GB/T 2423.34-2024 Environmental testing - Part 2: Test methods - Test Z/AD: Composite temperature/humidity cyclic test
1 Scope
This document specifies a composite test procedure, primarily intended for component type specimens, to determine, in an accelerated manner, the resistance of specimens to the deteriorative effects of high temperature/humidity and cold conditions.
This test standard does not apply to specimens that are energized during the complete test. Specimens can be energized during the constant phases of the tests. Measurements on energized specimens are typically carried out during constant phases of the test unless specified otherwise.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
IEC 60068-1 Environmental testing - Part 1: General and guidance
IEC 60068-2-67 Environmental testing - Part 2-67: Tests - Test Cy: Damp heat, steady state, accelerated test primarily intended for components
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
——IEC Electropedia: available at http://www.electropedia.org/
——ISO Online browsing platform: available at http://www.iso.org/obp
4 General
4.1 Description of the test
Test Z/AD is a cyclic temperature/humidity test which is designed to reveal defects in test specimens caused by "breathing" as distinct from the absorption of moisture.
This process can be initiated by the forming of condensation on the specimen's surface. As the temperature on parts or the whole of the specimen's surface might be lower than the corresponding dew point at the humidity value, water can accumulate in small cracks or gaps on the specimen's surface.
Once the air temperature is reduced, the air in internal voids of specimen is contracted which results in a drop of pressure and drawing-in either wet air or condensed water through cracks or other leaks inside the specimen. The wet air will condense on inner walls of a void and may gradually fill it. During the temperature rising phase, the air in the void is expanded, this time with a lower dew point than during drawing-in, and partially escape out. This cycle is repeated, and water can be accumulated inside the specimen and may gradually fill its voids.
This so-called "breathing" effect is caused by changing the temperature inside the specimen in an atmosphere with high humidity. During the excursion to sub-zero temperature phase of the test, the water trapped in cracks and other voids freezes and due to the expansion of ice volume the cracks extend, and new cracks can form.
This test differs from other cyclic damp heat tests in that it derives its increased severity from:
a) a greater number of temperature variations or "breathing" actions in a given time;
b) a greater cyclic temperature range;
c) a higher cyclic rate of change of temperature;
d) the inclusion of a number of excursions to sub-zero temperatures.
The accelerated breathing and the effect of the freezing of trapped water in cracks and fissures are the essential features of this composite test.
It is emphasized, however, that the freezing effect will occur only if the fissure dimensions are large enough to allow the penetration of a coherent mass of water as is normally the case in fissures between seals and metal assemblies, or between seals and wire terminations.
The degree of condensation will depend mainly upon the thermal time constant of the surface of the test specimens and may be negligible for very small specimens but copious for large specimens.
Similarly, the breathing effect will be more apparent on specimens which contain relatively large air-filled or gas-filled voids, but again, the severity of the test will depend to some extent on the thermal characteristics of the specimens.
The condensation effect and the temperature distribution could be disturbed by functional checks, therefore the checks should be carried out during the constant phases of the temperature profiles (Figure 2 and Figure 3, areas B and F).
To simply the programming of the test sequences, the set points are given in Annex A.
4.2 Application of the test
For the reasons given above, it is recommended that this test procedure be limited to component type specimens when the construction of the specimens suggests a "breathing" type of damp heat test combined with icing and where the thermal characteristics are compatible with the rates of change of temperature, etc., of test Z/AD.
For solid type specimens, for example plastic encapsulated, where there may be small hairline cracks or porous material, the absorption or diffusion mechanisms will predominate and a steady damp heat such as test C of IEC 60068-2-78 is preferred for investigating these effects.
For larger specimens such as equipment or when it is essential for components to ensure
thermal stability during the various phases of the cycle, test Db of IEC 60068-2-30 should be employed, although due to the reduced number of cycles in a given period, the degree of acceleration may not be as fast. In this case, test Db should normally form part of a sequence such as that defined in IEC 60068-1.
As in other damp heat tests, a polarizing voltage or electrical loading may be applied to the specimens. In the case of electrical loading, the loading should be such that the temperature rise of the specimens does not unduly affect the chamber conditions.
From the above, test Z/AD should not be considered to be interchangeable with, or an alternative to, either steady-state or other cyclic damp heat tests, but the choice of test procedure should be made with due regard for the physical and thermal characteristics of the test specimens and the types of failure mechanisms which are significant for each particular case.
