Prosumer electrical installations

BS 7671 Requirements for Electrical Installations governs safety in fixed electrical installations and has a number of predecessors dating back to 1882, which have all provided the minimum criteria for safe electrical installations. As technology advances and design philosophy alters to suit, BS 7671 has to change to keep pace.
 
The current document is widely known as the 18th Edition of the Wiring Regulations and was published in 2018, with amendments in 2020, 2022 and 2024. The regulations in BS 7671, the subsequent amendments and the associated guidance notes, should be referred to by those that design, install, commission and maintain fixed electrical installations.

Throughout its history, the two primary considerations of the design hierarchy have been electrical safety (how the installation reacts to faults to protect people and property) and design capacity (to operate within constraints to prevent overloads and thermal issues, including electrical fires). 

Traditionally, electrical installations have been straightforward, especially in dwellings. There is a grid connection, an electrical energy meter, a consumer unit (CU), and final circuits to the point of use, such as sockets or lights. Energy flow has historically been in one direction, as illustrated in Figure 1.

 

Figure 1: Traditional electrical installation

 

Modern installations can often incorporate newer technologies, such as solar panels and wind turbines, to generate on-site renewable electricity. Battery systems can store surplus energy from this generation or take advantage of dynamic off-peak tariffs by charging at off-peak and discharging at peak times. Surplus energy can also be exported back out to the grid to generate income through export tariffs. This all complicates energy flow as parts of the installation can be subjected to bidirectional energy flow. This means the direction of energy flow can change at any given time, depending on the configuration of the system. To maintain safety criteria, the electrical installation, including protective devices and controls, needs to be designed to cope with this.

 

 Figure 2: Modern prosumer electrical installation

 

Amendment 2 of BS 7671:2018, published in 2022, introduced Chapter 82. This new set of supporting regulations:

“provides additional requirements, measures and recommendations for the design, erection and verification of all types of low voltage electrical installations…” 

This includes:

"local production and/or storage of energy in order to achieve compatibility with existing and future ways to deliver electrical energy to current-using equipment or to the public network by means of local sources.” 

Electrical installations that produce and consume electrical energy:

“are designated as Prosumers Electrical Installations (PEIs).”

One size never fits all and there are a number of evolving scenarios for electrical installations that provide end user flexibility and could ease pressure on the grid. A design hierarchy that shall always start with electrical safety and capacity is now supported by efficiency measures. For a low-risk installation with no critical loads, the design hierarchy could look like Figure 3.

 

Figure 3: Example design hierarchy

For a higher-risk installation, resilience and efficiency may swap places but safety and capacity are never compromised. 

Other considerations of prosumer electrical installations (PEIs) can include:

• Energy flow to the loads
• Energy flow from the photovoltaics (PV) to energy storage or to the distribution board (DB)/CU
• Bidirectional energy flow through the meter, the grid connection and to/from energy storage
• Energy used in real time for general tasks, or exported
• Energy can be stored from the grid at optimum tariffs and from solar.

The new IET Guide to Prosumer Electrical Installations has been developed for designers, installers and maintainers, to illustrate in more detail the additional electrical safety and capacity consideration of BS 7671 Chapter 82. The guide is not intended to be prescriptive, as PEI types can vary, but the guide does provide a strong framework for the reader. It includes the issues to be addressed and what electrical safety risks need to be mitigated.

All sections of Chapter 82 from BS 7671 are covered in the guide, which also contains illustrations to aid the reader. The guide discusses the concepts of PEIs and their interaction with the smart grid. Energy efficiency principles are also covered. 

Sections 4 and 5 of the guide focus on electrical safety and functional requirements. Topics covered include: earthing, functional safety, outage of public networks, selectivity of protective devices, design considerations, controls, measurements, maintenance, inspections and testing.

Sections 6 and 7 explore the types of PEI, their modes of operation, the implementation of energy storage in a PEI context and the principles of load planning, including load shedding in island mode and load shifting to minimize peak tariffs.

Section 8 explores additional topics from outside of Chapter 82, such as customer limitation schemes, and G98 and G99 applications. 

Various appendices provide supplementary information. Appendix D provides some calculation examples to map out loads through a 24-hour cycle and some illustrations of the selectivity issues that could be encountered in direct feed and island mode situations. Appendix E provides a series of checklists for PEIs including pre-installation (to assist with feasibility checks), installation commissioning activities in line with BS 7671, certification and administration (to assist with sign offs), and guidance on functional tests.

The publication is now ready to pre-order from the IET Book Shop and will be published in December.

The IET Guide to Prosumer Electrical Installations brings together all aspects of a prosumer’s installation. It covers not only safety requirements, but also functionality and energy efficiency. By integrating these elements and addressing the considerations of modern electrical systems, it serves as an invaluable resource for designers, installers and maintenance professionals. 

Acknowledgements

• Joe Cannon