A digging machine damaged in 11 kV electrical supply cable which supplies a complex of reprocessing and waste management plants at the UKAEA Dounreay site at Thurso, Scotland, because alternative electrical supply arrangements did not become available as intended. The ventilation fans lost power. Plants were evacuated. Power was restored to the fans 15 hours later. On re-entry the plants were found to be clean. The environment around and inside the complex was monitored and no release of radioactivity was detected. Event rated on the basis of "significant failure in safety provisions with sufficient defence in depth remaining to cope with additional factors".