IP ratings: what they measure and when
IP66 describes protection against dust and powerful water jets under defined test conditions in IEC 60529. It is not a guarantee of a particular service life or proof of vapour-tightness. The evidence needs to identify the specimen, assembly, conditioning and applicable luminaire requirements under IEC 60598-1. (IEC 60529) (IEC 60598-1)
Long-term sealing also depends on materials, assembly, maintenance and exposure. Ask for evidence covering the intended conditions and configuration.
Seal material ageing
EPDM and silicone compounds can have good weathering and ozone resistance. Sealing still depends on grade, temperature, geometry, assembly and compression. Compression set is the residual deformation after defined compression, time and temperature followed by release and recovery; it is not the same as the number of thermal cycles. (Parker)
Compression set is the residual deformation after a defined compression, time and temperature, followed by release and recovery. It is not simply the number of thermal cycles. A loss of sealing force can also involve stress relaxation, joint movement, installation or material damage; the actual seal needs assessment. (Parker)
The breathing effect
A luminaire in service is not in thermal equilibrium. Solar gain during the day heats the housing; the LED load adds further heat during operation; overnight temperatures then cause the housing to cool. As the air inside contracts during cooling, the internal pressure drops below ambient. If the seal has developed any microscopic imperfection, this pressure differential draws external air into the housing — much as a syringe draws fluid when the plunger is pulled back.
The problem is not the air itself. The problem is the water vapour it carries. Relative humidity describes how much water vapour air holds relative to its maximum capacity at a given temperature — and that maximum drops sharply as temperature falls. The temperature at which air becomes fully saturated and water vapour begins to condense into liquid is called the dew point. When a luminaire cools sufficiently overnight, the air inside often passes through its dew point, and water condenses on the coldest internal surfaces: typically the inside face of the glass, the optical elements, or the PCB.
Moisture can enter through leakage, pressure changes or diffusion, or remain after manufacture and servicing. Condensation forms when an internal surface falls below the air’s dew point. An aged seal is one possible cause, but each fault needs investigation in the context of design and exposure.
In practice
- Typical seal materials in outdoor luminaires are EPDM rubber and silicone. Both age under UV radiation, ozone, heat, and repeated compression — measurable as compression set under ASTM D395 — causing sealing contact force to reduce over time, regardless of how well sealed the luminaire was at delivery.
- IP66 describes protection against dust and powerful water jets under defined test conditions in IEC 60529. It is not a guarantee of a particular service life or proof of vapour-tightness. The evidence needs to identify the specimen, assembly, conditioning and applicable luminaire requirements under IEC 60598-1. (IEC 60529)
- The greater the temperature swing between day and night or between seasons, the more breathing cycles the luminaire undergoes — making climates with large seasonal variation, such as Scandinavia, particularly relevant for this failure mode.
How condensation causes failures
Fogging, droplets, deposits and intermittent faults can indicate moisture. Water with contamination can cause leakage currents or short circuits. A fault that disappears as the luminaire warms up is not conclusive: component and connection faults can also depend on temperature. Check alternative causes before confirming the diagnosis.
Optical surface contamination is a second consequence. Condensation on the inside face of a polycarbonate lens or toughened glass cover reduces light transmission. Mineral deposits left behind as condensate evaporates accumulate over seasons and further degrade output.
Driver component corrosion is a third pathway. Electrolytic capacitors and other driver components are not designed for exposure to liquid water. Corrosion of their terminals accelerates the ageing process described in the companion article on driver capacitors and lifespan.
Toughened glass vs polycarbonate lenses
Polycarbonate performance depends on grade, UV stabilisation, coatings and exposure. Yellowing affects optical performance, while the effect of cracks on sealing depends on their depth and location. A fixed ten-year loss of IP protection cannot be inferred from the material name; inspect the actual cover, joints and seals.
Toughened glass is generally more UV-stable than polymer covers and does not share polycarbonate’s UV-degradation mechanism. Its long-term optical and surface condition still depends on glass quality, coatings, contamination, cleaning and mechanical exposure. For installations where long-term watertightness matters — coastal roads, marine environments and industrial sites with chemical atmospheres — the cover material is a relevant part of the moisture-management specification, not merely an aesthetic choice.
Maintenance access and re-gasketing
An accessible housing can allow seal replacement where the design and manufacturer’s instructions provide for it. Use the specified parts and assembly procedure, and verify that the required ingress protection is maintained after service.
For a permanently sealed assembly, repair options depend on the manufacturer’s design and service instructions. If the specified sealing cannot be restored by an approved repair, replacement of the affected assembly may be needed. Include access, spare parts and repair policy in the lifecycle assessment.
Practical implications
An IP rating is a meaningful starting point. IP66 confirms that a new luminaire, correctly assembled, offers an appropriate baseline of protection. But it is not a 20-year guarantee. The actual long-term watertightness of a luminaire depends on seal material and geometry, lens material, the quality of pressure-equalisation provision, whether conformal coating provides secondary protection for the electronics, and whether the housing can be re-sealed in the field. (Parker)
When evaluating luminaires for long-service applications, the questions below give more useful information than the IP class alone:
- What seal material is used, and what is its stated service life or temperature rating?
- Is there a pressure-equalising membrane, or is the housing solely reliant on the seals themselves?
- Is the PCB conformally coated as a secondary moisture barrier, or is the seal the only line of defence?
- How is seal performance verified after ageing — not just the IP class of a new luminaire?
Next level of understanding
Moisture that reaches the electronics rarely stops at a short circuit on a connector — it usually strikes the most voltage-sensitive component first.
Condensation and contamination can cause corrosion of conductors and connections, leakage currents and damage across the electronics. Thermal ageing of driver capacitors is a separate mechanism that also needs consideration.
Service Life & Reliability
How driver capacitors affect service life.
Thermal ageing of electrolytic capacitors is one of the mechanisms behind why a luminaire's real service life can fall short of the data sheet figure.
Summary
A corrosion-resistant housing material addresses one problem — whether the housing itself degrades — but does not resolve this mechanism. The electronics inside are metallic regardless of housing material, and are equally affected by condensate once moisture has found a path through cable entries and seals.
IP66 describes protection against dust and powerful water jets under defined test conditions in IEC 60529. It is not a guarantee of a particular service life or proof of vapour-tightness. The evidence needs to identify the specimen, assembly, conditioning and applicable luminaire requirements under IEC 60598-1. (IEC 60529)