmers, chargers.

  - Mandatory for: washing/dishwashing machines (inverter-controlled motors), air conditioners, computers, server rooms

• Recommended by IEC for all residential socket circuits
• Price: 30-50% higher than AC

  Type B

  Responds to AC + pulsating DC + smooth DC leakage current.

  Used where DC leakage is possible: solar inverters, EV charging stations (Mode 3, IEC 61851), VFDs, UPS, medical equipment.

  - Mandatory for EV charging stations (IEC 61851-1)

• Mandatory for transformerless solar inverters
• Price: 3-5× higher than AC

  Type F (new)

  Responds to AC + pulsating DC + mixed-frequency currents (up to 1 kHz). Intermediate between A and B. For single-phase inverter circuits (inverter AC compressors, VFD washing machines).

  Type B+ (newest)

  Extended B: additionally responds to currents up to 20 kHz. For high-frequency inverters.

  Comparison table

  | Characteristic | Type AC | Type A | Type F | Type B | Type B+ |

|---|---|---|---|---|---| | Sinusoidal AC | ✅ | ✅ | ✅ | ✅ | ✅ | | Pulsating DC | ❌ | ✅ | ✅ | ✅ | ✅ | | Freq. up to 1 kHz | ❌ | ❌ | ✅ | ✅ | ✅ | | Smooth DC | ❌ | ❌ | ❌ | ✅ | ✅ | | Freq. up to 20 kHz | ❌ | ❌ | ❌ | ❌ | ✅ | | Price (relative) | 1× | 1.5× | 2.5× | 4× | 6× | | Typical use | Heaters, lamps | Sockets, appliances | Inverter AC, washers | EV, solar inverters | Medical, industrial VFDs |

RCD selection for an apartment

Lighting (LED): Type A (LED drivers produce pulsating leakage)

Socket circuits: Type A — mandatory per modern standards

Washing/dishwasher: Type A (or F for inverter-motor models)

Electric stove: Type A (electronic controls)

Air conditioner: Type F (inverter compressor generates mixed-frequency currents)

EV charger: Type B (IEC 61851-1, mandatory)

Common mistakes

1. Type AC for the whole apartment — doesn't protect against pulsating leakage, RCD may "go blind" from core saturation

2. Type B where Type A suffices — 4× overpayment

3. Incomer RCD rated too low — nuisance tripping from cumulative natural leakage

How GorkyCAD helps

- Auto-selects RCD type based on consumer composition per group

• Recommends Type A/F when inverter-based equipment is present
• Warns if Type AC is on a group with a washing machine