In-depth focus on the latest update to the IET Wiring Regulations
Read our guide to the changes announced in Amendment 3 and discover the background to their introduction
In the UK, it is increasingly common for a modern home to have an additional means of generation of electricity provided over and above the normal supply from the distribution network operator (DNO). This can help to reduce the consumer’s electricity consumption from the grid and reduce associated suppliers’ energy costs.
Many new housebuilders specify the installation of roof-mounted solar photovoltaic (PV) systems to provide such capability and, in older properties, many homeowners have had such systems retrofitted over the last 10 years or so.
These systems are designed to operate in parallel with the DNO supply and are designated as microgeneration systems as they have a limited generation capability of not more than 4 kW or 16 A of electricity to satisfy certain DNO requirements.
Competence
Electrical contractors and electricians involved in the design, installation and commissioning of microgeneration systems should be suitably competent to carry out this type of work and will require to have a good working knowledge of the requirements given in BS 7671:2018+A2:2022 (IET Wiring Regulations 18th Edition) to ensure the safety and proper operation of installed systems.
The current requirements that need to be considered include:
Section 551 – Low voltage generating sets
Section 551 applies to low voltage generating sets generally. Additional requirements are given in Regulation 551.7 for installations where the generating set may operate in parallel with other sources of supply, including systems for distribution of electricity to the public.
Section 712 – Solar PV power supply systems
Section 712 is one of the special installations or locations given in Part 7 of BS 7671. Regulation 712.551 provides additional requirements for installations where the generating set may operate in parallel with other sources of supply, including systems for distribution of electricity to the public.
Chapter 82 – Prosumer’s low voltage electrical installations
A prosumer is defined in Part 2 of BS 7671 as “an entity or party which can be both a producer and a consumer of electrical energy”.
Chapter 82 includes requirements for local production and/or storage of energy in installations. A prosumer’s electrical installation (PEI) may comprise a number of supplies, including the grid supply, which are managed through an electrical energy management system (EEMS) to achieve safe operation, sustainability and efficient use of energy when integrated into a smart grid.
Alternative means of supply can be provided by microgeneration systems that operate in parallel with the grid supply or independently, e.g. during a grid fault or where the grid is deliberately disconnected by the prosumer or DNO.
In such installations, energy can also be stored and used during peak times to offset consumption from the grid supply.
Developments and correct connection
Developments in the technology associated with such systems are significant and mean that, unlike traditional consumer installations where the flow of current is normally in one direction – i.e. from the DNO supply to the connected electrical loads – electricity in a prosumer installation can also flow from the microgenerator supply back to grid supply.
This evolution in electrical installations therefore needs careful consideration by those involved with microgeneration and prosumer installations to ensure the correct selection and erection of equipment and, most importantly, its correct connection.
This is especially true when considering the suitability of protective devices to be installed, e.g. residual current devices (RCDs) such as residual current circuit breakers (RCCBs) and residual current breakers with over-current (RCBOs), or miniature circuit-breakers (MCBs), which are an essential part of the safety measures required in any electrical installation.
The suitability and correct installation of protective devices has been the source of much recent debate in the industry and electrical contractors and electricians should therefore also be aware of the following:
BEAMA Technical Bulletin
As we outlined in the April/May issue of cabletalk, BEAMA has produced a 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 micro-generator, or grid (mains) supply, a link to which can be found on the opposite page.
This should be considered as it discusses requirements for the correct connection of RCDs and MCBs where power supplies are provided from various sources of supply.
It mentions requirements in BS 7671:2018 + A2:2022, specifically Regulation 551.7.1, which requires that when a generating set is used as an additional source of supply in parallel with another source – e.g. a PV system in conjunction with a grid supply – the RCD providing additional protection in accordance with Regulation 415.1, SHALL disconnect all live conductors, including the neutral conductor.
This is necessary because there is a theoretical electric shock risk which could exist under certain conditions where a single-pole RCBO has been installed to facilitate connection of the supply circuit from a microgenerator systems.
This is also outlined on page 14 of Electrical Safety First Best Practice Guide No 3, Connecting a microgeneration system to a domestic or similar electrical installation in parallel with the mains supply, a link to which is available on the opposite page.
BEAMA’s bulletin also highlights requirements given in relevant product standards to spell out the differences that exist between unidirectional protective devices and bidirectional protective devices and that under NO circumstances should the load terminals of a unidirectional protective device be connected to a source of supply.
To distinguish between the supply and load terminals of a unidirectional protective device, the product standard requires the device to have designated marked terminals – typically either ‘line’ and ‘load’ or ‘in’ and ‘out’ or by directional arrows, as shown in Figure 1, left.
Taking account of manufacturer’s instructions and compliance with Regulations 134.1.1 and 510.3 in BS 7671 is therefore particularly important to ensure that these devices are appropriately connected.
If a supply were to be inadvertently connected to the load terminals of a unidirectional protective device, this could result in damage to the device, resulting in its failure to operate correctly during a fault.
This could be a critical safety issue, especially where RCD protection is provided via an RCCB or RCBO for additional protection or fault protection for the supply where required.
Where a supply requires to be connected to the load terminals of a protective device, as may be the situation where a microgenerator has been installed, a suitable bidirectional protective device is necessary and should be installed.
Bidirectional protective devices are available from most major manufacturers – but be aware that the product standard does not require these to have a specific mark or markings, as connection of a supply can be made to either set of terminals.
Electrical contractors or electricians may be aware of certain manufacturers’ protective devices which are marked as if they are unidirectional, however the specific manufacturer may provide information that the device has been tested to operate bidirectionally.
This would mean that the device does not comply with the product standard and the use of such products would need to be regarded as a departure from BS 7671:2018 + Amendment 3:2024.
This is not necessarily a problem, and a simple risk assessment should be carried out by the designer to ensure that the installation is no less safe than would otherwise be achieved by using a compliant product.
The provision of documented evidence such as a declaration that the device has been tested to operate bidirectionally may be sufficient to enable the designer to consider this achieved. However, it would be recommended that such evidence is appended to the relevant BS 7671 certification issued for the installation.
BS 7671:2018 incorporating Amendment 3:2024
Having become aware of all the issues discussed in this article, JPEL/64 – the joint BSI/IET committee responsible for the maintenance of BS 7671 – has acted to ensure they are addressed in BS 7671 by publishing BS 7671:2018 incorporating Amendment 3:2024.
This was published for immediate use from 31 July and comprises a single additional regulation and two additional definitions.
The new Regulation 530.3.201 requires that selection and erection of equipment for protection shall take account of appropriate use of either a unidirectional protective device or a bidirectional protective device.
An informative note located below the regulation advises that product standards as listed in Appendix 1 for some protective devices – including RCCBs, RCBOs, circuit breakers and arc fault detection devices (AFDDs) – require these devices to be marked to indicate if they are unidirectional, e.g. ‘in’ and ‘out’ or ‘line’ and ‘load’ or arrows.
The first additional definition provided is for a unidirectional protective device, which is defined as: “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.”
The second definition is for a bidirectional protective device, which the amendment says is: “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.”
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WANT TO KNOW MORE?
Amendment 3:2024 is a separate publication from BS 7671:2018+ A2:2022 and is free to download by going to bit.ly/IET-A3
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