This specification applies to electric generators and motors of such size that the maximum sustained output, i.e., the rated load plus the sustained overload given in Clause 59, exceeds 2.5 kW, 2.5 kVA, or 2.5 B.H.P. per revolution per minute; also to direct coupled exciters without limit of size, having windings insulated with Class A and Class B materials, as defined in Appendix I, but excluding alternators of the steam turbine driven type, single phase generators and motors, rotary converters and motors for traction purposes. This specification may also be applied to machines of smaller ratings by special arrangement between the manufacturer and purchaser.