A temporary power supply for a UK outdoor event needs three things in place before anyone energises a board: per-generator load measured against capacity, a BS 7909 distribution chain signed off by a competent person, and 110V CTE supply for portable tools. Get those right and most field-day failures never happen. The recurring questions on an AGF or ISO 20121 audit are about data: can you show what each generator carried, and can you export it.
STAT
The UK event industry burns an estimated 380 million litres of diesel a year, and analysis of more than 20 million monitoring data points suggests around 40 percent of that could be cut with no loss of power and no extra blackout risk. The gap is almost always measurement, not hardware. Source: Powerful Thinking and A Greener Future, The Show Must Go On (2024 figures)
Key takeaways
| Topic | In brief |
|---|---|
| Per-generator load | Measure load against capacity per generator, logged and exportable, not read off a nameplate. |
| BS 7909 baseline | Every distribution chain BS 7909-compliant and signed off by a competent person before energisation. |
| 110V CTE tools | UK practice is 110V centre-tapped-to-earth for portable tools; board monitoring sits on the supply side, not at the tool. |
| Sustainability data | AGF and ISO 20121 events increasingly ask for per-generator energy data; MID-grade metering makes it auditable. |
How much load is on each generator, and how close is it to capacity?
This is the question that prevents most field-day failures, and you cannot answer it from a nameplate. You answer it from instrumentation at the distribution board, recorded continuously, and exportable to the event’s sustainability report.
The efficiency case is well documented. Powerful Thinking’s analysis of festival power found that generators are routinely oversized, because need is calculated from peak values rather than actual running values. A generator running below about 30 percent of its capacity starts to waste fuel per kWh, and below 10 percent it can waste 60 percent or more. The most efficient band is roughly 40 to 80 percent load. If you cannot see live load against capacity for each generator, you cannot keep your machines in that band, and you cannot prove your CO2 figures afterwards.
If you cannot show per-generator load against capacity from a logged file, your sustainability reporting becomes the venue’s estimate rather than your measured data.
What does BS 7909 require for the distribution baseline?
BS 7909 (Temporary electrical systems for entertainment and related purposes, Code of practice) is the recognised benchmark for temporary electrical systems at UK events. It is a code of practice, not law. The legal duty sits in the Electricity at Work Regulations 1989, and BS 7909 is the accepted way to meet it. Every distribution chain on site should be built to BS 7909, and a competent person should sign it off before you energise. RCD and RCBO selection follows from BS 7909 together with BS 7671 Section 704 for site installations.
Two questions to ask each contractor at sign-off:
- Is the device-protection chain documented end to end?
- Is each downstream RCD test logged with date, tester, and result?
How does 110V CTE change what you can monitor?
UK practice is 110V centre-tapped-to-earth (CTE) supply for portable tools, separated from the 230V or 400V generation supply by a transformer. CTE keeps the maximum voltage to earth at around 55V, which is what protects the operator at the tool. This is HSE-recommended practice, the standard way to meet the Electricity at Work Regulations 1989 for portable tools, rather than an absolute legal line in itself.
It also sets a limit on monitoring. Distribution monitoring at the board sits on the supply side of the transformer. It does not see what happens at the tool on the CTE side. The fault you catch at the board is a supply-side fault. The fault at the tool is still the domain of the manual test routine and the competent person’s inspection. If a vendor claims their platform sees CTE tool circuits directly, ask them to demonstrate it on a live tool. Most platforms do not, and saying so plainly is part of honest sizing.
What does HSG141 actually expect on a managed site?
HSG141 (Electrical safety on construction sites, HSE guidance, second edition April 2023) is guidance, not law, and it sits on top of the Electricity at Work Regulations 1989. What it sets out is a regime of reduced low voltage, RCD protection, and scheduled inspection and testing by competent people. It does not, on its own, call for instrumented continuous logging or alarms.
So treat continuous voltage and current logging, overvoltage and undervoltage alarms, and a retained fault history as good practice for a well-managed site, not as a box HSG141 ticks for you. The value is real. A logbook from a competent person is the baseline, and instrumented logging is what lets you demonstrate the duty of care quickly when an auditor or an incident asks for it.
How do you stop cables becoming the failure mode?
This is the unglamorous point that bites every event. Pre-walk the entire cable run before the public arrives. Mark ramps, fix tape, and raise overhead crossings to the documented minimum height for vehicle movements. The failure here is rarely electrical. It is usually someone tripping on a cable that ran in the rain, so treat cable management as a safety task with the same discipline as the distribution design.
What sustainability data will the event ask you for?
CO2 per generator, per device, and per circuit is increasingly expected on AGF-certified events and ISO 20121-compliant productions. A Greener Future’s festival certification assesses Energy and Power as one of its core categories, and reporting against CSRD or the EU Taxonomy raises the bar again.
This is where MID-grade metering matters. MID, the Measuring Instruments Directive 2014/32/EU, defines the accuracy classes that make energy data billable and auditable. A spreadsheet from a generator hire firm is an estimate an auditor can challenge. MID-grade kWh data is not. On the Spine platform, MID-approved metering is available as an accessory for exactly this reason.
What should the pre-energisation walk-around cover?
The morning of the build, the questions that surface every problem in advance:
- Where is the single logbook for this event’s electrical system?
- Who is the on-call competent person from 22:00 to 06:00?
- What is the fallback plan if generator 2 trips at 19:00 on Saturday?
- Are all distribution boards labelled with their load capacity, in the same units the audit asks for?
If any of those four does not have an immediate answer, sort it before energisation, not after.
The board tells you what the supply is doing. The walk-around tells you what the people and the cables are doing. You need both before the gates open.TSR-Elsite
Summary
A temporary event power supply comes down to measured load per generator, a BS 7909 distribution chain signed off before energisation, 110V CTE for tools, and energy data you can export. The regulations and codes (HSG141, BS 7909, BS 7671 Section 704) are the recognised ways to meet the Electricity at Work Regulations 1989, not separate legal hurdles. If you want a monitoring layer that logs per-generator load against capacity, stores the fault history for the project lifecycle, and exports MID-grade kWh data, see Spine for events or get in touch: sales@tsr-elsite.fi or +358 9 4555 588.
Frequently asked questions
Does HSG141 require continuous electrical monitoring at events?
No. HSG141 is HSE guidance for construction sites and sets out reduced low voltage, RCD protection and scheduled inspection and testing. Continuous logging and alarms are good practice that help you demonstrate the Electricity at Work Regulations 1989 duty of care, not a specific HSG141 requirement. Events are governed in practice by BS 7909.
What is the difference between BS 7909 and BS 7671 at an event?
BS 7909 is the code of practice for temporary electrical systems for entertainment. BS 7671 Section 704 covers construction and demolition site installations and informs RCD selection. On an event build you typically work to BS 7909 for the temporary system and lean on BS 7671 for device protection. Both are standards, not law.
Why use 110V CTE for tools instead of 230V?
Centre-tapped-to-earth keeps the maximum voltage to earth at around 55V, which roughly halves shock severity at the tool. It is the recognised way to meet the Electricity at Work Regulations 1989 for portable tools on UK sites. Exception: some fixed or specialist equipment runs at 230V or 400V with its own protection.
Can a monitoring platform see my 110V CTE tool circuits?
Usually not. Board-level monitoring sits on the supply side of the transformer, so it sees supply-side faults, not what happens at the tool. Tool-side faults stay with the manual test routine and competent-person inspection. Be sceptical of any vendor who claims otherwise without a live demonstration.
What energy data do AGF and ISO 20121 events expect?
Increasingly, per-generator and per-circuit energy and CO2 data rather than a single site estimate. A Greener Future assesses Energy and Power as a core category, and ISO 20121 is the event sustainability management standard. MID-grade metering makes the numbers auditable.
How much diesel can better power management actually save?
Industry analysis suggests around 40 percent of UK event diesel could be cut with no loss of power, largely by right-sizing generators and keeping them in their efficient load band. The saving comes from measurement and load management, not from buying new generators. Exception: small builds with a single correctly sized generator have less to gain.