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IET Guide to Temporary Power Systems – an update

The IET’s Guide to Temporary Power Systems is undergoing a long-awaited update. Much has changed since the first edition published in 2012, not just in respect of the British Standards BS 7671 and BS 7909, but also with the temporary power industry in general.

Perhaps the most significant progression has been in renewable energy; the revised guide has a section on battery storage which is increasingly common in events, construction and similar. With the draft guide now out for public review, it has seen a raft of changes and this article gives an overview of what the more significant ones are.

The use of transportable battery 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 shut-downs 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.

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, 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’s Practitioner’s Guide to Temporary Power. Importantly, the requirements for effective electrodes are explained along with the typical failure modes experienced with temporary systems, demonstrating the relationship between the protection installed on the generator output and the value of the electrode resistance achieved on site. This section reflects the latest best practice which has yet to be reflected in relevant standards.

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 and the appropriate requirements for protective bonding. In a similar vein, the need to join electrical environments 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 updated and now includes information on the hazards associated with neutral switching on 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 the thunderstorms 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. 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 RCDs on socket-outlets rated at 32 A or less. BS 7909:2023 has also seen a major revision, the last being 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.

While the underlying theme of the book is temporary power for events, the advice and guidance on temporary power, generally from generators to cable selection and design methodologies, will be useful and appropriate for many other sectors using temporary electrical systems. This is reflected in the revised title for the book: Guide to Temporary Electrical Systems.