Found in almost all homes and commercial buildings, residual current devices (RCDs) play a key role in ensuring the safe operation of electrical circuits. Choosing the right RCD depends primarily on its intended use, but several other factors also come into play. Here are some tips to help you select the right RCD for your needs.
What Is the Purpose of a Residual Current Device?
Installed in the electrical panel—specifically between the main circuit breaker and the individual branch circuit breakers—RCDs are designed to protect people from the risk of electric shock. This modular device automatically cuts off the power supply if it detects a short circuit, current leakage, or electrical overload. Often required by local electrical codes, RCDs come in various types depending on the devices and circuits they are connected to. One RCD can protect up to eight separate circuits.
What Are the Different Types of RCDs?
There are several types of residual current devices, each defined by two key features: their current rating (in amperes) and their sensitivity or trip threshold. Each type is suited to different applications, which is why it's important to understand their differences.
Generally, Type AC RCDs are the most common and are used to protect standard circuits supplying outlets, ovens, lighting, shutters, and similar equipment.
Type A RCDs are intended for more specific applications, such as circuits supplying household appliances like cooktops, washing machines, or electric vehicle charging points. These devices are more sensitive and are capable of detecting both alternating (AC) and pulsating direct (DC) residual currents.
There are also Type Asi RCDs, which are commonly used for equipment such as heat pumps or industrial washing machines. They are also suitable for dedicated EV charging outlets and single-phase wallboxes or charging stations up to 7 kW. For three-phase charging stations (3P+N) rated at 11 or 22 kW, a Type B RCD is required.
If you’re unsure which RCD to choose for your installation, it’s best to consult a qualified electrician.
How to Choose the Right Residual Current Device
When selecting an RCD, there are several factors to consider. These include the type of application, the model, the current rating, and the sensitivity level.
The sensitivity of an RCD is determined by measuring the difference between the current entering and leaving the installation. This difference, measured in milliamperes (mA), must not exceed a certain threshold—otherwise, the RCD will trip. This threshold is known as the trip current or residual operating current.
The current rating of an RCD indicates the maximum current the device can handle to protect your electrical system. Common ratings include 25 A, 40 A, and 63 A. While the brand is not a technical specification, it is still an important consideration. Trusted brands like Schneider Electric and Legrand are known for producing reliable and durable RCDs.
How to Connect a Residual Current Device
Before connecting an RCD, always prioritize safety by disconnecting the power supply entirely.
According to widely accepted safety guidelines, it is recommended to install at least two RCDs in an electrical system for effective protection. Typically, this setup includes one Type A device and one Type AC device.
Each RCD can protect up to eight circuits. Here is an example of how circuits might be distributed:
- Lighting circuit: 30 mA – Type AC
- Socket outlets: 30 mA – Type AC
- Heating circuit (e.g., convector heaters): 30 mA – Type AC
- Appliance circuit: 30 mA – Type A
- Heat pump: Type A or Type Asi
- EV charging station: Type Asi (single-phase) or Type B (three-phase)
To ensure continuous operation of your lights and appliances, it is strongly advised to separate socket and lighting circuits across different RCDs.
Lastly, test your RCDs regularly using the built-in test button to confirm they are functioning correctly. If you have any doubts or experience issues, don’t hesitate to contact a qualified electrician.
