Guide to Temporary Electrical Systems, 2nd Edition

The IET’s Guide to Temporary Electrical Systems has finally arrived after undergoing a long-awaited update. As regular readers of Wiring Matters will appreciate, the field of electrical safety constantly evolves, with standards, equipment, tools and techniques all improving and changing over time. Since the first edition in 2012, the world of temporary power has changed considerably, though not necessarily in how it is used; after all, the need for a temporary supply and associated distribution is a requirement as old as the need for electrical installations in buildings.

The use of temporary power is very widespread, from railways, military services, construction, festivals, street markets, temporary funfairs, exhibitions and more. While the underlying theme of the book is temporary power for events, the advice and guidance covering generators to cable selection, and design methodologies to hybrid system deployment, are universal and apply to any other sector using temporary electrical systems because the design requirements are drawn from BS 7671.

Compared to the first edition, two significant changes are noteworthy. Firstly, much work is underway, both nationally and internationally, regarding generator use and their earthing arrangements; the understanding of system topologies and protection is now much advanced. While much of the standards development is still underway and yet to be published, the Guide to Temporary Electrical Systems gives guidance based on the latest knowledge in this area and is unique in the UK.

The second significant progression has been in renewable energy. Clearly, the days of the combustion engine are waning and hydrogen and solar are becoming more established as energy sources, though they are still limited in capacity at a practical level. Renewable fuels for diesel engines are popular, but the veracity of the environmental credentials for vegetable oil-based diesel has recently come under the spotlight, with some companies refusing to use it.

Rather than the energy sources, perhaps the biggest benefit has been the development of large-scale battery storage, with capacities in the region of up to 400-500 kVA available in transportable formats. The use of such systems allows hybridization of generator packages which is important in the move to Stage V emissions compliant diesel engines. Running such generators on low loads often results in unexpected shutdowns as the exhaust temperatures do not get hot enough for the emissions filters to work effectively; hybridizing a system with energy storage allows the generator to work hard for shorter durations, bringing the exhaust temperature up and alleviating the problem. As the generator will usually operate in parallel with the battery unit, it is important to consider earthing arrangements and the effects of circulating currents. Another key aspect with battery systems is their performance under fault conditions – many inverters will not deliver the current required to operate the protective devices in a distribution, so managing protection for faults requires more diligence on the part of the designer.

Along with these issues, the updated guide looks at the considerations around energy storage capacities, as well as how battery units can integrate into generation systems, such as working in a simple hybrid mode or as a mini-grid with other generators operating in a load-on-demand configuration, for example.

On the subject of generators, information on generator control and operation has been updated and now includes guidance on parallel operation of sets, leading power factors and the stability of generator sets powering capacitive loads, such as switched-mode power supplies, as commonly found in most modern equipment and lighting. The (often vexed) question over generator earthing – or lack of – has been significantly revised to take into account the previous work published in the IET Practitioner’s Guide to Temporary Power Systems. Importantly, the requirements for specifying appropriate earth electrode resistances are explained, along with the typical failure modes experienced with temporary systems.

Another section to receive a makeover is “Mobile and transportable units”, which range from temporary site huts and office cabins through to complex broadcast vehicles. In general, the requirements relating to equipment selection detailed in Part 717 of BS 7671 are quite straightforward, but the design is sometimes less so. There are diagrams in Part 717 which give examples of how the supply and earthing arrangements to a unit may be configured but understanding the benefits and drawbacks of each can be a challenge. These are all explained in the revised section, in particular, the benefits of transformer-based designs with respect to mitigating shock risks in the event of supply neutral (PEN) conductor failure where units are connected to the public distribution.

“Protective earthing” has been revised to provide more guidance on determining extraneous-conductive-parts in and around temporary structures and the appropriate requirements for protective bonding. In a similar vein, the need to join electrical environments (in essence, separate electrical sources and distributions) is often necessary where multiple supplies are operating in proximity to each other. This has been considered in more detail (including fault current paths), building on the guidance in BS 7909.

Supply resilience is increasingly important in installations as well as with temporary systems. The section on uninterruptible power supplies has been expanded and updated and now includes information on the hazards associated with neutral switching on uninterruptible power supply (UPS) inputs for example, as well as looking at how inverters behave when faults occur on the output, which can vary between designs. Supply resilience in the event of emergencies is important for events where there may be large numbers of the public present. Systems may have to be kept operational for crowd control purposes and this is very important for outdoor events where thunderstorms can roll in. The guide has a completely revised section on management of the temporary systems when lightning is forecast, taking into account recent industry guidance on the subject.

While these are headline changes, there are of course a significant number of updates arising from the revisions to BS 7671 and BS 7909, and other relevant guidance. The former has undergone a range of changes since the first edition of the book, with salient topics including cables in escape routes, the use of arc fault detection devices, surge protective devices and the application of 30 mA residual current devices (RCDs) on socket-outlets rated at 32 A or less. BS 7909:2023+A1:2024 is the first major revision of the standard since the last one in 2011. Many of the changes reflect those in BS 7671, whereas others relate to generator operation, earthing and renewables, for example, which are explored in more detail in this guide.

The IET Guide to Temporary Electrical Systems, 2nd Edition is now available to purchase from the IET Bookshop.