Low voltage (less than 1000 VAC) types are common in domestic, commercial and industrial application, and 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.
There is three main types for MCB's: 1. Type B - trips between 3 and 5 time full load current; 2. Type C - trips between 5 and 10 times full load current; 3. Type D - trips between 10 and 20 times full load current; In UK all MCB's MUST be selected in accordance with BS 7671.
MCCB (Molded Case Circuit Breaker)—rated current up to 2500 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 low-voltage switchboards or switchgear cabinets.
The characteristics of Low Voltage 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 electric motor operators so they can trip (open) and close 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, such as DC for subway lines. Direct current requires special breakers because the arc is continuous—unlike an AC arc, which tends to go out on each half cycle. A direct current circuit breaker has blow-out coils that generate a magnetic field that rapidly stretches the arc. Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel.
The 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
