RCD Type A, F or B - which one do you actually need?
Variable frequency drives and modern electronics create residual currents that a basic RCD can no longer detect reliably. This guide explains when Type A is enough and when Type F or the all-current sensitive Type B is mandatory under EN 62423.
View RCDsWhy is Type A no longer enough with drives?
An RCD trips when a residual current flows to earth. What matters is the waveform of that current: Type A only detects sinusoidal AC and pulsating DC residual currents. Variable frequency drives, PV inverters and switched-mode power supplies, however, produce smooth DC residual currents and high-frequency mixed frequencies that a Type A delays or even blocks.
A smooth DC residual current can pre-magnetise the toroidal core of a Type A device. The RCD then fails to detect even a dangerous 50 Hz AC fault - personnel protection is lost without any warning. That is exactly the hazard EN 62423 addresses.
When to choose Type F and when Type B?
Type F bridges A and B. It handles mixed frequencies up to around 1 kHz and a smooth DC fault component up to 10 mA, is surge-resistant and ideal for single-phase drives such as washing machines, heat-pump circulation pumps or small single-phase frequency converters. But as soon as three-phase drives with a high DC link voltage are involved, only the all-current sensitive Type B will do.
- Type A: classic final circuits without power electronics.
- Type F: single-phase drives, modern appliances, mixed electronic circuits.
- Type B: three-phase frequency drives, PV inverters without isolating transformer, industrial motors.
- Type B+: additionally fire-safety optimised, trips at higher-frequency faults (recommended for enhanced fire protection).
- Rated residual current 30 mA for personnel protection, 300 mA only for pure fire or plant protection.
What about selectivity and installation?
Beyond the waveform, selectivity matters. If a final circuit uses a Type B and an upstream section is also protected, a time-selective Type B marked S with a higher rated residual current ensures that only the affected circuit trips on a fault, not the whole installation.
Frequently asked questions
Can I just fit Type B everywhere?
Technically yes, Type B detects every residual current waveform. But it is far more expensive than Type A or F, so you deploy it specifically where smooth DC residual currents occur - mainly three-phase drives and inverters.
What happens if only a Type A is fitted?
A smooth DC residual current can pre-magnetise the Type A core and blind it to AC faults. Personnel protection can then fail silently while the installation keeps running.
Is Type F enough for my heat pump?
For single-phase units with power electronics, Type F is usually sufficient and surge-resistant. Three-phase drives almost always require Type B per the manufacturer's data - the operating manual is binding.
What does Type B+ mean?
Type B+ is a Type B with extended fire protection: it trips at higher-frequency residual currents with a reduced limit, cutting the fire risk in critical areas.
Looking for the right RCD?
We supply RCDs of Type A, F and B plus all-current sensitive Type B+ devices - matched to the residual current waveform of your drives and electronics.
Standard-tested
Types A, F and B to EN 61008 and 62423.
All-current sensitive
Type B and B+ detect residual currents down to DC.
Correctly rated
Selection by waveform and leakage current.
Expert advice
We help with selectivity and device choice.