The releasing element auf all ESTI Excess Temperature Protectors is the ESTI cartridge. This is a small spherical glass bulb, filled with a chemically inactive and insulating fluid. Whenever the cartridge is heated the fluid contained in it expands. As soon as a predetermined critical temperature is reached the ESTI cartridge bursts with absolute certainty, due to the excess internal pressure.

This bursting of the ESTI cartridge causes the release of electrical and mechanical switches.