The bathtub curve — a model that actually fits
Reliability engineers have used the bathtub curve since the mid-twentieth century to describe the failure rate of a product over its operating life. The name comes from the shape: failure rate starts high (the left rim of the bath), falls to a low and relatively stable plateau (the flat base), then rises again towards end of life (the right rim).
For street luminaires the model maps onto three distinct phases, each with different causes, different implications, and different countermeasures:
Infant mortality
Manufacturing defects, transport damage, incorrect installation. Failures concentrated within the first 6–18 months. The root cause is almost always a latent defect that operating conditions brought to the surface.
Useful life
A low, roughly constant failure rate due to random events: impact damage, electrical surges, extreme weather, or simply statistical component failures. This is the phase MTBF (Mean Time Between Failures) describes.
Wear-out
Failure rate rises as components reach their chemical and mechanical limits: electrolytic capacitors dry out, gaskets lose their compression set, LED lumen output falls below threshold. This phase follows a deterministic timescale, not random chance.
In practice
- MTBF describes Phase 2 only. It says nothing about how long the luminaire lasts before wear-out begins — a common misuse of the figure in tender documents.
- Phase 1 failures are largely preventable through incoming inspection, controlled installation procedures, and a commissioning checklist that checks IP integrity on-site before deployment.
- Phase 3 is predictable, not random. Knowing the rated temperature of driver capacitors and the ambient conditions makes it possible to model the onset of wear-out with reasonable accuracy.
Root causes — what actually starts the process
Across the three phases the majority of luminaire failures in the field trace back to a small set of root causes:
- Moisture ingress: the single most common root cause of premature failure in LED street luminaires. Once moisture reaches the PCB or driver, it enables the oxidation and short-circuit pathways that can cascade to total failure. IP ratings are measured on new products in laboratory conditions — they say nothing about how long the seal remains effective.
- Thermal stress: luminaires experience large diurnal temperature swings — warmer during the day, cooler at night — and seasonal swings on top of that. Each cycle is a mechanical loading event for gaskets, circuit boards, solder joints, and plastics. Over several years the cumulative fatigue becomes a failure driver.
- Corrosion: in metallic housings, electrochemical corrosion operates continuously in the presence of moisture and electrolytes. Where the housing has begun to corrode it typically has also ceased to be watertight — so corrosion and moisture ingress become mutually reinforcing failure modes.
- Driver component ageing: electrolytic capacitors — the highest-failure-risk component in an LED driver — age through a chemical process that is accelerated by temperature above the rated operating point. A capacitor running consistently 10°C above its rated temperature has roughly half the service life of one running at the rated temperature.
- LED lumen depreciation: LEDs do not typically fail suddenly; they dim gradually. When output falls below the maintained illuminance level specified in EN 13201, the installation has effectively failed its specification even if the luminaire is still lit.
How failure chains develop
What makes field diagnosis difficult is that most failures do not have a single cause — they have a chain. An early breach in the housing seal (perhaps from a gasket that has compressed over time) allows humid air into the enclosure. Thermal cycling condenses that moisture onto the coldest surfaces — typically the driver PCB. Condensation initiates corrosion of PCB traces and begins to degrade solder joints. Eventually either a solder joint cracks or a short circuit develops, and the luminaire goes dark. The logged failure is "driver failure." The root cause was a gasket specification that was inadequate for the operating temperature range.
EPDM compression set after 6–8 years of thermal cycling
Humid outdoor air enters the enclosure through a compromised seal
Dew point reached on the coldest surface in the enclosure
Trace oxidation, electrolytic migration, weakening of solder joints
The three sub-topics that explain failure in depth
The individual failure mechanisms — lumen depreciation, moisture ingress, and driver ageing — each have their own logic and their own timescales. Three articles in this knowledge base go into each in detail.
Service Life · 03
100,000 hours — what it actually means
LM-80 test duration, TM-21 projection limits, and why most long-hour claims cannot be independently verified.
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Service Life · 06
Moisture, condensation and failure modes
How humid air enters an enclosure, when it condenses, and what that means for the electronics inside.
Read →
Service Life · 08
Why driver capacitors determine service life
The Arrhenius principle, electrolytic capacitor ageing, and why driver temperature management matters more than headline MTBF.
Read →
Procurement implications
Understanding the bathtub curve and the failure chains behind it changes what you ask for in a specification. A luminaire rated at a high MTBF tells you about Phase 2 (random failures), but says nothing about Phase 3 (wear-out). The figures that matter for total cost of ownership are the ones that characterise wear-out: the L70 threshold hours for the LED module, the rated temperature and expected service life of the driver's electrolytic capacitors, and the material and rated lifespan of the housing seals.
A specification that captures those data points — alongside IP rating, corrosivity class, and IK rating — gives a considerably more complete picture of long-term performance than watt-per-lumen and purchase price alone.
Summary
Luminaire failure follows a pattern that can be modelled and, to a significant extent, planned for. Infant mortality failures are preventable through quality control and careful installation. Random Phase 2 failures are low-frequency and statistically manageable. Wear-out — Phase 3 — is deterministic and driven by the long-term behaviour of specific components: capacitors, gaskets, and LED modules. Identifying which components set the pace of wear-out, and asking for specification data for each, is the most productive place to direct procurement attention.