Understanding Bidirectional Protective Devices for Modern Electrical Systems

The increasing prevalence of electrical energy storage systems and small-scale generation capable of feeding into the public distribution system has made bidirectional power flow in some conductors more common. Batteries, classified as generators under BS 7671 (551.1.1(v)), should be viewed as both a supply and a load (see regulation 823 note). This raises important considerations regarding the suitability of protective devices such as circuit-breakers, residual current circuit breakers (RCCBs), and residual current operated circuit breakers with integral overcurrent protection (RCBOs). Additionally, it is crucial to ensure that all necessary conductors are disconnected when protective devices operate under fault conditions to meet requisite disconnection times.

Suitability for Use with Bidirectional Power Flow

BS 7671 does not currently define "unidirectional" or "bidirectional," although these terms are used in the context of overcurrent protective devices (712.533.101(d)), information exchange (825.1(viii)), and conducted transients - classification AM (Appendix 5). Common definitions are as follows:

• Unidirectional: Operating in one direction only.
• Bidirectional: Operating in either direction.

Sections 551 and 712 of BS 7671:2018+A2:2022, covering low-voltage generating sets and solar PV supply systems, do not provide specific requirements or prohibitions regarding protective devices in these applications, aside from mandating bidirectional overcurrent protective devices on the DC side of solar PV systems (712.533.101(d)). Overcurrent protective devices in prosumer electrical installations must be suitable for all directions of current flow and polarity (826.1.2.2). If suitability is uncertain, the manufacturer's instructions must be followed (134.1.1; 510.3).

A technical bulletin from BEAMA warns that connecting a generator or battery output to a protective device's outgoing (load) terminals can render the device inoperable. Circuit-breakers marked with "in" and "out" or "line" and "load" may have impaired arc extinguishing or short-circuit operation if connected incorrectly and triggered by a fault. Similarly, incorrect connections of unidirectional RCDs can cause damage when operated or tested, potentially leaving no visible damage and potentially impairing operation.

Product Standards and Marking Requirements

According to Clause 6.1 of BS EN 60898-1:2019:

• If distinguishing between supply and load terminals is necessary, markings must indicate the supply with arrows pointing towards the circuit-breaker and the load with arrows pointing away.

For RCCBs and RCBOs, Clause 6 of BS EN 61008-1:2012+A12:2017 and BS EN 61009-1:2012+A12:2016 specifies:

• Supply and load terminals must be clearly marked if distinction is necessary, e.g., with "line" and "load" or directional arrows.
• Markings must follow Table Z3, which may require a wiring diagram on the side, back, or inside the cover of the device.

Unidirectional RCBO Failure Mode

A unidirectional RCBO is designed for the supply to be connected to designated incoming terminals and the load to outgoing terminals. Connecting a solar PV system to the load terminals can maintain voltage across electronic components such as silicon control rectifiers (SCR) or trip relays after tripping, risking damage due to the components' short time rating. Some RCBOs include a switching contact linked to both line and neutral, ensuring disconnection upon tripping and protecting internal circuitry.

Selecting Protective Devices for Solar PV Power Systems

In prosumer installations (Section 823), bidirectional overcurrent protective devices are necessary (826.1.2.2). Where RCDs are used to protect AC supply cables connected to generating sets, all live conductors, including neutral, must be disconnected to achieve required disconnection times (551.7.1(ii)). The Best Practice Guide 3 by Electrical Safety First highlights that RCDs in PV/battery systems must not rely solely on inverter functions for additional protection. BS EN 61009-1 classifies RCBOs that disconnect all live conductors as two-pole RCBOs with characteristics suitable for TN and TT systems.

Summary

Unidirectional circuit-breakers and RCDs can suffer internal damage if connected incorrectly when bidirectional current flow is expected. Such damage is often undetectable through visual inspection, and reset capability does not ensure operability. For installations involving small-scale generation or battery storage, protective devices must handle bidirectional power flow and polarity. RCDs used for additional AC supply protection must disconnect all live conductors, and not all RCBOs meet this requirement. In cases of doubt, manufacturers' advice should be sought.