Circuit breakers are used in nearly all electrical systems — from residential dwellings to electric utility facilities. We rely on them to protect our systems from damaging overcurrents and short circuits. Understanding circuit breaker construction and operation is key to realizing their limitations and proper usage. This article covers basic components and operation of popular types of circuit breakers, including molded-case (MCCB), insulated-case (ICCB), low-voltage power (LVPCB), medium-voltage air-magnetic (MVACB), and medium-voltage vacuum (MVVCB) circuit breakers.

Key Components

MCCBs contain five components — a frame, operating mechanism, interrupting structure, trip unit, and terminal connections. These components are shown in Fig. 1.

    1. Frame — houses and supports the components and also provides insulation to contain the arc.
       
    2. Operating Mechanism — opens and closes the contacts.
       
    3. InterruptingStructure — includes the arc chutes and all current-carrying parts except the trip unit. Arc chutes are designed to interrupt the arc quickly, usually 1.5 to two cycles for MCCBs and ICCBs.
       
    4. Trip Unit — senses abnormal current flow and causes the operating mechanism to open the contacts. MCCB trip units are usually of the thermal-magnetic type.
       
    5. Terminal Connections — provides a suitable connection from the breaker to the conductor. Molded-case circuit breakers typically bolt directly to the bus.

Most manufacturers’ MCCBs have similar components and similar appearance.

ICCBs have the same basic construction as MCCBs, but use solid-state and digital trip units (as opposed to thermal-magnetic trip units) and have much higher interrupting ratings. ICCBs are often draw-out type, as opposed to bolt-in, although they can be either.

LVPCBs (Photo 1 at right) are also known as air-frame and draw-out circuit breakers. There are five major assemblies on a typical LVPCB:

    1. Disconnects or Stabs
      • Main disconnects — connect the circuit breaker to the main bus.
      • Secondary disconnects — connect the circuit breaker to the control circuits.
      • Ground disconnect — connects the circuit breaker to the ground bus.
    2. Contacts
      • Arcing — transfers the arc to the arc runners in the arc chute.
      • Main — carry the main load current.
      • Auxiliary — make and break the control circuits.
    3. Arc Extinguishers or Arc Chutes — interrupt and contain the arc.
       
    4. Operating Mechanism — opens and closes the contacts.
       
    5. Overcurrent Trip Device — modern circuit breakers have digital trip units, although older units could be air dashpot or oil dashpot types. Typical functions are long-time delay (LTD), short-time delay (STD), instantaneous (INST), and ground fault (GF).

Medium-voltage circuit breakers have the same basic components as their lower voltage counterparts, but use protective relays that are separately mounted in the switchgear. MVVCBs use a vacuum bottle instead of contact assemblies and arc chutes found on MVACBs. Photo 2 (right) shows a typical MVACB, while Photo 3 shows an MVVCB.

Main Disconnects (stabs)

The main disconnects connect the circuit breaker to the bus. LVPCBs typically use either vertical or horizontal rows of straight, spring-loaded fingers to make contact with the bus bar, although on larger frame sizes they may have round or “tulip-style” disconnects. Photo 4 shows the backboard assembly of an LVPCB with primary, secondary, and ground disconnects clearly visible.

Most medium-voltage metal-clad circuit breakers will use “tulip-style” disconnects (Photo 5) and have some type of spring around them to ensure good contact with the bus. Medium-voltage, draw-out circuit breakers have the primary disconnects in the switchgear guarded by a shutter mechanism installed in the switchgear cubicle. As the circuit breaker is removed (racked out), the shutter closes. When the circuit breaker is racked into the “connected” position, the shutter opens, allowing the primary disconnects to make.

Secondary Disconnects (stabs)

The secondary disconnects bring power from the control circuits into the circuit breaker. Photo 4 shows an LVPCB and its secondary disconnects. The typical arrangement for this breaker is to connect and disconnect these stabs as the breaker is racked in and out of its cubicle. Photo 6 shows the typical arrangement for an MVACB, which often has to be manually connected.

Ground Disconnect (contact shoe)

The frame of the circuit breaker must be connected to the ground bus so that a short circuit or fault will be taken to ground immediately and allow the protective devices to operate as quickly as possible. The ground disconnect connects the frame of the circuit breaker to the ground bus (Photo 7 at right). The ground disconnect is the first connection made when racking a breaker in and is the last connection broken when racking it out. This ensures that the frame of the breaker is grounded whenever there is a possibility of the frame being energized.

Arcing Contacts

Arcing contacts are designed to prevent the main contacts from being damaged and can be made of alloys of silver, 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 the arc across them. When the circuit breaker closes, the arcing contacts make first, again drawing the arc across them. This prevents the main contacts from carrying the arc and preserves them.

The contact surfaces are shaped so that they have a rubbing motion, referred to as “wipe.” Wipe helps clean the contact surface, and is caused by one of the contact surfaces being contoured and the other surface being flat. When the contacts close, the contoured surface will have a wiping motion against the flat surfaced contact. Arcing contacts will usually have an arcing “horn” on the very top of the contact structure. The arcing horn aids in transferring the arc from the arcing contact to the arc runner in the arc chute.

Main Contacts

The main contacts are constructed of a softer alloy using less tungsten or zinc and more silver. They carry the load current, so they must have a lower resistance to current flow. The mains are larger, which also decreases their resistance.

Auxiliary Contacts

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 (Photo 8 at right). Medium-voltage metal-clad switchgear will typically have the auxiliary contacts mounted in the switchgear, rather than the breaker frame. Auxiliary contacts are mechanically driven from the operating mechanism and are used for control and indicating circuits. They are connected to the operating mechanism by a linkage and operate at the same time the main contacts do.