In this article, Andrew Duffen, Technical Commercial Engineer at NAPIT, seeks to unravel the mysteries of unidirectional and bidirectional protective devices.
With the introduction of Amendment 3 of BS 7671, the requirement for the installation of bidirectional protective devices for all sources of supply, including grid connections, generating sets, i.e. battery storage, solar PV and electric vehicles, with the ability to power back to the charger (vehicle-to-grid), has been addressed.
Amendment 3 is a standalone document and has to be included with the current version of BS 7671:2018+A2:2022 and Corrigendum (May 2023). It is a free-to download PDF and should be appended to your brown copy of BS 7671:2018.
A key principle of this amendment is the safety and compliance of electrical installations as more users become prosumers in this modern world, the growth in the renewable energy market in the UK and the rising cost of energy.
There are two definitions for BS 7671 in Amendment 3, describing both types of protective devices.
What is a unidirectional protective device?
For a unidirectional protective device, BS 7671 Amendment 3 states:
“A protective device where it is intended by the manufacturer that a source of supply is only connected to one defined set of connection terminals.”
Unidirectional protective devices are labelled to show the line and load terminals and are intended to function when the source of supply connects solely in one direction, from the supply to the load. It is essential to pay attention to the connection terminations as specified on the device, shown in Fig 1.
An example of this type of device is a Residual Current Breaker with Overcurrent (RCBO). Not all RCBOs are unidirectional; some RCBOs now incorporate technology that prevents the RCBO from being damaged when the source of supply is derived from either direction, making them bidirectional protective devices.
What is a bidirectional protective device?
The definition of a bidirectional protective device in Amendment 3 is defined as:
“A protective device where it is intended by the manufacturer that a source of supply is connected to either or both sets of connection terminals.”
Therefore, a bidirectional protective device does not have line and load terminal markings, which allows it to safely accommodate a source of supply from either direction without risk of damage. An example of this type of device would be a RCCB, see Fig 2.
Generally, RCCBs within consumer units are in two-module-sizes, and do not usually have an in and out indication; therefore, they are classified as bidirectional protective devices, though they could be marked to display where the neutral or line should be terminated.
RCCBs installed in split load consumer units are not suitable for the connection of solar PV or battery storage systems.
How will Amendment 3 affect existing installations?
Careful consideration should be given to existing installations in regards to carrying out electrical installation condition reports (EICR). Inspectors carrying out these reports will need to give a classification code if a unidirectional protective device was installed with connections for more than one source of supply, such as a generating set. NAPIT as part of the Wiring Regulation Advisory Group (WRAG) have created a question and answer on this topic with industry consensus on the impact on existing installations.
To correctly assign classification codes when conducting EICRs, NAPIT and WRAG recommends checking with the manufacturer to establish if the protective devices being installed are bidirectional or unidirectional protective devices and to receive a declaration of conformity which shall be appended to the EICR, as shown in Fig 3.
If a declaration of conformity is provided, then no classification code should be recorded on the ECIR. If no declaration is received and if the RCD is used only for ‘Additional Protection’ it should be a classification code C3, or if the RCD is used for ‘Fault Protection’ then a classification code C2 would be recorded on the EICR.
Conclusion
After the release of Amendment 3, all sources of supply for connection to either set of terminals, including generation sets, must now be protected by a bidirectional protective device.
Designers, installers and inspectors must be aware of Amendment 3, ensuring that all electrical installations that have sources of supply operating in either direction, such as generating sets, comply with the amendment. Regulation 530.3.201 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.”
With rapid growth in the renewable energy industry, bidirectional protective devices will become the standard.
In regard to existing installations of generation sets, where it is unclear if the protective device is either unidirectional or bidirectional, inspectors must confirm the protective device type with the manufacturer, with either written or published confirmation of device type.
Once this information is gained, it will aid the inspector in determining any potential classification code required for an EICR.
Further information on uni and bidirectional devices can be found in the On-site Solutions publication, available at NAPIT Direct.
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