Why does one downlight keep failing when the others are fine?

Downlights in a row see the same supply, the same switch and usually the same lamps, so when one behaves differently the cause is almost always physical and local. That makes it one of the more satisfying faults to find, because there are only a handful of possibilities and most are visible from above.

Safety

Step by step

1. Confirm it really is one fitting.Write down which position fails each time. Two failures in the same hole is a pattern; two failures in different holes is a lamp quality question instead.
2. Look from the loft if the ceiling has a void above.Isolate the lighting circuit first. You are looking for insulation packed against the fitting, a fire hood that has slumped onto it, a joist or a duct immediately alongside, or a fitting sitting in a pocket with no air.
3. Check the insulation clearance.Fittings that are not marked insulation-coverable need a clear gap around and above them. Loft insulation laid over the top is both a fire risk and the commonest cause of a single downlight cooking its lamps. A proper loft cap or an IC-rated fitting is the answer.
4. Check the fitting's own heat path.GU10 LED lamps rely on their heatsink shedding heat into the air behind the ceiling. A fitting with a tight bezel, a deep can, or a lamp pushed hard into a recessed housing runs far hotter than its neighbours.
5. Inspect the lamp holder.With the circuit isolated, look at the GU10 holder contacts. Blackened, pitted or discoloured contacts mean the connection has been arcing, and that generates local heat that kills lamps. A discoloured holder is replaced, not cleaned.
6. Inspect the connector behind the fitting.Many downlights are connected with a small push-fit or screw connector in the void. A loose or badly made connection there heats up and stresses the lamp. If you find one that is discoloured or loose, isolate and get an electrician — this is a fixed wiring connection.
7. Check the fire hood if there is one.Fire hoods maintain the ceiling's fire resistance around a downlight, but they must be the correct size and must not sit hard against the fitting. A slumped or undersized hood is a common cause of one fitting running hot.
8. Escalate on any sign of heat damage.Browning of the ceiling around the bezel, a burnt smell, a discoloured holder, or plasterboard that is warm to the touch means stop. Isolate the circuit and call an electrician. Downlight fires start exactly this way.

Tips

• Use lamps of the same brand and model across a row. Different heatsink designs run at different temperatures and the difference shows up as one fitting failing first.
• If the ceiling is below a bathroom, consider water. A slow leak reaching a downlight causes failures and RCD trips and is easy to miss.
• If a whole row of halogen downlights is being replaced with LED, do it as a set with fittings rated insulation-coverable and fire-rated. The energy and safety gain is significant and it removes several of these problems at once.

Common mistakes

• Just fitting another lamp and moving on — Each replacement is a data point being thrown away, and if the cause is a poor connection, each new lamp is being asked to run through an arcing contact.
• Pulling insulation back and leaving it loose — It slumps back within months. Either a rigid loft cap or an insulation-coverable fitting is needed, not a temporary gap.
• Ignoring browning on the ceiling around the bezel — That is heat damage to the plasterboard from the fitting. It does not improve and it is a genuine fire indicator.

If it doesn't work

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