A succession of external electrical power disturbances at a nuclear research facility required to place backup electric generating sets online to continue operations. The site featured 2 distinct 20-kV lines, labelled A and B, that could be used indifferently and that depended on the external electrical network, with just one needed for power supply. Moreover, the plant was equipped with electric generating sets serving as a relay in case both lines were out. On 28th May, a malfunction on line A led to switching over the site’s electrical power onto line B. An unsuccessful attempt was made to return to line A on 31st May at 4 pm at the request of the electricity network operator: line B was still responsible for powering the site. At 11:34 pm, a power loss on line B was observed: electric generating sets started up and took over during the line interruption, which lasted approx. 8 min, then line B was once again operational. A second line B power loss was recorded at 11:59 pm, again activating the generating sets, which this time took over for 20 min of downtime. Line B became operational at 12:19 am. During the various malfunctions to the external supply, overall site operations were not affected. Some measures were nonetheless adopted (e.g. sufficient diesel reserves) to ensure the generating sets functioned in the event of an extended loss of both lines. Following the site operator’s investigation of the electrical network, the cause of line A malfunctions was found to be a defective cable insulation at the level of a junction box (outside the facility). This defect was repaired on 1st June. The first line B loss was due to power outage at the Chatillon transformer station: this station was relayed by another backup station, which made it possible to quickly re-establish the line B supply. The second line B interruption stemmed from a malfunction arising at the backup transformer station (an oil pump). Given the degradation in the plant’s electrical power supply, this event was rated at level “0” on the International Nuclear Events Scale.