The paper describes the results of field tests, carried out on a section of the Central Electricity Generating Board's system during the summer of 1961, to measure the behaviour of interconnected generating sets, including a 200 MW unit, under the influence of transient faults and to compare with predicted performance by different methods of analysis. The results stressed that a rapid clearance of close-up 3-phase faults is essential with modern generators to prevent transient instability, but the dynamic severity of the fault attenuates rapidly away from the point of its application. They also confirmed previous findings, namely that the conventional method of analysis by network analyser or digital computer predicts the amplitude of generator-rotor first swing with good accuracy, and thereafter gives a pessimistic indication of system-recovery. Micro-models and analogue computers give a greatly improved prediction of system post-fault recovery.