• • • HAZARDOUS AREA EQUIPMENT • • •
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. 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. For more information, visit
www.segen.co.uk.
electricalengineeringmagazine.co.uk
ELECTRICAL ENGINEERING • APRIL 2025 27
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