Circuit breakers have replaced fuses in most electric circuits. As these are easy to select, install, and maintain, circuit breakers are more commonly employed as protection against over-current and shorts. For different applications, there are various types of circuit breakers available in the market. We will discuss two of them.
Air Circuit Breakers
Air circuit breakers use air as the medium for extinguishing the arc formed when the breaker trips. The air is usually kept in compressed form inside a cylinder. As the breaker trips and the contacts separate, the compressed air, blown through specially designed nozzles, drives the arc into a special arc chute.
Electric circuits working in the Low Voltage (LV) range use air circuit breakers extensively and they can interrupt currents of several thousand amperes. Air circuit breakers work with an inherent current sensing mechanism, which is similar to the thermo-magnetic release.
To weaken the arc, the air circuit breaker uses a component called the arc chute. This has a number of splitters to split the arc into several sections—increasing the length of the arc—thereby quenching it. The arc chute is usually made of composite refractory material, and pressurized air drives the arc, formed during the separation of the fixed and moving contacts, into this arc chute.
The industry uses air circuit breakers widely for protection of facilities and transmission lines, and these breakers are available in both three- and four-pole versions. Air circuit breakers protect electric equipment such as motors and transformers as well. Ships and mines also use air circuit breakers for protection.
Magnetic Hydraulic Circuit Breakers
Magnetic Hydraulic Circuit Breakers use the principle of magnetic effects of current to operate. Although similar in operation to the standard magnetic circuit breaker, the magnetic hydraulic circuit breaker differs as the latter has a hydraulic time delay mechanism.
The delay comes from forcing the core to move through a cylinder filled with silicone fluid. During over-current, the magnetic field it creates pulls the core into the cylinder. As the core has to pass through the silicone fluid in the cylinder, it slows down, introducing the time delay.
Therefore, after a momentary over-current has passed, the core can return to normal, without the circuit breaker tripping. With a persistent over-current, the core ultimately reaches the coil.
As the core enters the coil, it changes the reluctance of the magnetic circuit. This increases the resultant flux, which ultimately attracts the armature causing the protecting device to trip and thereby, separate the contacts.
With the contacts separating, the over-current reduces to zero. This kills the magnetic field, releasing the core to return to its original position.
This feature is a great advantage of the magnetic hydraulic circuit breakers—they automatically reset immediately after tripping. Other circuit breakers with thermal over-current elements require a cooling period before they can be reset.
Independent of the ambient operating temperature, magnetic hydraulic circuit breakers offer accurate and reliable over-current protection. The industry uses them extensively as they are relatively inexpensive and available for both AC and DC applications.
