Abstract: A three-wire alarm system is disclosed which features a plurality of plug-in strips each carrying a plurality of three-connector receptacles. These strips may be interconnected one to the next and to a battery-powered alarm box by identical jumper cables with three-pinned plugs. Other cables with identical plugs are fastened to valuable items which a merchant desires to display. When these items are placed on display, they are plugged into a nearby plug-in strip. The items may be plugged in without setting off the alarm; but removing any plug or cutting, shorting, or breaking the cables produces an alarm that can be turned off only with a key.The three-contact receptacles of the invention contain a spring that (1) serves to interconnect two of contacts when the receptacle is not in use, (2) contacts the pin mating with the third contact when a plug is inserted, and (3) is dragged by that pin into contact with one of the other contacts when the plug is withdrawn thereby causing an alarm.