Jake Green, Head of Technical Engagement with Scolmore Group, considers conditions where current may flow in more than one direction, and where it may be necessary to select protective devices that are ‘bidirectional’ in nature rather than ‘unidirectional’.
BEAMA issued a technical bulletin in 2024 detailing a concern where certain protective devices failed to safely operate when required where current flowed in either direction.
An electrical installation having more than one source of supply current may create the conditions where current can flow in more than one direction. Where this is the case, it is necessary that protective devices are able to operate correctly in whichever direction current flows.
Amendment 3
To address the issue of current flowing in two directions Amendment 3 of BS 7671 (BS 7671: 2018+A3:2024) was issued on 31st July 2024 and is to be implemented immediately. The amendment introduced one new requirement (Regulation 530.3.201) and two new definitions.
- Bidirectional protective device – 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.
- Unidirectional protective device - 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.
Regulation 530.3.201 requires that due account is taken by the designer and installer of the likely direction of current flow and select protective equipment accordingly.
Direction of connection
Single-pole, single-pole and neutral (two-pole) and double-pole devices are typically labelled with ‘In’ or ‘Supply’ and ‘Out’ or ‘Load’.
Where devices are labelled ‘In/Out’ or ‘Supply/Load’ they will always be unidirectional in nature, and under certain conditions should not be selected for use. Unidirectional devices can always be used where there are no alternative sources of supply. Bidirectional devices will not be specifically labelled with in and out and are likely to be labelled with ‘L/N’ or ‘L1/N1’ and ‘L2/N2’. Bidirectional devices are likely to be necessary where multiple sources of supply exist.
Where the normal supply operates in parallel with a second source of supply
Consider an example where a solar PV system has no battery storage system installed. In this condition power flow will be in one direction only (unidirectional); that is from the solar PV generator into the inverter and then from the inverter to the means of protection in the consumer unit/distribution board. In such a condition the protective measure of ADS applies, and the conditions detailed in Regulation 434.2.1 would apply.
However, even in this condition, Regulation 530.3.201 will still apply even though the source of supply is unidirectional. A bidirectional protective device would be required since within a consumer unit/distribution board the supply from the inverter is likely to be connected to the load side of the protective device whilst the supply side protective device is connected to the busbar within the consumer unit.
Consider a second example, where a solar PV system has an attached battery storage system installed. In this condition, power flow will be bidirectional; that is current can flow from the solar PV generator to the normal supply or to the battery storage system, and also from the normal supply to the battery storage system. In such a condition, the protective device will be required to be bidirectional in nature.
Fig 2 details the likely direction of current flow where a solar PV installation contains a battery storage system.
[1] Permits a device used for fault protection to be installed between a point of a reduction in cross-sectional area and the position of the protective device where:
- The length of conductor does not exceed 3 m;
- Installed in such a manner as to reduce the risk of fault to a minimum; and
- Installed in such a manner as to reduce to a minimum the risk of fire or danger to persons.
Fig 2 Likely directions of current flow
Where a prosuming installation exists, and an electrical installation containing a solar PV system and battery storage system is a prosuming installation, Regulation 826.1.2.2 requires account to be taken of all directions of current flow and polarity.
Other requirements
In addition to the general requirements of detailed above, further requirements exist in relation to the use of RCDs and protection against thermal effects.
Regulation 551.7.1 requires two conditions to be fulfilled:
- Protection against thermal effects (Chapter 42) and protection against overcurrent (Chapter 43) must remain effective in all situations, and
- Where an RCD provides additional protection (415.1) for a circuit connecting the generator set to the installation, the RCD must disconnect all live conductors (including the neutral).
The effect of the second bullet point is that where a solar PV system or similar is installed the protective device must be capable of switching the neutral conductor.
Periodic inspection and testing
When carrying out periodic inspection and testing in those installations where Regulation 530.3.201 now applies great care will need to be taken regarding the relevant coding associated with the presence of unidirectional/bidirectional devices. Whilst a Code 1 (immediate danger) will rarely be necessary there may be situations where either a Code 2 or Code 3 will apply. The inspector will need to consult the current guidance issued within the industry for help in selecting the appropriate code.
Conclusion
With the increasing installation of prosuming electrical installations it will be necessary for designers and installers to utilise bidirectional protective devices.