Cyclic, Steady-State and Composite Damp Heat Tests
Cyclic, steady-state and composite damp heat tests
Damp heat tests are part of the IEC 60068 environmental testing series and examine product behaviour when temperature and humidity act together. Steady-state, cyclic and composite tests create different conditions; they are not automatically interchangeable. Build the test plan around the general test framework and the product’s actual service conditions.
The main differences
| Test | Relevant publication | Main condition |
|---|---|---|
| Steady-state damp heat | IEC 60068-2-78:2025 / Cab | High humidity at constant temperature, without condensation on the specimen. |
| Cyclic damp heat | IEC 60068-2-30:2025 / Db | High humidity with temperature cycles, generally producing surface condensation. |
| Composite temperature/humidity cycle | IEC 60068-2-38:2021 / Z/AD | Combines hot-humid and cold conditions in a defined programme. |
Edition check: The 2025 Db edition replaced the 2005 edition; the 2025 Cab edition replaced the 2012 edition. When reading an old report, retain its actual edition reference. Do not present newer requirements as conditions applied during the earlier test.
Why should the steady-state test avoid condensation?
Cab investigates high humidity at constant temperature. Introducing a cold specimen into humid conditions incorrectly can create surface water and change the intended exposure. Preconditioning and thermal stabilization therefore matter. Distinguish operating temperature from storage temperature; surviving storage does not by itself demonstrate operation under the same conditions.
How do cyclic and composite tests differ?
Db uses high humidity and temperature changes within a 12 h + 12 h cycle. Temperature rises, holds and falls follow the applicable edition’s profile. Matching the total number of hours does not make it equivalent to a steady-state damp heat test.
Z/AD primarily assesses component-type specimens for accelerated deterioration from heat, humidity and cold. Specimens energized throughout the complete test are outside its scope. They may be energized during constant phases. This does not permit continuous powered operation through every transition.
Does the humidity percentage determine condensation?
Relative humidity must be read together with temperature. Condensation can form when a surface falls below the air’s dew point; the surrounding air does not have to be at 100% RH. A humidity sensor reading may not describe the conditions at a cold metal surface.
Site example: A cabinet moved from cold storage into a warm, humid space can develop condensation before its internal parts warm up. Measuring chamber air does not show that every specimen surface has the same temperature. Consider temperature and humidity chamber verification together with specimen placement and temperature monitoring.
How are severity and acceptance determined?
Define severity, duration and acceptance criteria before testing. Record temperature, humidity, cycle count, power state, specimen preparation, recovery and measurement times. Assessments such as function, insulation, measurement drift or visible damage are selected according to the relevant product specification.
“Still working” may be insufficient. A sensor can continue producing readings while exceeding its permitted error. Compare results with initial measurements and predefined limits. The difference between an environmental class and a test method and conversion from environmental class to test help explain this selection.
Can these tests replace other environmental tests?
Damp heat does not fulfil every purpose of cold testing, dry heat testing or change-of-temperature testing. Salt mist creates a different exposure. IEC 60529 / IP degrees of protection classify enclosure protection; an IP rating alone does not demonstrate successful completion of these three damp heat tests.