SiteThe main distribution board has been fine for years. Then one afternoon the lights in the warehouse flicker, there's a smell, and the incomer to the conveyor panel trips. When the sparky gets the cover off, one phase lug on the busbar is black. The insulation on the cable has cooked back 50mm.
It didn't happen that afternoon. That termination had been working loose and getting hotter for months. Nobody could see it, because it was behind a closed door doing its job.
That's what thermal imaging is for. Heat shows up long before the failure does. A camera lets you see it.
What a thermal survey finds
Most electrical failures in a panel give off heat first. A thermal camera picks up:
- loose or corroded terminations on busbars, isolators and contactors
- an overloaded cable or breaker
- an imbalance between phases on the same circuit
- a failing fuse or a contactor with worn contacts
- a drive or soft starter running hotter than its twin
- blocked panel fans and filters, and the heat that builds behind them
The best finds are the ones you'd never get any other way. A terminal that's tight to the eye but hot under load. One phase of three that's running warmer than the others on a motor feed.
The law behind it
You don't have to use a thermal camera. Nothing in the law asks for one. But the Electricity at Work Regulations do require that systems "shall be maintained so as to prevent, so far as is reasonably practicable, such danger" (regulation 4(2)).
HSE's memorandum on the regulations says inspection, and testing where needed, is a core part "of any preventive maintenance programme". It adds that practical experience may change how often you do it (HSR25, paragraph 68). Thermal imaging is one of the most useful inspections you can add to that programme. For the wider duties, see our guide to the Electricity at Work Regulations for maintenance managers.
The doors-open problem
Here's the catch. To see a termination clearly, you usually need the door open or the cover off. With the panel live and under load. That's the whole point, because a loose joint only gets hot when current flows through it.
So a thermal survey is live work near exposed conductors, and that brings regulation 14 into play. Work on or near live conductors is only allowed when it's unreasonable for them to be dead, reasonable to work near them live, and suitable precautions are in place (regulation 14). HSE's guidance on safe working practices adds that work near live exposed conductors "should rarely be permitted" (HSG85).
A thermal survey can pass that test, because the job can't be done dead. But it needs planning:
- a written risk assessment and method for each panel
- someone competent on that kind of switchgear doing the survey, or with the thermographer
- the right PPE for the fault level of the board
- barriers to keep other people back while covers are off
- a clear rule about what the surveyor touches, which should be nothing
Don't let someone with a camera wander round opening panels on their own. And don't let a good survey talk you into live fixing. If it finds a hot joint, the repair is done dead, with proper isolation. HSE is clear that the inconvenience of working dead, "sometimes arising from commercial and time pressures, will very rarely outweigh the risks associated with live work" (HSG85). Our guide to safe isolation covers the repair side.
Make the cover come off less
If you survey the same boards every year, think about fitting infrared windows to your main panels. The thermographer then shoots through the window with the door shut. It cuts the live exposure on every survey after that.
Windows don't suit every panel, and they only show what's in their line of sight. But on a main board you survey every year, they're worth pricing.
Getting a survey that's worth having
Thermal surveys vary a lot. A good one gives you more than a stack of colourful pictures. Ask for these things:
Survey under real load. A panel at 6am on a Monday tells you very little. Book the survey when the site is running hard, and ask the surveyor to note the load on each circuit.
Compare like with like. The most useful finding is often the difference between three phases on the same circuit, or two identical drives side by side.
Mind the shiny bits. Bare copper, tinned busbars and stainless steel reflect heat and give false readings. A good thermographer knows this and works round it.
A ranked list of actions. Each finding should come with a photo, a normal photo of the same spot, a temperature, a load and a priority. "Repair at next stop" is useful. "Monitor" with no date isn't.
Dust, heat and the rest
Panels in dusty areas need extra care. HSE's memorandum says most industrial enclosures "do not resist the entry of fine dusts", and recommends regular inspection and cleaning where dust builds up (HSR25, paragraph 98). A survey often finds the panel fan filter blocked solid. That's a five-minute job that stops a summer of drive trips. Our post on hot weather breakdowns has more on panel cooling.
HSE's general advice on testing is a good rule for how often to survey. Kit in a harsh environment "should be tested more frequently", and you should change how often you check "according to the number and severity of faults found" (HSE, electrical safety FAQs).
The survey is the easy part
Thermal surveys go wrong in the same way as every other kind of report. The PDF comes in, gets saved, and the red findings wait for a quiet week that never comes.
Every hot joint is a planned job: isolate, remake the termination, check it, survey again. That needs an electrician with time. If yours is tied up on breakdowns, the repair list gets older and the joints get hotter. Sooner or later one of them picks the busiest shift of the week to fail. Then you're paying for a burnt-out panel, a long stop and an emergency callout, for a fault you already knew about.
What to do on Monday
- List your main distribution boards and critical motor control panels, and when each was last surveyed.
- Pull the last survey report and check every finding has a repair date and a name.
- Ask whoever surveys for you for their risk assessment and method for working near live parts.
- Price infrared windows for the one or two boards you open most often.
* HSE guidance correct as of 6 October 2026. Paragraph numbers refer to HSR25, third edition (2015). HSG85 is the third edition (2013). Check the Electricity at Work Regulations on legislation.gov.uk and HSE's electrical safety pages for the latest.