Small circuit breakers (PIA)
Introduction
A thermomagnetic switch, thermomagnetic switch, thermal key, breaker or taco[1] is a device capable of interrupting the electric current in a circuit when it exceeds certain maximum values. Its operation is based on two of the effects produced by the circulation of current in a circuit: the magnetic (Ampère's law) and the thermal (Joule effect). The device therefore consists of two parts, an electromagnet and a bimetallic sheet, connected in series and through which the current flows towards the load.
It should not be confused with a differential switch.
Like fuses, magneto-thermal switches protect the installation against overloads and short circuits.
Operation
Short circuit
As the current circulates through the electromagnet, it creates a force that, through a suitable mechanical device (M), tends to open contact C, but it can only open if the current I that circulates through the load exceeds the set intervention limit (the overcurrent reaches the short-circuit limit).
This level of intervention is usually between three and twenty times (depending on the letter B, C, D, etc.) the nominal intensity (the design intensity of the magneto-thermal switch) and its action is approximately 25 thousandths of a second, which makes it very safe due to its reaction speed.
This is the part intended for protection against short circuits, where a very rapid and high increase in current occurs. Short circuits are
increases in intensities caused by accidental direct contact
between phase and neutral.
Overload
The other part is made up of a bimetallic sheet (represented in red) that, when heated above a certain limit (overload current, but less than the short circuit), undergoes deformation and moves to the position indicated in the dashed line which, through the corresponding mechanical device (M), causes the opening of contact C.
This part is responsible for protecting against currents that, although they are higher than those allowed by the installation, do not reach the level of intervention of the magnetic device. This situation is typical of an overload, where consumption increases as devices are connected.