Abstract: A method of testing operation of an electrical circuit involves supplying a test signal to circuit terminals (1 to 4), which signal may be a pulsed signal, to the circuit, thereby to switch off a switch component (ZD1). When the circuit is in this condition, further test signals may be applied to the terminals (1 to 4) to test operation of other circuit components (11, 12, 13, ZD2). The method is particularly applicable to circuits, e.g. diode safety barriers, that are encapsulated and require testing after encapsulation, when only terminals (1 to 4) are exposed for connection into an external circuit.