method of operating a mains - powered processor - controlled electrical apparatus when the power fails and is subsequently restored and apparatus therefor . in the case of a power outage data identifying the program in progress is transferred for salvage to a memory . when power is restored and if the duration of the mains - power failure is smaller than a predetermined value , physical parameters which define the instantaneous state of the apparatus are sensed in response to an instruction from the processor in order to determine the conditions under which the program may be resumed by the apparatus .