Residual Current Device Types: The Difference Between AC, A, F and B
Residual current device types: As drives and electronic loads spread, choosing the correct RCD type has become a precondition for personal safety.
ContentsResidual current device types define which waveform of leakage a device is able to detect. In installations full of variable speed drives, LED drivers and charging points this distinction is no longer academic: with the wrong type, a device may fail to trip while leakage is present.
What a residual current device does
The device measures the vector sum of the currents entering and leaving a circuit. If the sum is not zero, part of the current is returning through earth. When that difference exceeds the setting, the circuit is opened. Overcurrent protection cannot perform this role, because leakage is normally far below the trip threshold of a breaker.
Types and the leakage they detect
The types build on one another: each higher type detects everything the one below it detects, and more.
| Type | Typical application | Notes |
|---|---|---|
| AC | Classic resistive and motor loads | No longer preferred for new work in many countries |
| A | General socket outlets, lighting, domestic electronics | The common minimum choice |
| F | Single-phase circuits with frequency converters | Washing machines, air conditioners, small drives |
| B | Three-phase drives, charging stations, photovoltaics | Detects smooth DC leakage |
The blind spot problem
The rectifier in a frequency converter changes the shape of the leakage current. Once the leakage contains a smooth DC component, the toroid of a type AC or type A device saturates, and the device not only misses that leakage but can also become insensitive to other alternating leakage on the same circuit. Type selection on drive-fed circuits should therefore be confirmed against the manufacturer’s installation guide.
Setting: 30 mA or 300 mA?
30 mA is the threshold accepted as additional protection for people, and it is used on socket circuits, wet areas and portable equipment. 300 mA serves fire protection and is chosen at incoming and distribution level. Where the two are used in series, the upstream device must be a selective (type S) time-delayed unit, otherwise both trip together. For the coordination logic see selectivity.
Points to watch on the panel side
Most site problems can be prevented at the design stage:
- The neutral must pass through the device and must not be linked to another neutral downstream
- On long cable runs, capacitive leakage can trip a 30 mA device; split the circuit
- Place surge protection upstream of the device
- Test the trip button periodically and record the result
- On critical process loads, monitoring only (RCM) may be preferable to tripping
For surge arrester placement, see SPD selection.
Key takeaways: residual current device types
- What a residual current device does — The device measures the vector sum of the currents entering and leaving a circuit.
- Types and the leakage they detect — The types build on one another: each higher type detects everything the one below it detects, and more.
- The blind spot problem — The rectifier in a frequency converter changes the shape of the leakage current.
- Setting: 30 mA or 300 mA — 30 mA is the threshold accepted as additional protection for people, and it is used on socket circuits, wet areas and portable equipment.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Can type B be used everywhere?
Technically yes, but it costs considerably more. It is therefore reserved for circuits where smooth DC leakage is possible, with type A or F used elsewhere.
What should be done about frequent tripping?
Measure the leakage current first. On long runs with many electronic loads, natural leakage can approach the threshold. Splitting the circuit across several devices is usually the lasting solution.
Does an RCD replace earthing?
No. It complements the earthing system. In an installation without proper earthing, protection remains incomplete.
Related articles
- Surge Protective Device (SPD) Selection: Type 1, Type 2 and Type 3
- Choosing the Incoming Device: MCCB or ACB?
- Busbar Trunking or Cable? Choosing the Right Distribution Method
- All technical articles
Devpan grades residual current protection according to load type and earthing arrangement. Share your circuit schedule and we will define the right types and thresholds with you.
