A damp heat test is an environmental reliability test that holds a specimen at elevated temperature and high relative humidity — either steady-state or cyclic — to accelerate the moisture-driven failure modes (insulation breakdown, corrosion, delamination, mold, polymer drift) that would otherwise take years to appear in service. The two governing methods are IEC 60068-2-78 (Test Cab — damp heat, steady state, IEC webstore) and IEC 60068-2-30 (Test Db — damp heat, cyclic, 12 + 12 h cycle, IEC webstore), adopted in China as GB/T 2423.3 and GB/T 2423.4 respectively. They are not interchangeable: Cab drives moisture by diffusion/absorption without condensation; Db drives moisture by condensation and the "breathing" of enclosures under a 24-hour temperature cycle. Choosing the wrong method manufactures a result that does not match the field failure it is supposed to predict.
What Is the Difference Between Test Cab and Test Db?
The single most consequential decision in damp heat testing is which failure mechanism you are trying to accelerate, because that decides Cab vs Db — and the SERP routinely conflates them.
- Test Cab — steady-state damp heat (IEC 60068-2-78). The specimen sits at a fixed temperature and relative humidity for the whole test. There is no condensation on the specimen — the chamber is held just below the dew point. Moisture enters the specimen by diffusion through polymers and along interfaces (absorption). Cab answers: will this material/package drift, swell, lose insulation resistance, or grow dendrites under sustained humid exposure? It is the test for potting compounds, conformal coatings, encapsulants, insulation systems, and any product whose life is governed by slow moisture uptake.
- Test Db — cyclic damp heat (IEC 60068-2-30). The specimen is run through a repeating 24-hour cycle of temperature and humidity. During the temperature fall the chamber crosses the dew point, so liquid water condenses on the specimen, and any sealed enclosure "breathes" humid air in and out as pressure changes with temperature. Db answers: will this product survive condensation, breathing, and cyclic moisture pumping? It is the test for outdoor enclosures, connectors, cable assemblies, and anything with a sealed volume.
The practical rule: if your field failure is bulk moisture absorption, you need Cab; if your field failure is surface condensation or enclosure breathing, you need Db. A report that says "tested to IEC 60068 damp heat" without naming Cab or Db, the severity, and the duration is unverifiable. Damp heat is one of the climatic tests in the IEC 60068 series that sits alongside high-temperature testing and IP water-resistance testing — temperature tests drive thermal aging without moisture, IP tests drive direct water jets without temperature cycling, and only damp heat drives the humidity + temperature combination that accelerates insulation and corrosion failures.
What Conditions and Durations Apply?
The two methods define standardized severity levels, not arbitrary numbers — selecting outside them makes the test non-comparable across labs and products.
Test Cab (IEC 60068-2-78 / GB/T 2423.3) — steady-state severity levels:
| Condition | Temperature | Relative humidity |
|---|---|---|
| Cab 40/85 | 40 °C ± 2 K | 85 % ± 3 % RH |
| Cab 40/93 | 40 °C ± 2 K | 93 % ± 3 % RH |
| Cab 55/93 | 55 °C ± 2 K | 93 % ± 3 % RH |
| Cab 85/85 ("85/85") | 85 °C | 85 % RH |
Preferred durations: 4 days, 10 days, 21 days, 56 days (1344 h). The combination 85 °C / 85 % RH for 1000 h is the workhorse severity for semiconductors and PV modules (see below); the milder 40 °C / 93 % RH for 4 days is a common telecommunications severity (referenced from ETSI EN 300 019-2-2).
Test Db (IEC 60068-2-30 / GB/T 2423.4) — cyclic, 24-h cycle (12 h + 12 h):
| Variant | Upper temp | Lower temp | RH |
|---|---|---|---|
| Variant 1 | 55 °C ± 2 K | 25 °C ± 3 K | up to 95 % RH |
| Variant 2 | 40 °C ± 2 K | 25 °C ± 3 K | up to 95 % RH |
A single 24-h Db cycle: temperature rise from 25 °C to 55 °C over ~3 h at ≥ 95 % RH, hold at 55 °C / 90–95 % RH for ~9 h, fall from 55 °C to 25 °C over 3–6 h (condensation occurs here), hold at 25 °C / ≥ 95 % RH to complete 24 h. The cycle count is 2 or 6 cycles for most product specs, extended for harsher applications.
The fact the SERP obscures: damp heat severity is not "hotter and wetter is tougher." Cab 85/85 stresses polymer diffusion hard but produces no condensation; Db 55 °C produces condensation and breathing that 85/85 never will. They attack different physics — a product can pass 85/85 and fail Db, or vice versa. This is also why damp heat is paired with photoaging test when polymer drift is the concern (humidity plus UV, not heat alone), and why the post-exposure electrical findings often need follow-up electronic component failure analysis to locate whether the moisture-driven degradation is in the bulk dielectric, an interface, or a metallization.
How Is the Damp Heat Test Run and What Is Checked After?
The chamber must meet the IEC 60068-2-78 / -2-30 construction and control requirements: uniformity of temperature and humidity within the working space, recovery time after door opening, and no direct spray of water onto the specimen (the humidity is delivered as vapor). The specimen is energized or loaded only if the relevant specification requires it; most often it is held unpowered and inspected electrically afterward.
Conditioning and recovery matter as much as the exposure itself: after removal the specimen is allowed to recover under standard atmospheric conditions for a defined period before measurement, so that transient surface moisture does not corrupt the electrical readings.
Typical pass/fail checks after damp heat:
- Insulation resistance and dielectric withstand (the headline electrical checks — moisture is the dominant killer of insulation resistance).
- Functional / performance test — does the device still meet spec?
- Visual inspection — corrosion, blistering, delamination of coatings, migration of metals (dendrites), mold growth, swelling of polymers, fogging of optical surfaces.
- Mechanical — embrittlement or loss of adhesion in adhesives, potting, and laminates.
The acceptance threshold is product-specific (set in the relevant product standard), but the measurement method after exposure is standardized — which is why a damp-heat report must cite the severity, the duration, and the post-test pass criteria.
What About PV Modules — IEC 61215 and the 1000 h 85/85 Test?
The most demanding damp heat application is photovoltaic module qualification. IEC 61215-2 (IEC 61215-1-1 for crystalline silicon, published by IEC) mandates a damp heat test at 85 °C / 85 % RH for 1000 hours — the so-called "85/85 1000 h" — as one of the sequence tests for design qualification. The module must not show more than the allowed maximum power degradation (typically ≤ 5 %) after the exposure, with no visible defects (delamination, cell corrosion, bubbles, EVA browning).
The reason 85/85 for 1000 h is so aggressive: it is an accelerated aging test, not a storage test. The Arrhenius relationship maps the 1000 h at 85/85 to a target service lifetime (commonly 25–30 years), but the mapping depends on the activation energy of the dominant degradation reaction in the module materials. This is where the SERP content stops — but the engineering reality is that the same 1000 h corresponds to very different real-world years depending on the module's activation energy and the climate it is deployed in. Lower activation energy materials degrade faster, so the same chamber hours translate to fewer field years in hot-humid climates (Florida, Singapore) than in temperate ones.
How Do You Choose Between Cab, Db, and the PV 85/85 — and What Fails Each?
The decision tree is driven by which failure mechanism governs the product's field life, not by which standard is more familiar:
| Field failure mode | Governing mechanism | Test to use |
|---|---|---|
| Insulation resistance drop, dielectric failure over years | Bulk moisture absorption/diffusion into polymers, potting, laminates | Cab (steady-state) |
| Conformal-coating breakdown, dendritic growth on PCBs | Slow ionic migration under sustained humidity | Cab |
| Connector corrosion, enclosure sweating, breathing ingress | Condensation and enclosure pressure breathing | Db (cyclic) |
| Cable jacket cracking at interfaces, water tracing | Cyclic wet/dry fatigue | Db |
| PV module 25–30 year lifetime qualification | Accelerated polymer/hot-melt degradation | IEC 61215 85/85 1000 h (a Cab-derivative) |
| Combined humidity + corrosive gas or salt | Synergistic corrosion | GB/T 2423.50 (damp heat + salt mist) or IEC 60068-2-52 salt mist |
A common mistake visible in the SERP: specifying "damp heat" generically on a datasheet. A product that must survive tropical outdoor cycling needs Db; a sealed electronic module whose life is set by potting-compound absorption needs Cab. They are not substitutes.
Frequently Asked Questions
What standard governs damp heat testing?
Two methods cover almost all industrial damp heat testing: IEC 60068-2-78 (Test Cab, steady state) and IEC 60068-2-30 (Test Db, cyclic), adopted in China as GB/T 2423.3 and GB/T 2423.4. For photovoltaic modules the accelerated 85 °C / 85 % RH / 1000 h damp heat test is mandated by IEC 61215-2.
What is the difference between Test Cab and Test Db?
Cab holds the specimen at a fixed temperature and humidity with no condensation — moisture enters by diffusion/absorption, modeling long-term insulation drift. Db runs a 24-hour cyclic profile (25 ↔ 55 °C) so that condensation forms and sealed enclosures breathe, modeling condensation and moisture pumping. Choose Cab for absorption-driven failures, Db for condensation-driven failures.
What are the standard damp heat severities?
Cab severities: 40 °C / 85 % RH, 40 °C / 93 % RH, 55 °C / 93 % RH, and 85 °C / 85 % RH, with preferred durations of 4, 10, 21, or 56 days. Db variants use an upper temperature of 40 °C or 55 °C with a 25 °C lower temperature, at up to 95 % RH, for 2 or 6 cycles (24 h each).
Why is 85 °C / 85 % RH / 1000 h used for PV modules?
It is the accelerated aging test mandated by IEC 61215-2 for PV design qualification. The 1000 chamber-hours are mapped to a 25–30 year service lifetime via the Arrhenius relationship, but the mapping depends on the activation energy of the module's dominant degradation reaction — so the same 1000 h represents different field years for different materials and climates.
What is checked after a damp heat test?
The headline checks are insulation resistance and dielectric withstand (moisture is the dominant cause of insulation failure), followed by functional/performance test, visual inspection (corrosion, delamination, mold, dendrites, polymer swelling), and mechanical checks on adhesives, potting, and laminates. Acceptance thresholds are set by the relevant product specification.
Can a product pass 85/85 and fail Db — or vice versa?
Yes. 85/85 stresses diffusion-driven absorption but produces no condensation; Db at 55 °C produces condensation and breathing that 85/85 never does. They attack different physics, so a product can pass one and fail the other. That is exactly why the test method must be chosen to match the field failure mechanism.
Our Testing Capabilities
Beijing ZKGX Research (ISO/IEC 17025 testing laboratory) provides damp heat testing across the steady-state, cyclic, and accelerated-aging ranges, with chambers calibrated to the IEC 60068 and GB/T 2423 control requirements:
- Steady-state damp heat (Test Cab) to IEC 60068-2-78 / GB/T 2423.3 — severities 40 °C / 85 % RH, 40 °C / 93 % RH, 55 °C / 93 % RH, 85 °C / 85 % RH; durations 4 / 10 / 21 / 56 days (and 1000 h for PV qualification).
- Cyclic damp heat (Test Db) to IEC 60068-2-30 / GB/T 2423.4 — 24-h cycles at upper temperatures of 40 °C or 55 °C, 2 or 6 cycles, with condensation and recovery verification.
- PV module damp heat to IEC 61215-2 — 85 °C / 85 % RH / 1000 h with maximum-power degradation measurement and visual defect inspection.
- Combined / synergistic — damp heat + salt mist (GB/T 2423.50), damp heat followed by cold (sequence testing) to expose crack-initiation interactions.
- Post-test evaluation — insulation resistance, dielectric withstand, functional test, visual/microscopic inspection, adhesion and coating assessment.
If you have a product to qualify against IEC 60068-2-78 or -2-30, a PV module to put through IEC 61215 damp heat, or a field-failure mechanism to reproduce, contact our testing team to scope the method (Cab vs Db), the severity, the duration, and the post-test acceptance criteria.