IEC 61300-2-48:2009 details a procedure for determining the suitability of a fibre optic device or closure to withstand variations in humidity and temperature that may occur during operation, storage and/or transport. The test is intended to indicate the performance of such devices when exposed to heat and humidity followed by short-term freezing. The main changes to this second edition are the addition of the Category O cycle procedure and the severity reconsideration.