This specification applies to power transformers, reactors and earthing transformers, having windings insulated with Class A, b, H and/or C materials, rated at 1 kVA or over if single phase, and 2 kVA or over if polyphase, and without limitation to voltage rating. It applies also to transformers for use on single wire earth return (S.W.E.R.) systems. The specification refers principally to transformers for operation on sinle-phase and symmetrical three-phase systems, but the requirements are generally applicable to transformers for other polyphase systems. Requirements for mining-type transformers and transformers for mercury-arc rectifier equipment are covered in so far as they are invoked by BS 355, BS 1698, and BS 2538.