QualifiedMedium risk

High-bay or floodlight not working / cutting out

A high-bay (warehouse) or floodlight is out or cuts out after warming up — pointing at the driver, heat, supply, or a daylight/photocell control.

Start fault-finding

Safety first

Often at height and on three-phase distribution — safe access and isolation essential. Drivers/fittings get hot.

Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.

Full detail — causes, the why, and common mistakes.

Likely causes

Ranked from most to least likely.

1
Driver failed or thermally cutting out
Most likely
A failed or overheating LED/HID driver cuts the light out, sometimes recovering when cool.
2
Photocell / daylight control
#2
A photocell holding the light off in daylight (or faulty) explains 'not working'.
3
Supply / circuit fault
#3
Lost supply or a tripped circuit for that lighting run.
4
HID lamp/ignitor end of life (older fittings)
Least likely
Older metal-halide/HID lamps cycle or fail to strike near end of life.

Reports are saved on this device to reflect what you actually find.

Testing sequence

Work through one test at a time. Expected reading and what each result means.

Test 1 of 2

Check for a photocell/daylight control and whether it's holding the light off; confirm supply.

Expected reading

No photocell hold; supply present.

Your actual reading

If it passes

Powered and not held off — check the driver/lamp.

If it fails

Photocell hold or no supply — address that.

Often leads toDownlights cutting out then coming back All lights in one area out (others fine)

View all expected readings at once

1. Check for a photocell/daylight control and whether it's holding the light off; confirm supply.

No photocell hold; supply present.

2. Observe the cut-out pattern (thermal) and check/replace the driver (or HID lamp/ignitor on older fittings).

Steady operation with a healthy driver/lamp.

Fault-finding flowchart

The same logic as a decision tree.

1
start
High-bay/flood not working
→ step 2
2
decision
Is supply present and no photocell holding it off?
Yes→ step 3No→ step 4
3
decision
Is the driver/lamp healthy (no thermal cut-out)?
Yes→ step 5No→ step 6
4
result
Photocell hold or no supply — address that.
5
result
Operating normally — done.
6
result
Failed/overheating driver or end-of-life lamp — replace.

Common mistakes apprentices make

Not checking the photocell is holding it off in daylight.
Ignoring a thermal cut-out pattern.
Working at height without safe access.
Assuming the lamp on an LED fitting (it's the driver).

When to stop & escalate

Height access, three-phase lighting circuits, and fitting replacement are licensed work with safe-access planning.

If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.

Related faults

Lighting

Downlights cutting out then coming back

Downlights switch off after a while then come back once cooled — classic thermal cut-out behaviour, usually from heat build-up around the fitting or driver.

4 causes3 test steps

QualifiedMedium risk

Lighting

All lights in one area out (others fine)

Every light in one part of the house is dead while power points and other lighting still work — points at that lighting circuit's protective device or a shared fault.

4 causes3 test steps

ApprenticeMedium risk

Lighting