Any electrical network needs protection against over currents - currents whose strength exceeds the working value of the electrical circuit. They occur as a result of a short circuit or overload and can damage circuit elements or cause a fire to the insulation of the conductors.
To protect electrical circuits from possible damage during short circuit and overload, special devices called circuit breakers are used.
To protect power grids rated up to 125 A, compact modular circuit breakers are used. In circuits where currents up to 1600 A flow, medium-sized protective devices are used - molded case circuit breakers. Protection of systems with currents up to 6,300 A is the prerogative of automatic air circuit breakers.
Purpose of automatic air circuit breakers
Air circuit breakers are used for:
· infrequent shutdowns and inclusions of power electric networks (in manual mode);
· opening of electrical circuits during overload or short circuit (in automatic mode);
· automatic de-energization of electrical equipment in order to protect it from breakdowns at low voltage;
· circuit protection in various systems of automatic input of backup power, for example, in those where contactors and magnetic starters are used.
Why is the circuit breaker is called air?
The use of the word "air" in the name of a circuit breaker is associated with the method of an electric arc extinction inside an apparatus.
When opening and closing the switch contacts, an electric arc arises between them, which must be quickly quenched in order to prevent damage to the device.
The poles of an automatic air circuit breaker are not located in separate chambers, but in a single space filled with air and divided by partitions. Thus, the quenching of the arc discharge in them occurs in the air. Hence the name of the device.
Main features of the air circuit breaker
One of the main characteristics of an automatic air circuit breaker is the rated current of the releaser. This is the current for which the appliance is designed. It flows through the switch without causing it to trip. The device is turned off only if the rated current is exceeded.
Another important characteristic of the circuit breaker is the ultimate switching capacity (USC). This is the largest short circuit current that the "air breaker" is able to turn off.
Another important parameter of the automatic device is switching (electrical) wear resistance. Its value reflects the number of on and off cycles - the service life of the device.
Air circuit breaker control
The switch can be controlled both manually and remotely. To manually start it up, you need to use the special handle to cock the spring of the release mechanism, and then press the power button. The "air breaker" is taken out of operation by pressing the shutdown button.
The device is remotely switched on by means of a motor drive, which automatically retracts the spring after power supply, and an activation coil, which closes the contacts. Another coil that opens the contacts is responsible for disconnecting.
Drawout Air Circuit Breakers
Power air circuit breakers are available in fixed and drawout versions. A drawout device is a system consisting of the apparatus itself and the basket in which it is installed.
The basket is attached to the rails of the input-distribution device. It has guides and a chassis for easy rolling air circuit breaker in and out.
Moving the device forward or backward in the basket is carried out by rotating the special handle. The drawout switch can be in three positions: "Operating", "Test", "Disabled".
In the operating position, all circuits are turned on - the main and auxiliary: the device operates in normal mode. When the circuit breaker is pulled out, the main circuit contacts open, and the "air breaker" is in a position that allows testing. Further rotation of the handle of the retractable mechanism already leads to the opening of the contacts of the auxiliary circuit, which allows maintenance work on the circuit breaker.
Application examples of the air circuit breakers
High-current air circuit breakers can be used wherever it is required to provide protection for power circuits, both at civil and industrial facilities.
For example, they are used as input machines in transformer substations (TS) and main distribution boards (MDB) of apartment buildings, into which electric energy is supplied from the TS.
Widely used "air breakers" are also used in industry, providing protection for steel-smelting furnaces, rolling mills and other industrial electrical equipment.