This article describes what must be taken into account when selecting protective devices for use in applications where bidirectional power flow is to be expected in normal operation.
Introduction
As the installation of electrical energy storage systems and small-scale generation capable of feeding into the public distribution system becomes more widespread, it is increasingly common to have bidirectional power flow in some conductors within an installation.
Batteries are considered to be generators for the purposes of BS 7671 (551.1.l(v)) and should also be considered as both a supply and a load (see note to regulation 823).
This raises issues in respect of the suitability, or otherwise, of protective devices such as circuit-breakers, residual current circuit-breakers (RCCB) and residual current operated circuit breakers with integral overcurrent protection (RCBO).
Additionally, in such applications, consideration must also be given to which conductors must be disconnected when a protective device operates under fault conditions. For example, it may be necessary to disconnect all live conductors so that a requisite disconnection time is achieved. This is also discussed.
Suitability for use where bidirectional power flow is to be expected
Currently BS 7671 does not contain definitions for the terms unidirectional or bidirectional, although these terms are used within that standard in respect of overcurrent protective devices (712.533.lOl(d)), exchange of information (825.l(viii)) and conducted transients - classification AM (Appendix 5).
In the absence of definitions, in common parlance:
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unidirectional means capable of operating or moving in one direction only, and
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bidirectional means capable of operating or moving in either direction.
Neither Section 551 nor Section 712 of BS 7671:2018+A2:2022, which give the general requirements for low voltage generating sets and the particular requirements for the installation of solar PV supply systems respectively, contain any specific requirements, or prohibitions, in respect of protective devices used in such applications other than requiring bidirectional overcurrent protective devices on the DC side of a solar PV system (712.533.lOl(d)). However, it is a requirement that overcurrent protective devices in prosumer's electrical installations are suitable for all possible directions of current flow and polarity (826.1.2.2).
When working on such installations, where there is any doubt regarding suitability of a protective device for use in a particular application, the manufacturer's instructions shall be taken into account (134.1.1; 510.3).
The BEAMA technical bulletin Connection of Unidirectional and Bidirectional Residual Current Devices (RCDs) and Miniature Circuit-Breakers (MCBs) to power supplies e.g. battery storage, Photovoltaic (PV) systems, Electric Vehicles (EV) to home, a microgenerator, or grid (mains) supply advises that, in some cases,
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connecting the output of the generator or battery to a protective device's outgoing terminals, designated as 'out' or 'load' will result in damage rendering the device inoperable and
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​​​​​​​circuit-breakers with terminals marked 'in' and 'out' or 'line' and 'load' may have their arc extinguishing and/or short-circuit operation characteristics impaired if they are connected incorrectly and the device operates under a fault condition.
Furthermore, in some cases, when a unidirectional residual current device is connected incorrectly, testing the device with a test instrument or even merely operating the 'test' button may also render the device inoperable.
For both circuit-breakers and RCDs
(RCCBs and RCBOs) it is unlikely that there will be any visually observable signs to indicate that damage has occurred. Furthermore, even if the 'test' button can be reset this does not mean that the device is still capable of operating.
Product standards
Clause 6 .1 (Standard marking) of BS EN 60898-1:2019 requires that:
If it is necessary to distinguish between the supply and the load terminals, the former shall be indicated by arrows pointing towards the circuit-breaker and the latter by arrows pointing away from the circuit-breaker.
In the case of RCCBs and RCBOs, Clause 6 (Marking and other product information) of BS EN 61008 1:2012+A12:2017 and BS EN 61009-1:2012+A12:2016 both state:
If it is necessary to distinguish between the supply and the load terminals, they shall be clearly marked (e.g. by "line" and
"load" placed near the corresponding terminals or by arrows indicating the direction of power flow).
An example of typically used arrangements is shown in Fig 1.
Fig 2 shows the terminal markings on a bidirectional RCBO.
Clause 6 of both BS EN 61008 1:2012+A12:2017 and BS EN 61009-1:2012+A12:2016 also states that the devices covered shall be marked in accordance with the Table Z3 therein.
In both cases this states that, unless the correct mode of operation is evident and there is insufficient space in a visible position on the device, a wiring diagram should be included either:
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on the side or on the back of the device, visible only before the device is installed, or
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on the inside of any cover which has to be removed in order to connect the supply wires.
Examples of such wiring diagrams are shown in Fig 3.
Unidirectional RCBO failure mode
Fig 4 shows a circuit-diagram for a 1-ph and switched neutral unidirectional RCBO.
Unidirectional devices are designed for the supply to be connected to the incoming (supply) terminals (shown here as Lrn and Nrn) and the load to be connected to the outgoing (load) terminals (shown here as LouT and N ouT). Where a solar PV system is connected to the outgoing terminals a voltage can remain present across the internal electronic components such as silicon control rectifiers (SCR) or thyristors in the signal amplifier circuit or the trip relay solenoid for up to 1 second' after the device has operated. In some designs of RCBO, this continued presence of voltage after the device has tripped can result in damage to such electronic components, which are typically short time rated.
One solution adopted by manufacturers to address this issue is to include a switching contact linked to the contacts in the line and neutral conductors, as shown in Fig 5. With this arrangement, when the device trips the supply to the amplifier and solenoid is also disconnected.
Selecting protective devices for the AC side of a solar PV power supply system
Where the solar PV system forms part of a prosumer's electrical installation (see Section 823), the overcurrent protective device must be suitable for bidirectional operation (826.1.2.2).
When a generating set, such as a solar PV system, is used as an additional source of supply in parallel with another source, where it is necessary to install an RCD to provide additional protection for an AC supply cable connecting a generator set to the installation, the RCD shall disconnect all live conductors, including the neutral conductor (55 1.7.l(ii)). This is necessary in order for the RCD to be able to operate within the requisite time to provide additional protection. This necessity is discussed in detail in Best Practice Guide 3 - Connecting a microgeneration system to a domestic or similar electrical installation (in parallel with the mains supply), published by Electrical Safety First. It should be noted that BS 7671 does not recognise inverters of PV or battery storage systems as a means to provide additional protection.
According to BS EN 61009-1, an RCBO which disconnects all live conductors is classified as two-pole RCBO with one overcurrent protected pole, where the unprotected pole is a switched neutral, and the device has the following characteristics:
• overload/short-circuit protection in L, breaking both L and N as they are mechanically linked
• earth-leakage (RCD) protection breaking both L and N
• switching off the device breaks L
and N, thus making it suitable to provide isolation of TN and TT system earthing arrangements.
Many RCBOs do not switch the neutral and so would not be suitable for use to provide additional protection to the AC supply cable to the PV system.
Summary
Unidirectional circuit-breakers and residual current devices (RCCBs and RCBOs) may suffer damage to the internal electronic circuitry sufficient to impair their functionality if the output of a generator or battery storage inverter is connected to their load terminals. It is highly unlikely that this damage will be detectable during a visible inspection. Where bidirectional current flow is expected in normal use, such as where an installation includes small-scale generation or battery storage, suitable protective devices must be installed which take account of all possible directions of power flow and polarity. Any RCD (RCCB or RCBO) installed to provide additional protection to the AC supply cable must be of a type which disconnects all live conductors. Not all RCBOs switch the neutral. Where there is any doubt as to the suitability of a particular protective device for use in applications where bidirectional power flow is expected, the advice of the manufacturer(s) of the device(s) in question should be sought.
1 Engineering Recommendation G98 Requirements for the connection of Fully Type Tested Micro-generators (up to and including 16Aperphase) in parallel with public Low Voltage Distribution Networks on or after 17 May 2019, published by the Energy Networks Association, advises that, for certain types of PV inverter, maximum shut down time on loss-of-mains may be up to 1 second.
