A recurring service call across Thanet: "my new LED downlights keep dying, but the electrician says the wiring is fine." Almost always this is the LED's internal thermal cutout tripping - not a wiring fault, not a driver failure, and often not even the fitting itself. Here is what's happening and the fix.
The thermal cutout mechanism
Modern GU10 and integrated LED downlights include a self-resetting thermal cutout - a small bimetallic switch or PTC element that opens the circuit inside the fitting when the LED junction temperature exceeds about 85-95°C. The LED goes dark. A minute or two later, once it cools, the cutout closes and the LED comes back on. Nine times out of ten this is diagnosed as "faulty fitting" when the fitting is doing exactly what it was designed to do.
Why Thanet lofts trigger it
Two conditions push the LED junction temperature over the cutout threshold:
- Loft insulation covering the fitting from above. Thanet had a substantial post-2013 wave of retro-fit loft insulation top-ups under ECO and follow-on schemes. Where a first-floor kitchen ceiling has downlights below and the loft above has been topped up to 270-300mm mineral wool, the fitting can no longer dump heat upward. It cooks.
- Fire-hood restricting airflow. The intumescent fire-hood required by Approved Document B compliance is by design a sealed dome above the fitting. It restores the fire rating but restricts convective cooling. Fine at 5W, marginal at 8-10W drivers.
The IC-rated fix
An IC-rated ("insulation contact rated") LED downlight is designed to run at full output with insulation directly on top of the fitting, no cutout tripping. Look for "IC" or "IC-4" on the datasheet, plus a maximum wattage figure for insulation-covered operation. Not every fitting supports this - many older stock GU10 downlights explicitly require a 100mm minimum clearance.
If your existing fittings aren't IC-rated but you cannot easily replace them, the alternative is a cowl cap - a small metal or intumescent dome fitted from the loft side over the fitting, keeping insulation off it while preserving the fire rating. Cheaper than replacing the fitting.
Fire-rating stays mandatory
Under Approved Document B, any ceiling below a bedroom or a landing that forms an escape route must retain a 30-minute (typical domestic) or 60-minute fire rating. Cutting a hole for a downlight breaches that rating unless the fitting is fire-rated to the same period, or a compliant fire-hood is fitted above. Do not remove fire-hoods to fix a thermal cutout problem - install IC-rated fittings inside a compliant hood instead.
Quick self-check
If your LED downlights cycle off-on every few minutes, especially in a room that was recently insulated above, it's thermal cutout - not driver failure. Turn the room's lights off for an hour and try again; if they come on and stay on cool, you've confirmed it.