Alarm system

An alarm system, comprising:

• a signal line (2, 4) for indicating damage to and/or an attempt to interfere with said line, said line being suitable for incorporation in a net-structure particularly intended for use as a barrier net for denying foreign objects, such as underwater vessels, frogmen and the like access to water passageways, channels, etc., wherein the signal line contains a fluid and is connected to sensors (8-11) constructed to generate an alarm signal in response to a change in pressure and/or in the flow conditions in said line;
• the signal line including a flexible tubing constructed to prevent the flow channel extending therethrough from being completely closed by squeezing or compression forces, said fluid comprising a coloured indicating agent subjected to a pressure of such magnitude that in the event of perforating damage to the tubing, such as a fracture or rupture, the agent will exit from the tubing and indicate visually the location of the fracture signalled by one of the sensors, and the indicating agent having a density below that of water, so that when the alarm system is used under water said agent will flow to the surface of the water.