What Is an RCD? How It Works, Types and Ratings Explained
An RCD is one of the most important protective devices in a modern electrical installation. It monitors current flowing in the live conductors and disconnects the supply when it detects an imbalance that indicates current may be taking an unintended path.
What does RCD stand for?
RCD stands for residual current device. It is the generic name for a family of devices that respond to residual current. In everyday trade use, “RCD” often means an RCCB fitted upstream of several circuits, but the wider term also includes RCBOs and other residual-current devices.
The protection required depends on the circuit, the connected equipment and the installation design. BS 7671 and the equipment manufacturer’s instructions should be considered together rather than treating one RCD type as suitable for every job.
How does an RCD work?
Under normal conditions, the current travelling out through the line conductor is balanced by the current returning through the neutral. The RCD passes these conductors through a sensing arrangement and monitors the difference.
If some current flows to earth or along another unintended path, the currents no longer balance. Once the residual current reaches the device’s operating threshold for the required time, the RCD trips and disconnects the supply.
A commonly specified 30 mA device is used for additional protection in many applications, but “30 mA” is not the current rating of the circuit. It is the rated residual operating current, shown as IΔn. The device also has a separate current rating, such as 63 A, 80 A or 100 A.
RCD, RCCB and RCBO: what is the difference?
| Term | What it does | Overcurrent protection? |
|---|---|---|
| RCD | Generic term for a device that responds to residual current. | Depends on the device. |
| RCCB | Residual-current protection for one or more downstream circuits. Often labelled and described simply as an RCD. | No. Separate overload and short-circuit protection is required. |
| RCBO | Combines residual-current, overload and short-circuit protection for an individual circuit. | Yes. |
For a direct comparison, see our guide to RCDs, MCBs and RCBOs. You can also browse our full ranges of RCDs and RCBOs.
What are the main RCD types?
The type marking describes the residual-current waveforms the device is designed to detect. Modern electronic loads can introduce pulsating DC, mixed-frequency or smooth DC components, so correct type selection matters.
| RCD type | Designed to respond to | Typical selection context |
|---|---|---|
| Type AC | Sinusoidal AC residual current. | Only where the connected fixed equipment is known not to contain DC components. |
| Type A | AC and pulsating DC residual current. | Common modern loads containing electronic controls, subject to equipment requirements. |
| Type F | Type A waveforms plus certain mixed-frequency residual currents. | Some single-phase variable-speed drives and frequency-controlled equipment. |
| Type B | AC, pulsating DC and smooth DC residual currents, plus specified higher-frequency components. | EV charging, PV, drives and other equipment where smooth DC residual current can occur. |
| Time-delayed / selective | Delayed operation to support discrimination with downstream devices. | Upstream protection where the installation design requires selectivity. |
These are selection categories, not a simple quality ladder. A Type B device is not automatically the correct choice for every electronic load. Check the equipment instructions, circuit arrangement and current edition of BS 7671. Our separate guides explain Type A RCDs, Type B RCDs and time-delayed RCDs in more detail.
How to read RCD ratings
| Marking or feature | What it tells you |
|---|---|
| Rated current (In) | The maximum continuous current the device is designed to carry under its specified conditions. |
| Rated residual operating current (IΔn) | The residual-current sensitivity, commonly 30 mA for additional protection but also available in other values. |
| Number of poles | Which live conductors the device switches. Two-pole and four-pole devices are common. |
| RCD type | The residual-current waveforms the device can detect: AC, A, F or B. |
| Time-delay marking | Whether operation is intentionally delayed for selectivity or another designed purpose. |
| Conditional short-circuit rating | The fault-current conditions the RCD can withstand when correctly coordinated with an upstream protective device. |
An RCCB’s current rating does not mean it protects against overload. It must be correctly coordinated with suitable overcurrent protection. Manufacturer data also sets conditions such as terminal capacity, mounting position and operating temperature.
What should be checked when selecting an RCD?
- Protection purpose: fault protection, additional protection, fire protection or a coordinated combination.
- Residual-current type: based on the equipment and possible DC or frequency components.
- Sensitivity and time characteristics: selected for the circuit and discrimination requirements.
- Rated current and fault coordination: checked against the circuit design and upstream protection.
- Poles and switching arrangement: appropriate for the supply and neutral requirements.
- Manufacturer compatibility: especially when fitting devices into an existing consumer unit or distribution board.
Standing leakage from connected equipment can contribute to unwanted tripping. Dividing loads across individual RCBO-protected circuits can improve fault localisation and continuity compared with one RCCB protecting a large group, although the correct arrangement is always design-specific.
RCD FAQs
Does an RCD protect against overload?
An RCCB does not. Overload and short-circuit protection must come from an MCB, fuse or another coordinated device. An RCBO combines both functions.
Is a 30 mA RCD always required?
No single rating covers every circuit. A 30 mA device is widely used for additional protection, while other sensitivities and time characteristics have different purposes. The installation design determines what is required.
Can Type AC be replaced with Type A?
Type A detects the waveforms covered by Type AC plus pulsating DC residual current, but replacement still needs to consider device compatibility, circuit design and manufacturer instructions.
Why does an RCD keep tripping?
Possible causes include an appliance or wiring fault, accumulated leakage across several loads, moisture, incorrect circuit separation or a device issue. Repeated tripping should be investigated by a competent electrician rather than repeatedly resetting the device.
Browse our RCD range for Type A, Type B, high-immunity and four-pole options, or explore all circuit protection devices. Check ratings, compatibility and manufacturer data against the installation design before ordering.
This guide is for general product-selection information. Electrical work should be designed, installed, tested and certified by a competent person in accordance with the current edition of BS 7671 and relevant manufacturer instructions.