The calculation of transient overvoltages due to circuit-breaker operations in power systems has always been of importance, but in the past attention has been focused on the restriking voltage transient occurring when a circuit breaker opens, since this intimately affects the duty and hence the design of the circuit breaker. More recently, owing to the rise in system operating voltage and to a desire to reduce capital costs by a reduction in system insulation level, the transient voltages arising when long transmission lines are energised and re-energised and the means for reducing these transient voltages have become of considerable importance. Although a number of methods for calculating switching transients exist, some are more accurate than others. The use of the most accurate methods, however, may not always be justified, owing to system-data limitations and the larger computation costs involved. The authors believe that the methods they have adopted and which are described, although not the most accurate, are well suited to the system data which are normally available at the present time. The paper outlines first a method based on a lumped-parameter representation of the system, and secondly a method based on a lattice-diagram solution of the transmission-line wave equations. Some of the facilities available with this second method are described, including the representation of lumped inductive and capacitive elements and transmission-line attenuation and distortion. After the results obtained using these methods have been compared, the effects of source inductance, line-connected shunt reactors and trapped-charge voltages on the transient voltages produced by the energisation of long lines are considered. Other effects which may modify these voltages are those of corona loss and reduced trapped-charge voltage, caused by the discharging effect of equipment connected to the line, e.g. shunt reactors and circuit-breaker-opening resistors. If the inclusion of these factors does not reduce the overvoltages sufficiently to enable the desired system insulation level to be obtained, the use of circuit-breaker-closing resistors or point-on-wave switching of the circuit breaker can be considered.