Magnetothermal switch

Thermomagnetic Interruptor Family 1, 2, 3 and 4 Polos

A thermomagnetic switch, thermomagnetic switch, thermal key or breaker, It is a device capable of interrupting the electrical current of a circuit when it exceeds certain maximum values. Its operation is based on two of the effects produced by current flow in a circuit: magnetic (Ampère's law) and thermal (Joule effect). The device therefore consists of two parts, an electromagnet and a bimetallic sheet, connected in series and through which the current going to the load flows.

Not to be confused with a differential switch.

Like fuses, magnetothermal switches protect the installation against overloads and short circuits.

Operation

Composition of a magnetothermal.

Operation of a magnetothermal.

Short Circuit

When the current circulates through the electromagnet, it creates a force that, by means of a suitable mechanical device (M), tends to open contact C, but it will only be able to open it if the current I flowing through the load exceeds the intervention limit fixed (overcurrent reaches short-circuit limit).

This level of intervention is usually between three and twenty times (according to letter B, C, D, etc.) the nominal intensity (the design intensity of the circuit breaker) and its actuation is approximately 25 thousandths of a second, which makes it very safe due to its speed of reaction.

This is the part intended for protection against short circuits, where a very rapid and high current increase occurs. The short circuits are increases in currents caused by accidental direct contact between phase and neutral.

Overload

The other part is made up of a bimetallic sheet (represented in red) which, when heated above a certain limit (overload current, but less than short circuit), suffers a deformation and goes to the position indicated in line of traces which, by means of the corresponding mechanical device (M), causes the opening of contact C.

This part is in charge of protecting from currents that, although they are higher than those allowed by the installation, do not reach the intervention level of the magnetic device. This situation is typical of an overload, where consumption increases as devices are connected.

Both devices complement each other in their protection action, the magnetic one for short circuits and the thermal one for overloads.

Manual disconnection

In addition to this automatic disconnection, the device is provided with a lever that allows manual disconnection of the current and the reset of the automatic device when a disconnection has occurred. However, this reset is not possible if overload or short circuit conditions persist.

It would even jump again, even if the lever was held with your finger, since it uses a separate mechanism to disconnect the current and lower the lever.

Polarity

The device described is a single-pole magnetothermic switch, since it only cuts one of the electrical supply wires. There are also bipolar versions and for triphasic currents, but essentially all are based on the same principles as the one described.

A switch is said to be omnipolar when it interrupts the current in all active conductors, that is, the phases and the neutral if it is distributed.

Features

The characteristics that define a thermomagnetic switch are:

Intensity or amperage

Depending on the cutting intensity, different calibers can be used. Typical values used in homes are 10 A, 16 A, 20 A, 25 A, 40 A. The device must be in accordance with the conductor section of the phase it protects.

The firing curve

The main codes are:

• B: quick shot
• C: normal shot
• D: slow shot
• Other curves: MA, Z.

Number of poles

• Unipolars: They protect only one phase.
• Bipolars: protect a phase and a neutral.
• Tripolars: protect three phases.
• Tetrapolars: protect three phases and a neutral.

Cutting power

The breaking capacity in terms of current of a circuit breaker is its breaking power and is usually expressed in kiloamperes (kA).

The overvoltage generated during a short circuit depends on the power and other characteristics of the phase in which it occurs.

For example, if it is calculated that in the event of a short circuit an overcurrent of 6 kA would be generated and the installed circuit breaker has a breaking capacity of 3 kA, in the event of a short circuit it will trip but it could happen that current passes through an electric arc.

For example, C-16A-IV 4 thermomagnetic switch, the 16A, a factor between 1.13 and 1.45 since the IEC-60898 and IEC-60947-2 standards indicate thermal actuation margins and margins of magnetic actuation and their respective tests to comply.

Resettable circuit breakers

For special applications, there are models that are equipped with a servomotor that resets the switch by itself, restoring service after a trip. There are those in which the rearm order is carried out manually from a distance, as in the case in which an operator from a control room is capable of firing or rearming a switch that is hundreds of kilometers away, or there are automatic reset, in which the breaker itself has an electronic control circuit that after a few seconds of the trip executes the order to reset the breaker by activating the servomotor; in the case of suffering a new shot, it can wait again for a while and order a new rearming a certain number of times before giving up. With this type of switches, it is avoided having to travel to distant facilities (television repeaters for example) to carry out a reset caused by transient shots; They are also used for the protection of security installations (CCTV, alarms) or for installations in which a power cut endangers people or property (elevators, medical and hospital equipment, cold rooms or freezers). It is becoming common to find monoblock resettable circuit breakers together with differential protection; In other words, the same device protects against short circuits, overloads and ground faults, being self-resetting for any of the three causes that can cause it to trip.