Circuit breakers are used in nearly all electrical systems, from residential homes to large electric utility facilities. They're essential to the protection of our systems from over-currents and short circuits. There are several types of circuit breakers used, depending on what kind of voltage, or load, you're using and the nature of your facility.
There are five popular types of circuit breakers:
Molded Case Circuit Breakers (MCCB): these circuit breakers work for smaller currents (100A-1000A), providing protection by combining a temperature sensitive device with a current sensitive electromagnetic device.
Insulated Case Circuit Breakers (ICCB): An ICCB is just an MCCB with an insulted frame. MCCBs use an iron frame and ICCBs take out the iron and use plastic, for insulation, in low-voltage scenarios.
Low-Voltage Power Circuit Breakers (LVPCB): these circuit breakers are considered the most rugged and versatile circuit breakers because they can withstand faults for up to 30 cycles (1/2 sec). They can handle from 800A-2500A, are field maintainable and very reliable.
Medium-Voltage Vacuum Circuit Breakers (MVVCB): This breaker uses the same components as the low voltage counterparts, except they use vacuums bottles instead of contact assemblies and arc chutes.
The arc and main contacts are seen in the mid-range to low voltage circuit breakers, or LVPCBs. They are also called air-frame or draw-out circuit breakers, since there is a connecting piece that jumps the current from one contact to the next. There are three types of circuit breaker contacts:
- Arcing, which transfers the arc, or the heated, glowing electrical current, to the arc runners (contacts) in the arc chute (contact connectors).
- Main, which carries the main load current.
- Auxiliary, which makes and breaks the control circuits.
Arcing contacts are designed to prevent the main contacts from being damaged and are made from silver alloys, cadmium, tungsten and zinc. The tungsten, cadmium, and zinc make the arcing contacts harder, so when the contacts open and close they will not deteriorate as quickly. When the circuit breaker opens, the main contacts part first and then the arcing contacts part, "drawing out" the arc across the air gap. When the circuit breaker closes, the arcing contacts meet first, creating the bridge for the arc to cross. This way, the main contacts are protected from carrying the arc, preserving them.
The contact surfaces are shaped so that they have a rubbing motion, called "wipe". Wipe helps clean the contact surface, where one surface is contoured while the other is flat. Sometimes, arcing contacts with have a "horn" to facilitate the arc transfer.
The main contacts are much simpler. They are constructed of a softer alloy using less tungsten or zinc, and more silver. They carry the main load current through the breaker, so they have a lower resistance to current flow. The mains are larger, which also decreases their resistance.
Auxiliary contacts control electrical functions within the circuit breaker, such as turning the spring charging motor on and off at the appropriate times. On LVPCBs, auxiliary contacts are mounted on the frame of the breaker, typically seen as a large handle on the side that one physically flips up and down to cut or restore power.
Medium-voltage metal-clad switchgears will typically have the auxiliary contacts mounted in the switchgear, seen by a metallic latch on the side. These contacts are mechanically driven from the operating mechanism and are used for control and indicating circuits. They are connected to the operating mechanism by an internal linkage and operate at the same time that the main contacts do.