As is known, simple bimetal thermal protectors are designed as current switches (Figure 1). Heat-sensitive bimetal is in the form of a single sided mobile tape, at the end of which there is a welded conductor contact. Because bimetal tapes can move in a relatively free manner, the pressure force of the contact constantly changes depending on heat. Thus, the resistance of contact naturally depends on temperature when in closed position. Under these conditions, pressure force may drop before reaching the closing temperature yet, which creates an arc at the end of high contact resistance (Figure 3). Due to this, the thermal protector may heat up enough to close before its time. At the worst scenario, contact ends may get welded into another, and thus the thermal protector does not open again and the protection function becomes permanently deactivated without notice. Therefore, such simple type bimetal protection systems provide a less safe protection in cases of overheating or fault compared to bimetal protectors not dependent on current.
In any case, such current switches display a sensitivity against vibration near the opening/closing temperature. The reasons of this are the minimum contact pressure force during opening/closing and the linear force-distance curve (Figure 4).
Due to adverse contact resistance conditions, such simple thermal protectors suffer the problem of overheating near the opening/closing temperature –especially in high current applications. As a result, the opening/closing temperature of conventional thermal protectors declines by an indefinite rate compared to thermal protectors with additional flexible disk.
In such simple bimetal thermal protectors, emergence of undesired arcs, contact wearing due to early opening/closing and resulting short life cycles are quite probable.
Figure 3: Thermal protector without extra flexible disk. The arc damage risk emerging due to this.
Figure 4: Force – Distance Curve (characteristic curve):
a simple disk (linear curve)
b additional flexible disk (nonlinear curve)