Low voltage circuit breakers

Low voltage (less than 1000 VAC) types are common in domestic, commercial and industrial application, include:
MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category. MCCB (Molded Case Circuit Breaker)—rated current up to 1000 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings. Low voltage power circuit breakers can be mounted in multi-tiers in LV switchboards or switchgear cabinets. The characteristics of LV circuit breakers are given by international standards such as IEC 947. These circuit breakers are often installed in draw-out enclosures that allow removal and interchange without dismantling the switchgear.

Large low-voltage molded case and power circuit breakers may have electrical motor operators, allowing them to be tripped (opened) and closed under remote control. These may form part of an automatic transfer switch system for standby power.
Low-voltage circuit breakers are also made for direct-current (DC) applications, for example DC supplied for subway lines. Special breakers are required for direct current because the arc does not have a natural tendency to go out on each half cycle as for alternating current. A direct current circuit breaker will have blow-out coils which generate a magnetic field that rapidly stretches the arc when interrupting direct current.
Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel.

Photo of inside of a circuit breakerThe 10 ampere DIN rail-mounted thermal-magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe. The design includes the following components:
Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off/tripped). Most breakers are designed so they can still trip even if the lever is held or locked in the "on" position. This is sometimes referred to as "free trip" or "positive trip" operation. Actuator mechanism - forces the contacts together or apart. Contacts - Allow current when touching and break the current when moved apart. Terminals Bimetallic strip Calibration screw - allows the manufacturer to precisely adjust the trip current of the device after assembly. Solenoid Arc divider / extinguisher