640,188. Detecting faults. AKT.-GES. BROWN, BOVERI, & CO. Nov. 6, 1947, No. 29589. Convention date, Nov. 6, 1946. [Class 37] [Also in Group XXXIX] Faults in an oil-immersed winding, particularly when testing with surge voltage, are detected by converting pressure surges caused 'by an electric discharge in the insulating liquid into electric indicating values by electroacoustic means. The high tension winding a, Fig. 1, of a transformer is housed in an oilfilled tank b into which projects an electroacoustic apparatus c. If during a surge test there occurs a flash-over or breakdown of the winding a, an electric discharge takes place and causes pressure surges in the oil which are transformed into electric impulses by the apparatus c. These are passed through an amplifier d and indicated by an instrument e or an oscillograph. As shown in Fig. 2, the apparatus c consists of a carbon powder or crystal microphone f which is mounted outside the tank b. The membrane of the microphone closes the upper end of an insulating tube g which is inserted into the tank through the filling tube h. The lower end of the tube g, which is immersed in the oil, may be left open or closed by a membrane i, pressure surges caused by a discharge being transmitted by the column of oil in the tube and the air above it to the diaphragm of the microphone f. The tube g may be filled with gas under pressure which causes the lower membrane to assume a spherical form as shown dotted and so form a greater surface for the transmission of the pressure surges. Alternatively a condenser microphone k, Fig. 3, may be used, the oscillatory circuit k, m being tuned to the frequency of the feeding source of current n. On surges impinging on the condenser k the capacity is varied and changes the natural frequency of the circuit. This change is indicated by an instrument e after amplification in an amplifier d. Alternatively the microphone may be mounted directly on the outside of the tank. Frequency filters may be inserted in the microphone leads to eliminate disturbing frequencies.