As electrical installations evolve to include renewable energy systems, battery storage and EV charging, the way power flows through circuits is changing. Rob Shepherd takes a deep dive into bidirectional protective devices and explains when and why they are required.
In a conventional installation, electricity flows from the supply network into the installation and uses protective devices with that direction in mind. Known as unidirectional devices, they typically include clearly marked line and load terminals. However, when embedded generation or storage is introduced, energy can flow in both directions. For example, solar PV panels may export surplus energy back to the grid and battery systems may discharge into the installation or even upstream. This is where bidirectional protective devices come into play.
Here and now
Bidirectional protective devices are an increasingly important component in modern electrical installations. As more systems incorporate multiple sources of power such as the aforementioned solar PV arrays, as well as technologies like battery storage and EV charging equipment, the traditional assumption of a single directional flow of electricity no longer holds true. This has implications for how protective devices are selected and installed.
In installations with solar PV, energy generated during daylight hours may be used on-site or exported to the grid. During export conditions, current flows in the opposite direction to that of a traditional supply. A standard unidirectional protective device may not operate as intended in this scenario, potentially compromising safety. Battery storage systems add another layer of complexity, as these systems can both absorb energy from the grid and supply energy back into the installation. During discharge, they effectively act as a source of supply, reversing the direction of current flow.
BS 7671 – the 18th Edition of the Wiring Regulations states: “Selection and erection of equipment for protection shall take account of appropriate use of either a unidirectional protective device or a bidirectional protective device.” A bidirectional protective device should be used when the manufacturer intends that a power supply may be connected to either or both sets of terminals. A unidirectional protective device should be used when the manufacturer specifies that the power supply is to be connected to only one designated set of terminals.
Direction of travel
One of the most noticeable aspects of bidirectional protective devices such as residual current devices (RCDs) and residual current circuit breakers with overcurrent protection (RCBOs) is the absence of line and load markings. This is unlike traditional unidirectional devices that usually require correct orientation to function properly.
This is because bidirectional units are designed to operate safely regardless of how they are connected, simplifying installation and reducing the risk of incorrect wiring in more complex systems. That said, the absence of line and load markings does not mean that an installation can be approached casually. Electrical contractors must still follow manufacturer instructions carefully, as there may be other requirements related to terminal layout, connection methods or compatibility with other components. The key thing to remember is that orientation is no longer critical for the device’s protective function.
Case in point
The need for bidirectional protection is particularly relevant in installations with inverters. Inverters used in solar PV and battery storage systems convert DC power into AC, enabling it to be used within the installation or exported. These inverters can introduce fault conditions that differ from those seen in purely grid supplied systems and a bidirectional RCD or RCBO ensures that earth leakage faults are detected regardless of whether the inverter is importing or exporting power.
EV charging equipment is also evolving beyond one way power delivery. Traditionally, chargers draw electricity from the grid to charge a vehicle’s battery, operating with a straightforward unidirectional flow. However, with the rise of vehicle to grid (V2G) technology, EVs are no longer just consumers of energy and they can also act as temporary energy storage systems. This means that electricity stored in a battery can be exported back to a building or even the wider grid when needed. As a result, electrical installations must now account for reverse current flow, which was not a consideration in conventional charging set-ups.
This introduces new challenges for electrical protection and system design. Without appropriate protection, reverse currents could lead to equipment damage, safety risks or instability in the electrical network. Therefore, incorporating bidirectional protection is becoming essential for future proof EV charging point installations, ensuring they remain safe, reliable and compatible with emerging smart energy systems.
Follow the rules
When selecting protective devices for installations that involve multiple energy sources, it is important to follow both BS 7671 requirements and the specific guidance provided by manufacturers. Devices must be carefully assessed to ensure they are suitable for the intended application. Confirming that equipment is explicitly rated for bidirectional operation is a critical step in preventing malfunction, equipment damage or risk to personnel.
As well as correct device selection, proper coordination between protective components is essential. Systems incorporating multiple devices must maintain effective discrimination and selectivity, even when current direction changes. This ensures that in the event of a fault, only the appropriate device operates while the rest of the system remains functional. Achieving this requires careful design, planning and, where necessary, testing under realistic conditions. Without proper coordination, faults may lead to unnecessary disconnection of supply or failure to isolate the issue, compromising both safety and system reliability.
Going concern
As the adoption of renewable energy and energy storage continues to grow, electrical contractors must adapt their approach to protection. By understanding when and why bidirectional devices are required, and by following current regulations and manufacturer recommendations, contractors can ensure safe, compliant and future ready installations.
Contributors
Rob Shepherd (author)
Rob is Consulting Editor for InstallerELECTRIC. He is a well-respected journalist with a strong history in the electrical sector.
