Abstract: In circuit testers, notably automatic test equipment for the in circuit testing of digital devices, overheating damage can result if outputs are repeatedly "overdriven", that is, while applying a test input to a first device, an output of a second connected device is forced into a state contrary to that it would normally adopt. Damage is avoided by enforcing a cooling interval between overdriving tests based upon a list of parameters representative of the overdriven device and its environment. In order to reduce the overall test time, a parameter is based on a measurement derived from a circuit of the type under test. A first test results in device heating to a value. A cooling interval is enforced so that a subsequent test which results in further heating of the device, may be made without exceeding device maximum allowable temperature .sup.T max.