naval mines were also used to protect krepost sveaborg from an attack from the sea. an outer line of contact mines was laid approximately outside the main line of defence. shore detonated controlled mines were used to create an inner mine barrier to protect the entrances to the outer anchorage. these mines were laid typically in three rows with separation between rows and between mines in a row. controlled mines required an extensive cabling system to the shore stations where the command to detonate the mines was given. fortified electrical stations provided the necessary electricity for the mine control network. electricity was also used for communication systems, searchlights, electrical ammunition lifts and in barracks and crew shelters.