ABOUT ten years ago it was found necessary to build short-circuit testing stations in order to find out the true rating for a large switch or current-breaker. In order to find out how a switch will break a large electric current at a low voltage or a current at a very high voltage, different methods have to be used and special large and expensive devices are necessary. It is very unsatisfactory to assign nominal ratings based on theoretical design considerations and experience gained by similar apparatus. The International Electrotechnical Commission has now published a specification for the short-circuit testing of circuit-breakers, and manufacturers have to issue a certificate with the apparatus stating that it has passed the required proving tests. Eight years ago Messrs. Reyrolle and Co. built the first short-circuit testing station at Hebburn-on-Tyne and since then, two more have been erected in Great Britain and others are being constructed or are under contemplation. According to the Electrical Power Engineer of December last, the certification of switches is not exclusively confined to those manufacturers who own testing stations in whole or in part. The facilities of all the British proving stations are open to other manufacturers for making tests. This entails a definite assurance from the testing stations of privacy in testing and secrecy of results of experiments making so much noise and requiring so large a space. All work is carried out on apparatus under a reference number, the manufacturer's name being omitted and also the designation of the apparatus. During testing all entrances to the station are locked. A separate office is provided for the use of customers wishing to discuss reports. Sometimes cinematograph films of the tests are shown at these offices.