IEC standard 60076-1 clause 5.5 explains the provision for unusual conditions in transformers. Regarding the frequent energisation of excessing 24 times per year is considered as unusual condition.Therefore the purchaser shall identify in his enquiry if such condition would be present which is not covered by the normal service condition.See below extract from IEC standard 60076-1 § 5.5 IEC standard 60076-1 § 5.5 5.5 Provision for unusual service conditions The purchaser shall identify in his enquiry any service conditions not covered by the normal service conditions. Examples of such conditions are: – external cooling medium temperature outside the limits prescribed in 4.2; – restricted ventilation; – altitude in excess of the limit prescribed in 4.2; – damaging fumes and vapours; – steam; – humidity in excess of the limit prescribed in 4.2; – dripping water; – salt spray; – excessive and abrasive dust; – high harmonic content of the load current exceeding the requirements of 4.2; – distortion of the supply voltage waveform exceeding the limits of 4.2; – unusual high frequency switching transients, see Clause 13; – superimposed DC current; – seismic qualification which would otherwise require special considerations in the design, see 4.2; – extreme mechanical shock and vibrations; – solar radiation; – regular frequent energisation in excess of 24 times per year

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