Metro Station Lighting Case

The problem as it presented itself

The environment, not the luminaire, was the constraint. The requirement combined a high ambient temperature with a corrosive atmosphere and a continuous operating cycle — three conditions that each reduce the rating independently, and that compound when combined. Quoting the catalogue rating for this installation would have produced a unit that met its datasheet and failed in the field.

The diagnosis came from listing the rated conditions and comparing them against the actual conditions one by one: ambient temperature, exposure to cleaning agents, ingress path, and duty cycle. Each was individually survivable; together they were not. The thermal path was the binding constraint, because continuous operation prevents the case from returning to ambient.

• Ratings are conditional. Every rating figure assumes an ambient and a duty cycle; both are part of the rating.
• Check the installed configuration, not the product. The rated state assumes correct gaskets, glands and mounting.
• Continuous operation changes the thermal case. Peak ratings do not apply to a 24-hour duty cycle.

The specification was corrected by selecting on the derated case rather than the nominal one, and by confirming the surface temperature class against the zone requirement. The product was unchanged; the way it was specified was.