The system used to relay monitoring information on a nuclear site was rendered inoperable by lightning during the night-time shift. Some units, such as the treatment of irradiated materials, were not beneficiaries, as activities that are executed with a permanent human presence, as opposed to the majority of other nuclear installations. Outside of working hours, relevant information such as alarm triggers are relayed to the site’s main supervisory station. On a Saturday towards the beginning of the evening, lightning knocked out the transmission system for around ten hours. Consequently, the line of defence, which relies upon human intervention in case of an operating anomaly, disappeared. Site security was merely dependent on the technical and operational devices in place, yet the exhaustiveness and efficiency of such devices were never completely assured. Once the information relay was discovered to be inoperable, the on-call personnel set up a human monitoring routine until the defective system could be restored and recalibrated. Analogous failures were identified on other site retransmission systems. Given the momentary degradation in the level of security for the particular installations, this incident was classified as a Level 1 on the INES (International Nuclear Event Scale).