Power Flow in Both Directions
When Is a Bidirectional RCBO Needed?
For many years, domestic electrical installations were designed around a straightforward assumption. Electricity entered the property through the incoming supply, passed through the consumer unit and travelled along each final circuit to the connected load.
Solar PV, battery storage and vehicle export systems are changing that familiar arrangement. An installation may now contain an additional source of supply, allowing power to move through parts of the installation in more than one direction.
However, the presence of power flow in both directions does not mean that every RCBO in the consumer unit must be bidirectional. The important question is not simply which way current may flow. It is whether the circuit arrangement could connect a source of supply to either set of terminals on the protective device.
This paper explains what bidirectional means, why the distinction matters and how electricians can determine whether a bidirectional RCBO is required.
The traditional direction of power flow
In a traditional domestic installation, the electricity supply enters the consumer unit from the distribution network. From there, it passes through the main switch, busbar and individual protective devices before reaching the connected loads.
Under this arrangement, each protective device has a clearly defined supply side and load side. The incoming supply is connected to the terminals identified by the manufacturer for the source, while the outgoing circuit is connected to the terminals intended for the load.
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The protective device is therefore installed in the direction anticipated by its manufacturer.
What has changed?
Many modern installations contain equipment that can generate, store or export electrical energy. This may include solar PV systems, battery storage systems and vehicles capable of exporting energy back into an installation.
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These technologies can create operating conditions in which power does not always travel from the distribution network towards the final circuits. At certain times, an additional source may supply loads within the property. Surplus energy may also pass back through the installation towards the distribution network.
Battery systems can make the arrangement more complex because their operating mode may change. A battery may absorb energy while charging and provide energy while discharging. The direction of power flow therefore depends on how the system is operating at that moment.
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This change has led to uncertainty about which protective devices are suitable. It has also created the assumption that any installation containing solar PV or battery storage automatically requires every RCBO to be bidirectional. That is not the case.
What is a bidirectional RCBO?
A bidirectional RCBO is a device that its manufacturer intends to be connected with the source of supply at either set of terminals.
A unidirectional RCBO is intended to have the source connected only to one defined set of terminals. These may be identified using markings such as line and load, in and out, or directional arrows.
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This distinction is important because bidirectional does not simply mean that current can physically pass through a device in either direction. Current passing through an RCBO does not, by itself, confirm that the device is suitable for every possible source connection.
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The defining point is the manufacturer’s intended connection. If the manufacturer declares that the source may be connected to either set of terminals, the device is bidirectional. If the source must be connected to one specified set of terminals, it is unidirectional.
Power flowing both ways is not the deciding factor
The presence of an additional source can cause power to flow in different directions through the installation, but this does not automatically determine the directionality required for every protective device.
Consider a solar PV system connected through its own dedicated circuit. When the system is generating, power may travel from the inverter through that circuit and into the consumer unit. From there, it may supply loads elsewhere in the property or be exported to the distribution network.
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That does not mean every RCBO protecting every other final circuit must be bidirectional. The electrician must identify which circuit connects the additional source and examine how that source is connected in relation to the protective device.
The decision must therefore be based on the actual circuit arrangement, not simply on the presence of solar panels, a battery or another source of energy.
When is a bidirectional RCBO needed?
Regulation 530.3.201 of BS 7671 requires the selection and erection of equipment to take account of the appropriate use of unidirectional or bidirectional protective devices.
Where the intended circuit arrangement could connect a source of supply to either set of terminals on an RCBO, the directionality of that device must be considered. A bidirectional RCBO may be required where the manufacturer permits the source to be connected at either terminal set and the proposed arrangement is permitted by BS 7671.
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This does not mean that using a bidirectional RCBO automatically makes every possible connection acceptable. The complete circuit arrangement must still comply with the current requirements of BS 7671 and the instructions provided by the manufacturers of the protective device and the connected equipment.
The electrician must assess the installation as a complete system. The source, operating modes, method of connection and position of the protective device all form part of that assessment.
Where should electricians look?
The first step is to understand the circuit rather than beginning with the product label.
Identify every possible source of supply. This includes the distribution network and any additional source such as solar PV, battery storage, a vehicle export system or another generating source.
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The next step is to establish which circuit connects that source and how power can move through the installation during every intended operating mode. A battery system, for example, may charge under one condition and discharge under another.
Once the circuit arrangement is understood, consider whether a source could be connected to either set of terminals on the RCBO. If it could, the electrician must confirm that the device is specifically declared suitable for that arrangement.
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The final decision must be based on the exact device being installed. Familiarity with another product from the same manufacturer, or with a device that appears physically similar, is not sufficient evidence.
How can directionality be confirmed?
Directionality should be confirmed using the markings on the device and the manufacturer’s technical information.
A unidirectional device may have defined line and load terminals, in and out markings, arrows or installation instructions that specify where the source must be connected.
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A bidirectional device should be clearly declared by the manufacturer as suitable for connection with the source at either set of terminals. If this cannot be established from the device markings, the product data or the manufacturer’s instructions, suitability should not be assumed.
The exact product reference must also be checked. Different devices within the same range may not necessarily have the same directional characteristics.
A practical selection process
The following process can be used when assessing whether a bidirectional RCBO is required.
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First, identify whether the installation contains an additional source of supply.
Next, establish which circuit connects that source and consider every intended operating mode.
Determine whether the circuit arrangement could connect a source of supply to either set of terminals on the RCBO.
Check the markings and technical information for the exact RCBO to confirm whether the manufacturer declares it suitable for that connection.
Finally, confirm that the complete arrangement complies with BS 7671 and the instructions supplied by the manufacturers of all relevant equipment.
The Navitas bidirectional RCBO solution
The Navitas bidirectional RCBO range is designed and declared for bidirectional operation. This provides clear confirmation that the source of supply may be connected to either set of terminals where the circuit arrangement and current requirements of BS 7671 permit it.
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Clear declaration removes the need to make assumptions based on the appearance of the device or the direction in which current may be flowing.
The use of a Navitas bidirectional RCBO does not remove the need to assess the complete installation. The circuit design, connected equipment and installation method must still comply with BS 7671 and the relevant manufacturer instructions.
Key points
Modern electrical installations may contain more than one source of supply, allowing power to move through parts of the installation in different directions.
This does not mean that every RCBO in an installation containing solar PV, battery storage or vehicle export must be bidirectional.
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A bidirectional RCBO is not defined simply by current being able to flow through it in either direction. It is defined by the manufacturer intending the source of supply to be connected to either set of terminals.
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The electrician must assess the actual circuit arrangement, identify every possible source and establish how each source is connected in relation to the protective device.
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Directionality must be confirmed using the markings and technical information for the exact RCBO. It should never be assumed from appearance, familiarity or current flow alone.
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The complete arrangement must comply with the current requirements of BS 7671 and the instructions supplied by the equipment manufacturers.
Conclusion
An installation needs a bidirectional RCBO where the circuit arrangement may connect a source of supply to either set of the device’s terminals and the manufacturer has declared the device suitable for that use.
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The decision should not be based simply on whether power can flow in both directions or whether the installation contains solar PV, battery storage or vehicle export. It must be based on the actual circuit arrangement, the intended operating modes, the device markings, current BS 7671 requirements and the manufacturer’s technical information.
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Understanding this distinction allows electricians to select protective devices with greater confidence and avoids both unnecessary replacement and the incorrect use of a unidirectional device.
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References
BS 7671:2018 Requirements for Electrical Installations, including current amendments.
IET, guidance concerning bidirectional and unidirectional protective devices.
Manufacturer technical information and installation instructions for the exact protective device and connected equipment.
Scope
This paper provides general guidance on the selection of bidirectional RCBOs. It does not replace an assessment of the individual installation, the current requirements of BS 7671 or the instructions supplied by the relevant equipment manufacturers.
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