The pattern of dark lights tells you where to look.
Learning Track · Module 6: Faults · Step 39 of 101
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| What you see | Likely cause | Send |
|---|---|---|
| Many lights in a row go dark together | Cable cut, joint failure, or blown fuse upstream | Cable crew with fault locator |
| Every third light is dark | One phase lost | Electrician to the cabinet |
| Full circuit dark | Breaker or earth leakage trip, contactor, controller, or supply | Electrician to the cabinet |
| One light dark, neighbours normal | Driver, LED module, node, photocell, or pole fuse | Lamp crew with spares |
| One light cycles on and off | Failing photocell, loose connection, or failing driver | Lamp crew |
USA: For utility lights on overhead lines, a row of dark lights usually means a problem on the secondary line or the transformer. Report it to the utility.
If current on one phase drops by a large step, the fault is on the cable. If it drops by one lamp’s current, the fault is at one pole.
Sources
- IEC 60364-6 Verification of low-voltage installations
- IEC 61643-11 Surge protective devices
- Maker installation and maintenance instructions
Changes: 30 October 2026: First publication: guides for Europe, the Middle East, South Asia, and the USA; failure library; tender clause library; comparisons; tools; downloads. See the changelog. Found a mistake? Report it.