WHEN an electric generator is accidentally short-circuited, huge currents are developed and unless the ‘pcircuit-breakers’ act promptly, serious damage may be done to the generator and there is a risk of fire. Until a few years ago, practical experience was the only guide to the rating of these circuit-breakers. The enormous currents required for testing purposes, in most cases, made the testing costs prohibitive. Proposals were made for a co-operative or national testing plant, but nothing materialised. In 1929 a private company, Messrs. A. Reyrolle and Co., Ltd., of Hebburn-on-Tyne, laid down their own testing station, which has proved capable of testing the largest circuit-breakers used in Great Britain. They have erected a miniature power station which has a capacity of 1-5 million kilovolt-amperes. Any short-circuit conditions which might possibly occur in practice can be produced in their testing room. The generators are driven by 5,500 volt motors connected with the public supply mains. Very large transformers are used to produce the heavy currents required. The observation gallery is built of reinforced concrete and has slits in the wall fronting the test bay, through which the behaviour of the apparatus under test can be safely observed. A system of traffic signal lights and alarm bells is installed outside the test bay to give warning when a test is about to be made and when all is clear. Short-circuit phenomena can be observed in time intervals as short as two millionths of a second. Messrs. Reyrolle's plant is one of the largest and best equipped short-circuit testing stations in the world. A scheme has now been initiated whereby full facilities are provided for utilising this testing plant for the benefit of the electrical industry. A company has been formed which will operate independently and will be in a position to issue national test certificates.