MTBF, lumen maintenance and system service life describe different things. A reliability figure needs a model, temperature, load and definition of failure. Assess the complete luminaire under its intended operating conditions.
The luminaire as a system
LED module, driver, optics, gaskets, connections and housing contribute to operation. When every critical part must work, the luminaire can be modelled as a series system. Failure of one part can then disable the system; no individual component curve describes the complete luminaire.
L70 means 70 percent of initial luminous flux for the object covered by the declaration. LED component data should not automatically be presented as life data for the complete luminaire. Gradual lumen depreciation and abrupt failures need separate treatment. LightingEurope.
The bathtub curve — a simplified model
The bathtub curve illustrates decreasing, approximately constant and increasing failure rates. It is a model, not a measured lifetime curve for VALDUR. The duration and shape of each phase must be assessed for the actual design. NIST.
What MTBF actually means
MTBF describes mean time between failures in repairable systems. MTTF describes mean time to failure for items that are not repaired. A luminaire with replaceable parts therefore requires a clear system boundary and maintenance strategy. NIST.
For an exponential model with a constant failure rate and mean time of 100,000 hours, the probability of a first failure within 1,000 hours is 1 − exp(−1,000/100,000), approximately 1 percent. This calculation assumes that model; it is not a VALDUR prediction or warranty. NIST.
A constant-failure-rate calculation alone cannot predict when wear-out will increase. Request its assumptions and evidence on ageing instead of reading a large MTBF number as a promised service life.
Failure mechanisms to assess
The following are assessment areas, with no ranking. The limiting mechanism depends on the design and operating environment; a general list provides no failure-frequency statistics.
- Moisture ingress: sealing surfaces, assembly, cable entries and ageing.
- Driver: component selection, measured temperature, loading and supply disturbances.
- Solder joints: temperature range, cycle count, time profile and joint design.
- Gaskets: compound, compression, elastic recovery and exposure.
- LEDs and optics: lumen depreciation, dirt and material changes.
- Surges: disturbance levels, protection and the electrical installation.
Sealing and ageing
EPDM generally has good resistance to ozone and weathering. Sealing performance still depends on compound, temperature, compression and design; the material name alone provides no guaranteed lifetime. Parker Hannifin.
An IP test describes the tested conditions. It does not automatically state sealing performance after a given number of years. Assess ageing and service separately. Any pressure-equalisation solution also needs verification in the complete luminaire.
Questions for a service-life assessment
- Which operating conditions and failure criteria apply to the life declaration?
- What driver evidence applies at the temperature measured inside the luminaire?
- Which thermal cycles, moisture conditions and maintenance actions are included?
- Which parts can be replaced, and which warranty terms cover the supply?
VALDUR in the project
POLAB declares L90/B10 for VALDUR. Read this together with the product documentation and warranty terms. Specific gasket compounds, membrane construction or capacitor B10 data require evidence for the actual design; they cannot be inferred from a general MTBF figure.
Frequently asked questions
Can MTBF values be compared directly?
Only if system boundaries, failure definitions, methods and operating conditions are comparable. Ask for the assumptions behind the figures.
Is B10 the same as L90/B10?
No. B10 in component failure analysis refers to a defined failure fraction. B10 in LxBy describes the distribution of gradual lumen depreciation among operating items. Always state the context.
Does a high MTBF mean all parts last equally long?
No. A system can experience few failures during a period and still require maintenance as a component ages.
Does L70 apply only to an LED module?
No. A declaration can refer to different objects. Check whether the data cover an LED component, module or complete luminaire and identify the temperatures and methods used.
Summary
Assess the luminaire using complementary evidence: reliability, lumen depreciation, ageing, environment and service. POLAB product declarations and warranties are separate from this general explanation of reliability concepts.