IEC 60068-2-38 pdf download – Environmental testing Part 2-38: Tests Test Z/AD: Composite temperature/humidity cyclic test

IEC 60068-2-38 pdf download – Environmental testing Part 2-38: Tests Test Z/AD: Composite temperature/humidity cyclic test

IEC 60068-2-38 pdf download – Environmental testing – Part 2-38: Tests – Test Z/AD: Composite temperature/humidity cyclic test
1 scope
IEC 60068-2-38 provides a composite test procedure,primarily intended for component typespecimens, to determine,in an accelerated manner,the resistance of specimens to thedeteriorative effects of high temperature/humidity and cold conditions.
2Normative references
The following referenced documents are indispensable for the application of this document.For dated references,only the edition cited applies. For undated references, the latest editionof the referenced document (including any amendments) applies.
IEC 60068-1,Environmental testing – Part 1: General and guidance
IEC 60068-2-30,Environmental testing – Part 2-30:Tests – Test Db: Damp heat, cyclic(12 h+12 h cycle)
lEC 60068-2-78,Environmental testing – Part 2-78: Tests -Test Cab:Damp heat,steadystate
IEC Guide 104,The preparation of safety publications and the use of basic safety publicationsand group safety publications
3 General
3.1 Description of the test
Test ZIAD is a cyclic temperature/humidity test which is designed to reveal defects in testspecimens caused by “breathing” as distinct from the absorption of moisture.
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 “pumping” 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 andfissures are the essential features of this composite test.
lt is emphasized, however, that the freezing effect will occur only if the fissure dimensions arelarge enough to allow the penetration of a coherent mass of water as is normally the case infissures 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 surfaceof the test specimens and may be negligible for very small specimens but copious for largespecimens.
Similarly,the breathing effect will be more apparent on specimens which contain relativelylarge air-filled or gas-filled voids, but again, the severity of the test will depend to some extenton the thermal characteristics of the specimens.
3.2Application of the test
For the reasons given above,it is recommended that this test procedure be limited tocomponent type specimens when the construction of the specimens suggests a “breathing”type of damp heat test combined with icing and where the thermal characteristics arecompatible with the rates of change of temperature,etc. of test Z/AD.
For solid type specimens,e.g. plastic encapsulated, where there may be small hairline cracksor porous material,the absorption or diffusion mechanisms will predominate and a steadydamp heat such as test C of lEC 60068-2-78 is preferred for investigating these effects.
For larger specimens such as equipment or when it is essential for components to ensurethermal stability during the various phases of the cycle,test Db of IEC 60068-2-30 should beemployed,although due to the reduced number of cycles in a given period, the degree ofacceleration may not be as fast. In this case, test Db should normally form part of a sequencesuch as that defined in lEC 60068-1.
As in other damp heat tests, a polarizing voltage or electrical loading may be applied to thespecimens. In the case of electrical loading, the loading should be such that the temperaturerise 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 analternative to,either steady-state or other cyclic damp heat tests,but the choice of testprocedure should be made with due regard for the physical and thermal characteristics of thetest specimens and the types of failure mechanisms which are significant for each particularcase.
4Description of test chamber
The exposure to moisture, followed by cold, can either be performed in one chamber or in twoseparate chambers.
4.1Chamber for exposure to moisture
The chamber for the exposure to moisture shall be so constructed that:
a) The temperature can be varied between 25 “C ± 2 K and 65°C ± 2 K in a period ofbetween’1,5 h and 2,5 h for both rising and falling temperatures.
b) The relative humidity can be maintained at (93 ± 3) % during the periods of constant or
rising temperature and between 80 % RH and 96 % RH during the falling temperatureperiods.
c) The conditions prevailing at any point in the working space are uniform and are as similar
as possible to those prevailing in the immediate vicinity of suitably located temperature-and humidity-sensing devices.
The air in the chamber shall therefore be continuously stirred at a rate necessary tomaintain the specified conditions of temperature and humidity.
d) The specimens under test shall not be subjected to radiant heat from the chamberconditioning processes.
e) Water used for the maintenance of chamber humidity shall have a resistivity of not less
than 500 L2m.