The purpose of this part of IEC 61300 is to determine the resistance of a fibre optic device to the deteriorative effects of high temperature, humidity and cold conditions. It is intended to reveal defects in a device under test (DUT) caused by breathing as opposed to absorption of moisture. The test covers the effect of the freezing of trapped water in cracks and fissures as well as condensation. However, the degree of condensation will vary depending on the size and thermal mass of the DUT. This test differs from other cyclic damp heat tests in that it derives its increased severity from: a greater number of temperature variations leading to pumping actions in a given time; a greater cyclic temperature range; a higher rate of change of temperature; the inclusion of a number of excursions to sub-zero temperature. This type of test is particularly important for fibre optic devices made of a variety of different materials.