Fusible Plug is one of several means to detect fire by sensing heat.
To make a complete heat detection system, fusible plug is installed along with pressurized tubing and a panel. Tubing will be pressurized by instrument air or nitrogen with constant pressure from the panel.
At the end of each tubing, fusible plug is located on the area where fire possibly present. It will melt at certain temperature during fire subsequently release the pressure within the tubing.
If the pressure falls below its set point, the panel will execute pre-determined action such as generate fire alarm, activating sprinkler system, etc.