About electrical protection devices for "dummies": circuit breakers

Many people remember Soviet circuit breakers - plugs. Instead of ordinary ceramic plugs, they were screwed into the shield of an electric meter. It was a compromise solution, which, in general, paid off. Indeed, thanks to this, plugs became "reusable", and without changing the existing design of the electrical panel. In general, the inventor of automatic protection devices is ABB, which patented a small-sized circuit breaker in 1923. Much time has passed since then, but the principle of operation of the circuit breaker has remained unchanged - the restoration of its normal operation with one movement of the hand.

A circuit breaker is an electrical switching device designed to conduct current in normal conditions and to automatically turn off electrical installations when short-circuit currents and overloads occur. The most common and popular today are circuit breakers that are mounted on a 35 mm DIN rail in a distribution panel.

The main parameter of circuit breakers is the rated current. This is a current whose value in a particular circuit is considered normal, i.e. for which electrical equipment is designed. For electrical installations in residential buildings, the rated current (In) of a circuit breaker can be 6, 8, 10, 13, 16, 20, 25, 32, 40, 63 A. Most often, circuit breakers are used in the range 16 - 63A as for single-phase consumers, and three-phase. There is also such a parameter as the rated voltage –220/230 V or 380/400 V.

The appearance of single-phase and three-phase circuit breakers is shown in the figure:

Circuit breakers break the circuit when the current in it exceeds the permissible value. This situation occurs when more than the allowed number of consumers is turned on or during a short circuit. At the same time, various processes occur, because of which it is necessary to use two types of protection in circuit breakers - thermal and electromagnetic.

When the current consumption is more than 3 times the nominal value, the thermal release of the circuit breaker trips. The principle of its action: the circuit breaks a bimetallic plate, which changes its shape from heating by passing current. The protective device can pass a current for a rather long time, slightly exceeding the rated current, which will avoid false alarms, but with a further increase in current, it will disconnect the load. Therefore, thermal protection has a rather large inertia with respect to overcurrents

At a significantly higher current (during short circuits), the inertia of the protection is a big minus, therefore, an electromagnetic release is used for this case. Unlike thermal, it has instant action.

The electromagnetic release consists of a solenoid (electromagnet), the core of which strikes a movable contact and opens the circuit. But it is not so simple. After all, the electromagnet should work at a certain current. The lower threshold, judging by the fact that thermal protection is triggered up to 3 In, will have exactly this value. What about the upper threshold? Here one more characteristic of AB comes up - the type of machine.

There are three types of circuit breakers - “B”, “C”, and “D”. Type B circuit breakers have an electromagnetic trip in the range of 3 to 5 In. Type C has a range of 5 to 10 In. Finally, type “D” is triggered in the range of 10 to 50 In.For a specific example, it will look like this: if we have two circuit breakers on 25A of class “B” and “C”, then when a short circuit occurs, the first one will turn off when the short circuit current reaches 75 A and 125 A, and the second from 125 A and higher. The short circuit current that the circuit breaker can handle without compromising performance determines “rated breaking capacity,” another characteristic of the circuit breaker. The better this parameter, the more reliable the switch. The process of releasing the contacts occurs very quickly, while the short circuit current does not have time to reach the maximum value.

Circuit breakers "B" and "C" are installed in the networks of residential buildings. Type "B" is used if there are no inrush currents due to the inclusion of any motors. Type “C” is recommended for protection of electrical receivers with small inrush currents. And the last type “D” is installed mainly in industrial premises where powerful engines are involved.

An important component of any circuit breaker is the arc extinction chamber. As you understand, during a short circuit an arc forms, and for how short it would not have existed, its effect negatively affects the overall reliability of the circuit breaker and, therefore, its service life. The arc extinction chamber consists of a set of parallel, metal plates isolated from each other. In it, the arc is divided into a sequence of many small arcs. They immediately go out due to the small amount of voltage between adjacent plates. This is the classic design of spark arresters.

In addition to automatic shutdown, a circuit breaker can be tripped manually. Therefore, the circuit breaker is called a switching protective device. Indeed, in addition to the protection properties, it provides the ability to de-energize the circuit in manual mode, which is necessary when repairing electrical equipment.

Choosing a circuit breaker, you should clearly know the parameters that we talked about above - rated voltage, rated current, and type of machine. Circuit breaker marking must contain the name or trademark of the manufacturer, the value of the rated voltage, the rated current, the letters B, C or D, indicating the type of circuit breaker, the rated breaking capacity in amperes and the connection diagram, if the correct connection method is difficult to understand from the appearance of the circuit breaker.

Mikhail Tikhonchuk

Continuation of the article: About electrical protection devices for "dummies": residual current device (RCD)