Abstract: In a security system in which at least one fibre-optic wave guide, which is carried in a strand, extends along a boundary, light is directed into one end of the guide and the light leaving the guide is detected by an optical detector. An indication is given when the intensity of the detected light falls below a predetermined threshold, so as to warn when the wave guide is disturbed significantly or cut through.In order to manufacture the strand, a fibre-optic wave guide is positioned against an elongate carrier and secured to the latter by bonding agent and/or adhesive tape.A dummy strip, having identical external appearance to the strand but not including a fibre-optic wave guide, can also run along the boundary in question.