The purpose of this part of IEC 61300 is to describe a test to determine the suitability of a fibre optic device to withstand the environmental condition of high humidity and change of temperature which can occur in actual use@ storage and/or transport. The test is primarily intended to determine the suitability of fibre optic components under conditions of high humidity ?C combined with cyclic temperature changes and@ in general@ producing condensation on the surface of the device under test (DUT). Absorption of moisture can result in swelling that would destroy functional utility@ cause loss of physical strength@ and cause changes in other important mechanical properties. Degradation of optical properties can also occur. Although not necessarily intended as a simulated tropical test@ this test can@ nevertheless@ be useful in determining moisture absorption of insulating or covering materials.