Circuit breakers are designed to protect electrical circuits from short circuits, current overload and other failures. They can operate without human intervention when certain preset values of current and voltage are reached. They act like a fuse with a fuse block, but don’t have the disadvantages of this fuse – the single response and the manual replacement of the fuse block. Circuit breakers can respond many times and the number of switching cycles is usually specified by the manufacturer. Depending on the number of poles, circuit breakers are divided into single-pole and multi-pole. The latter are used for the simultaneous disconnection of the phases of the electric circuit. According to their operating voltage circuit breakers are designed for low (up to 1000 V), medium (up to 35kV) and high voltage (110 -400kV).
Here we will take a look at circuit breakers for low voltage, but having large amperage. A circuit breaker consists of contacts (usually normally closed), a switching mechanism, terminals, bimetal plate (providing thermal shutdown), induction coil, arc-extinguishing chamber ( spark-extinguishing chamber) calibration screw (for setting the switching-off current), switching lever (for manual switching-on after troubleshooting) with „I“ and „O“ or „On“ and “ Off“ positions. The response of the circuit breaker for large currents functions through the electromagnetic system, which includes the coil, too. When the current in the circuit increases, a magnetic flux is induced in the coil. This magnetic flux creates an electromotive force, which opens, in turn, the contacts of the circuit breaker. The other construction system is based on the temperature rise of the bimetal plate. When heated, the bimetal plate becomes deformed and disconnects the contacts. In practice, circuit breakers combine both systems.
The electromagnetic system interrupts short-circuit currents, it is faster – the thermal protector is activated when there is a continuous current overload and the size of the current is slightly larger than that in normal operation mode. It is important to note that in a short-circuit mode, the currents that occur are several times larger than the rated current in the circuit. With such currents, upon opening of the contact terminals, the current continues to flow through the air gap forming an electric arc with a high temperature, which is able to melt the housing of the circuit breaker or weld the terminals if no constructive measures are taken. The technical solution for power circuit breakers with air cooling are arc-extinguishing chambers, which are designed to absorb the heat from the short-circuit currents. Arc-extinguishing chambers contain holes, equipped with metal plates, which take the heat of the arc. Medium and high voltage circuit breakers use other constructive solutions to deal with the heat arising from the formation of an electric arc. There, the working environment may be vacuum or inert gas. There are constructions in which the contact system is immersed in machine oil.
Low voltage circuit breakers are divided into: circuit- breakers with rated current up to 100A, circuit breakers with a cast package for rated currents up to 1600A, circuit breakers with rated current up to 6000A – these are generally air circuit breakers with adjustable switching-off parameters, vacuum circuit breakers for low and medium voltage for rated current up to 3000 A.
How to choose a circuit breaker?
When selecting a circuit breaker we must pay attention to the following main characteristics: do we need a single-pole or a multi-pole circuit- breaker (for three-phase voltage we usually choose a three-pole circuit breaker), rated current, rated voltage, is there a possibility for adjusting the switched-off quantities, maximum breaking capacity of the circuit-breaker (the largest current that the circuit-breaker can interrupt), working breaking capacity ( the current that the circuit-breaker can interrupt repeatedly without failure). When selecting a power circuit breaker we need to calculate what could be the largest short-circuit current which may occur in the circuit.
If there is a threat of short-circuit current over the maximum breaking capacity of the circuit-breaker, standard safety fuses can be placed to reinsure the system security. The logic here is not to prevent the damage of the circuit breaker but to prevent severe costly damages of the protected equipment due to unsuccessful interruption of the current. Moreover, when choosing a circuit-breaker it is important to know where it will be mounted. If the power circuit breaker is intended to be mounted on a DIN rail, its housing should be designed for installation on this type of rail. If the circuit-breaker will be installed in a terminal panel, we should check whether the current-carrying rails correspond to the circuit-breaker.
Sometimes the power circuit breaker is mounted directly to the protected electrical mechanism. The use of circuit breakers in automated systems for building installation is now commonplace and sometimes there is a sensor that signals remotely the state of the circuit-breaker to a centralized control center, so that if there is an indication for a broken circuit-breaker, the repair groups can take immediate measures and provide as soon as possible power to the residential or public building. Huge range of quality and reliable circuit breakers can be found here.