A security loop cable assembly is attached to the distal end of the main cable of an alarm system. The assembly has an enclosure with a main electrical connector inside the enclosure. The main cable terminates at the main connector. The enclosure releasably receives a polarized plug assembly that contains the ends of a loop cable. The plug assembly includes a housing and a case that are selectively joined. Inside the case two of the wires of the loop cable are permanently joined to opposite sides of an end-of-line resistor. The housing and case can be momentarily separated to thread one of them through an aperture in the protected object or gate. The plug assembly can be removably inserted into the enclosure, whereupon the main cable connector engages a loop connector to form an electrical circuit through the first and second wires and the end-of-line resistor.