Circuit breaker - what it protects and how it works

Circuit breakers are devices whose task is to protect an electric line from damage caused by a large current. It can be either short-circuit overcurrents or just a powerful flow of electrons passing through the cable for a sufficiently long time and causing it to overheat with further melting of the insulation. The circuit breaker in this case prevents negative consequences by cutting off the current supply to the circuit. Later, when the situation returns to normal, the device can be turned on again manually.

Circuit breaker functions

Protective devices are designed to perform the following basic tasks:

• Electrical circuit switching (the ability to turn off the protected area in case of power failure).
• De-energizing the entrusted circuit when short-circuit currents appear in it.
• Protection of the line from overloads when an excessive current passes through the device (this happens when the total power of the devices exceeds the maximum allowable).

In short, ABs simultaneously perform a protective and a control function.

The main types of switches

There are three main types of AB, differing from each other in design and designed to work with loads of different sizes:

• Modular. It got its name because of the standard width, a multiple of 1.75 cm. It is designed for small currents and is installed in household power supply networks, for a house or apartment. As a rule, it is a single-pole or double-pole circuit breaker.
• Cast. It is called so because of the cast body. It can withstand up to 1000 Amperes and is used primarily in industrial networks.
• Air. Designed to work with currents up to 6300 Amperes. Most often it is a three-pole automatic machine, but now devices of this type are being produced with four poles.

A single-phase protective circuit breaker is a circuit breaker that is most common in household networks. It can be 1- and 2-pole. In the first case, only the phase conductor is connected to the device, and in the second - also zero.

In addition to the listed types, there are also residual current devices, designated by the abbreviation RCD, and differential machines.

The first cannot be considered full-fledged AB, their task is not to protect the circuit and the devices included in it, but to prevent electric shock when a person touches an open area. The differential circuit breaker is an AB and RCD combined in one device.

How are circuit breakers arranged?

Let us consider in detail the device of the circuit breaker. The machine body is made of dielectric material. It consists of two parts, which are connected by rivets. If it is necessary to disassemble the body, the rivets are drilled out, and access to the internal elements of the circuit breaker is opened. These include:

• Screw terminals.
• Flexible conductors.
• Control handle.
• Movable and fixed contact.
• An electromagnetic release, which is a solenoid with a core.
• Thermal release, which includes a bimetallic plate and an adjusting screw.
• Gas outlet.
• Arc extinguishing chamber.

On the back side, the automatic safety fuse is equipped with a special latch, with which it is attached to the DIN rail.

The latter is a metal rail with a width of 3.5 cm, on which modular devices are attached, as well as some types of electric meters. To connect the machine to the rail, the body of the protective device should be wound over its upper part, and then click the latch by pushing the lower part of the device. You can remove the circuit breaker from the DIN rail by hooking the latch from the bottom.

The latch of the modular switch can be very tight. To attach such a device to a DIN rail, you must first hook up the latch from the bottom and put the protective device in place of the fastener, and then release the locking element.

You can make it easier - when snapping the latch, press firmly on its lower part with a screwdriver.

It is clear why a circuit breaker is needed, in the video:

How the circuit breaker works

Now let's figure out how the network protection circuit breaker works. It is connected by lifting up the control handle. To disconnect the AV from the network, the lever is lowered down.

When the electric protective circuit breaker operates in normal mode, the electric current with the control handle raised up is supplied to the device through the power cable connected to the upper terminal. The flow of electrons goes to a stationary contact, and from it to a mobile one.

Then the current flows through the flexible conductor to the solenoid of the electromagnetic release. From it, along the second flexible conductor, electricity goes to the bimetallic plate, which is included in the thermal release. Having passed along the plate, the flow of electrons through the lower terminal goes into the connected network.

Features of the thermal release

If the current exceeds the circuit in which the circuit breaker is installed, an overload occurs. The flow of high-power electrons, passing through the bimetallic plate, has a thermal effect on it, making it softer and forcing it to bend towards the tripping element. When the latter comes into contact with the plate, the machine is triggered, and the supply of current to the circuit stops. Thus, thermal protection helps to prevent excessive heating of the conductor, which can lead to melting of the insulating layer and damage to the wiring.

The heating of the bimetallic plate to such an extent that it bends and triggers the AB occurs for a certain time. It depends on how much the current exceeds the rating of the machine, and can take both a few seconds and an hour.

The thermal release is triggered when the circuit current exceeds the rating of the machine by at least 13%. After the bimetallic plate has cooled down and the current current is normalized, the protective device can be turned on again.

There is another parameter that can affect the operation of AB under the influence of a thermal release - this is the ambient temperature.

If the air in the room where the device is installed has a high temperature, then the plate will heat up to the tripping limit faster than usual, and may be triggered even with a slight increase in current. Conversely, if the house is cold, the plate will heat up more slowly and the time before the circuit is disconnected will increase.

The thermal release, as mentioned, requires a certain time during which the circuit current can return to normal. Then the overload will disappear and the device will not shut down. If the magnitude of the electric current does not decrease, the machine de-energizes the circuit, preventing the insulation layer from melting and preventing the cable from burning.

The overload is most often caused by the inclusion of devices in the circuit, the total power of which exceeds the calculated one for a particular line.

Nuances of electromagnetic protection

The electromagnetic release is designed to protect the network from short circuits and differs from the thermal one in terms of the principle of operation. Under the action of short-circuit overcurrents, a powerful magnetic field appears in the solenoid. It pushes the coil core to the side, which opens the power contacts of the protective device, acting on the release mechanism. The power supply to the line is interrupted, thereby eliminating the risk of fire in the wiring, as well as destruction of the closed installation and the circuit breaker.

Since in the event of a short-circuit in the circuit, an instantaneous increase in current occurs to a value that can lead to serious consequences in a short time, the operation of the machine under the influence of an electromagnetic release occurs in hundredths of a second. True, in this case, the current must exceed the nominal AB by 3 or more times.

Clearly about the circuit breakers in the video:

Arc chute

When the contacts of the circuit through which the electric current flows open, an electric arc arises between them, the power of which is directly proportional to the magnitude of the mains current. It has a destructive effect on the contacts, therefore, to protect them, the device includes an arc-extinguishing chamber, which is a set of plates installed parallel to each other.

Upon contact with the plates, the arc is fragmented, as a result of which its temperature decreases and attenuation occurs. The gases generated during the appearance of the arc are removed through a special hole from the body of the protective device.

Conclusion

In this article, we talked about what circuit breakers are, what these devices are and how they work. Finally, let's say that circuit breakers are not intended to be installed in the network as conventional switches. Such use will quickly lead to the destruction of the device contacts.