The paper describes the results of a series of short-line fault tests on the 275 kV CEGB system at Thorpe Marsh switching station by a collaborative committee. These tests were planned with two main objectives: cross-checking calculated system characteristics with measured values and the demonstration of circuit-breaker performance, both with as high a fault current as possible. Since the short-line fault has recently assumed greater significance owing to the increase of short-circuit currents, the tests were made for this condition. Details of the test arrangements and instrumentation, which included a special Faraday cage, are given. A fault current of up to 20 kA was obtained. Overhead-line transient and 50 Hz parameters are derived from the test results and compared with calculations. Similarly, the source-side transient recovery-voltage waveshapes are presented and compared with calculated waveshapes. Despite the complexity of the system, good agreement was obtained for these and for calculations of fault current. The calculation methods are described and references are given. Although the fault current represented only about 75% of the circuit-breaker rating under this condition, the tests gave a useful demonstration of the capability of circuit breakers developed by the unit-test principle.