Short answer is that the system is a dual loop type.
If one loop breaks you still have a servicable second loop.
The logic of the circuit detects a break when the gas pressure in the loop drops. The 'loop' is a sealed tube containing a gas and a gas impregnated metal core. When there is a temp rise then the impregnated metal core releases it's gas and consequently increases the loop gas pressure. This triggers the pressure switch, indicating a fire.
If the second loop gas pressure also fails within 20secs? Then the logic assumes a catastrophic engine burst and detects a fire.
There is a lot more to this but I'm busy. Hope this helps.
