Failures of insulation in service can be largely avoided by making field tests at regular intervals with suitable testing equipment. The paper deals with the field testing of the insulation of high-voltage switchgear, busbars and transformers, and describes the apparatus and methods employed by the authors on the Central Electricity Board's system.There are three main testing techniques: (a) the measurement of power factor; (b) the measurement of potential distribution; and (c) the measurement of insulation resistance; and practical testing equipments, designed to make such measurements and to satisfy the onerous conditions of field use, are described.Three wattmeter types of power-factor-measuring apparatus are described, two operating at 10 kV for bushing tests, and one operating at 50 kV for tests on busbar systems. Details are given of a modified form of bridge specially developed for field testing. The measurement of the potential distribution on bushings under service conditions is discussed, and reference is made to an electronic testing set for the measurement of insulation resistances up to 10 000 megohms at 5 000 V.More than 2 000 tests have been made on installed apparatus, and the results obtained are summarized. Twenty-four faulty bushings have been located by test and taken out of service before breakdown. Special test methods are described in order to facilitate making power-factor measurements on transformer bushings and orifice insulators.Information is also given on the power factor of oil in service.Methods are suggested of continuously supervising the insulation of important installations in cases where the low-voltage end of the insulators can be isolated from earth.Finally, the merits and demerits of the three main test methods are summarized in order to assist the maintenance engineer in choosing the method or methods which will best fulfil his requirements.