Residual Current Devices (RCDs) are critical components in electrical installations, providing protection against electric shocks and fire risks caused by earth leakage. However, when it comes to installing renewable technologies like solar PV, battery storage systems and EV chargers, choosing the correct RCD—and knowing whether you need one at all—can be more complicated than it seems.
Do You Really Need an RCD?
RCDs have become so commonplace nowadays that the decision to install RCCBs (residual current circuit breakers) and more recently RCBOs (residual current breakers with overcurrent protection) is almost automatic. However, there are specific circumstances that mean that you don’t need an RCD at all. The benefits of not installing an unnecessary RCD include cost and time savings, as well as the prevention of nuisance tripping.
When it comes to renewable energy systems, the decision to install an RCD isn’t always straightforward. For example, if the main fuseboard and solar inverter are installed in the same area (like a garage or plant room), and the inverter’s supply cabling is fully visible from the origin to the inverter, then it’s possible to argue that an RCD isn’t necessary. The reasoning is that the risk of the cable being damaged or disturbed is low when it’s visibly secured along a wall or inside surface trunking.
Interestingly, Section 8.8 of the IET Code of Practice (COP) for Grid Connected Solar PV confirms there is no fundamental requirement to install an RCD on the circuit feeding the inverter(s). In fact, it recommends designing systems so that inverters are not fed by an RCD, as this can prevent unnecessary tripping in the event of minor faults. However, EV chargers are a different case, and an RCD is always required for these installations.
Selecting the Right Type of RCD
Once you’ve determined that an RCD is necessary for your installation, the next step is choosing the right type. Historically, the most common RCDs were the Type AC devices, which are only suitable for circuits with purely resistive loads (like heating). However, most renewable technologies—such as solar PV and battery storage systems—incorporate DC (direct current) components. This is where problems may arise.
Type AC RCDs can struggle to detect faults in circuits that include DC current, as the RCD can become “blinded” by the DC component. This can either prevent the RCD from tripping when needed or cause it to trip unnecessarily (nuisance tripping). Therefore, Type AC RCDs are no longer suitable for circuits with DC components, which makes them inappropriate for solar PV and battery storage systems.
For renewable technologies, Type A RCDs are the appropriate choice. These devices are designed to handle up to 6mA of pulsed DC current, making them suitable for most systems involving solar inverters and battery storage. However, it’s important to always verify the manufacturer’s documentation for any specific RCD requirements for the devices you’re installing, as there may be specific limits or recommendations to consider. Refer to the selection tables in the IET COP books for more details.
Double Pole vs. Switched Neutral RCDs
Another important consideration is whether the RCD needs to be a double pole or a switched neutral device. According to the Wiring Regulations (BS7671), RCDs serving equipment like solar PV, battery storage, and EV chargers must disconnect all live conductors, including the neutral. This is a key safety feature.
In most cases, a double-pole RCD will meet this requirement, as it will disconnect both the live and neutral conductors. However, it’s important to check the specific details of the RCD you’re using. RCBOs often have a diagram on the side, indicating whether the device switches both the live and neutral conductors. Many RCBOs are single-pole with a switched neutral. These will disconnect both conductors in the event of a fault, but they will only provide overcurrent protection for the live conductor, not the neutral.
Bidirectional vs. Unidirectional RCDs
Some installations—particularly battery storage systems and EV chargers—can involve bidirectional power flow. In these systems, power may flow in both directions, which is important when considering RCD functionality. Unidirectional RCDs are designed for systems where power flows in only one direction, such as typical household appliances. But for systems that involve bidirectional power flow, like inverters that charge batteries or bidirectional EV chargers, it’s essential to use a bidirectional RCD.
Bidirectional RCDs are designed to function correctly regardless of the direction of the power flow, ensuring that the test function operates properly in both directions. In contrast, a unidirectional RCD exposed to bidirectional power may fail to function correctly if the power flow reverses, creating potential safety hazards. For instance, a unidirectional RCD exposed to bidirectional power could become permanently disabled without visibly tripping. This could leave the system user unaware that the RCD is no longer providing protection.
It is worth noting that on July 31, 2024, the IET released Amendment 3:2024 to BS7671:2018 (Requirements for Electrical Installations, IET Wiring Regulations), which provides greater clarity on the selection of unidirectional and bidirectional devices. In particular, Chapter 53 of the amendment (530.3.201) highlights specific product markings that must be included to indicate whether a device is unidirectional. These include directional indicators such as ‘In & Out,’ ‘Line & Load,’ ‘Supply & Load,’ or arrows indicating the power flow direction. If a device lacks any specific directional markings, it should be assumed to be bidirectional.
Summing Up
In conclusion, when installing renewable technologies such as solar PV, battery storage systems, and EV chargers, it’s vital to understand the considerations and specifications for RCDs. While it may be tempting to install an RCD by default, it’s worth asking whether it’s necessary in the first place—especially in cases where the inverter and fuseboard are in the same location and the wiring is fully visible. If an RCD is required, ensure you select the right type, such as a Type A for renewable systems, and verify that the device switches both the live and neutral conductors. Lastly, always ensure that you use a bidirectional RCD for systems involving reverse power flow, like battery storage and EV chargers.
By taking these factors into account, you can ensure that your installations are both safe and compliant with the latest regulations.
