IEC 60068-2-14 pdf download – Environmental testing – Part 2-14: Tests – Test N: Change of temperature

IEC 60068-2-14 pdf download – Environmental testing – Part 2-14: Tests – Test N: Change of temperature
1 scope
This part of lEC 60068 provides a test to determine the ability of components,equipment orother articles to withstand rapid changes of ambient temperature. The exposure timesadequate to accomplish this will depend upon the nature of the specimen.
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 (all parts)，Environmental testing
IEC 60068-2-1，Environmental testing – Part 2-1: Tests – Test A: ColdIEC 60068-2-2,Environmental testing – Part 2-2:Tests – Test B: Dry heatIEC 60068-2-17,Environmental testing – Part 2-17: Tests – Test Q: Sealing
lEC Guide 104,The preparation of safety publications and the use of basic safety publicationsand group safety publications
3Field conditions of changing temperature
lt is common in electronic equipment and components that changes of temperature occur.Parts inside equipment undergo slower changes of temperature than those on an externalsurface when the equipment is not switched on.
Rapid changes of temperature may be expected
– when equipment is transported from warm indoor environments into cold open air
conditions or vice versa,
when equipment is suddenly cooled by rainfall or immersion in cold water,- in externally mounted airborne equipment,
– under certain conditions of transportation and storage.
Components will undergo stresses due to changing temperature when high temperaturegradients build up in an equipment after being switched on,e.g. in the neighbourhood of highwattage resistors,radiation can cause rise of surface temperature in neighbouringcomponents while other portions are still cool.
Artificially cooled components may be subjected to rapid temperature changes when thecooling system is switched on.Rapid changes of temperature in components may also beinduced during manufacturing processes of equipment.Both the number and amplitude oftemperature changes and the time interval between them are important.
4General
4.1 Design of change of temperature tests
Tests Na,Nb and Nc comprise alternate periods at a high and at a low temperature with well-defined transfers from one temperature to the other. The conditioning run from laboratoryambient to the first conditioning temperature，then to the second conditioning temperature,then back to laboratory ambient is considered one test cycle.
4.2Test parameters
Test parameters comprise the following:- laboratory ambient;
– high temperature;- low temperature;
-duration of exposure;
– transfer time or rate of change;- number of test cycles.
The high and low temperatures are understood to be ambient temperatures which will bereached by most specimens with a certain time-lag.
only in exceptional cases may they be specified outside the normal storage or operatingtemperature range of the object under test.
The test is accelerated because the number of severe changes of temperature in a givenperiod is greater than that which will occur under field conditions.
4.3 Purpose and choice of the tests
Change of temperature testing is recommended in the following cases:
– evaluation of electrical performance during a change of temperature,Test Nb;- evaluation of mechanical performance during a change of temperature,Test Nb,
evaluation of electrical performance after a specified number of rapid changes oftemperature,Test Na or Test Nc;
evaluation of the suitability of mechanical components,and of materials and combinationsof materials to withstand rapid changes of temperature,Test Na or Test Nc;
evaluation of the suitability of construction of components to withstand artificial stressing,Test Na or Test Nc.
The change of temperature tests specified in the IEC 60068 series is not intended to evaluatethe difference in’material constants or electrical performance when operating undertemperature stability at the two extremes of temperature.
4.4Choice of the duration of the exposure
The duration of the exposure should be based on the requirements stated in 7.2.3，8.2.3 or9.2.2,or as stated in the relevant specification, keeping in mind the following points: