What are LV Circuit Breakers and How Do They Work?
Low Voltage (LV) Circuit Breakers are electrical protection devices designed to automatically interrupt current flow during overload or fault conditions in systems operating at 1000V or less. Unlike fuses which must be replaced after operation, circuit breakers can be reset after clearing a fault, providing both convenience and economy for electrical system protection.
LV Circuit Breakers work by detecting abnormal current conditions and mechanically separating contacts to interrupt the current flow. When excessive current passes through the breaker, it activates either a thermal or magnetic trip mechanism (or both). Thermal trips respond to prolonged overloads by using a bimetallic strip that bends when heated, while magnetic trips react to sudden high currents (short circuits) using an electromagnetic solenoid. Once tripped, the breaker mechanism locks open until manually or remotely reset, preventing automatic reconnection to a faulted circuit.
Key components and mechanisms in LV circuit breakers include:
- Contacts: Current-carrying parts that physically separate to interrupt current flow
- Arc Extinction System: Chambers or devices that safely contain and extinguish the electric arc formed during contact separation
- Trip Unit: The sensing element that detects abnormal conditions
- Operating Mechanism: Stores energy for rapid contact separation and maintains contact pressure when closed
- Terminals: Connection points for incoming and outgoing conductors
- Housing: Insulating enclosure containing the internal components
- Accessories: May include auxiliary contacts, shunt trips, undervoltage releases, or motor operators
Uses and Applications
LV Circuit Breakers serve critical functions across various electrical systems:
Residential Applications
- Main service disconnects for homes
- Branch circuit protection for lighting, receptacles, and appliances
- GFCI and AFCI protection for wet locations and arc fault prevention
- Dedicated protection for high-current appliances (HVAC, ranges, dryers)
- Subpanel mains for detached buildings or additions
Commercial and Institutional Buildings
- Main service entrance protection
- Feeder circuit protection
- Motor circuit protection
- Distribution panel mains
- Lighting circuit protection
- HVAC and mechanical equipment protection
- Emergency and standby power system protection
Industrial Applications
- Power distribution system protection
- Motor control center protection
- Machine tool and equipment protection
- Process control power protection
- Automated manufacturing line protection
- Large motor starting and protection
- Power factor correction equipment protection
Important Considerations Before Buying Used LV Circuit Breakers
Usage and Maintenance History
The operational history significantly impacts reliability and remaining service life:
- Verify age and total operation count if available
- Check for evidence of overheating, arcing, or mechanical damage
- Confirm if the breaker has interrupted fault currents and how many times
- Request maintenance and testing records
- Verify if the breaker has been reconditioned or refurbished
- Check contact condition and mechanism operation
- Inspect for signs of moisture damage or excessive dirt
- Confirm proper lubrication of mechanical parts
Specifications and Compatibility
Ensure the breaker meets your system requirements:
- Voltage rating must match your system (typically 120V, 208V, 240V, 277V, 480V, 600V)
- Current rating appropriate for the expected load
- Interrupting capacity (AIC/kA) adequate for your system's available fault current
- Physical dimensions and mounting type compatible with your equipment
- Terminal configurations suitable for your conductor sizes
- Trip characteristics appropriate for the application
- Frame size and type compatible with your switchgear or panelboard
- Required accessories (aux contacts, shunt trips, etc.)
Purpose and Application
Select the appropriate type for your specific needs:
- Standard thermal-magnetic vs. electronic trip units
- Fixed vs. adjustable trip settings
- Required protection functions (overload, short-circuit, ground fault)
- Environmental conditions at installation location
- Main vs. feeder vs. branch circuit applications
- Manual or remote operation requirements
- Special needs for selective coordination
- Compliance with specific code requirements
How Much Should I Budget for LV Circuit Breakers?
Pricing varies widely based on type, rating, and condition:
New LV Circuit Breakers
- Residential/small commercial molded case (15-100A): $20-$200
- Medium molded case (125-225A): $200-$1,000
- Large molded case (250-600A): $800-$3,500
- Power circuit breakers (800-1200A): $3,000-$8,000
- Large power circuit breakers (1600-5000A): $8,000-$30,000+
- Electronic trip units: 30-50% premium over thermal-magnetic
- Specialized breakers (GFCI, AFCI): 2-3x standard breaker cost
Used/Reconditioned LV Circuit Breakers
- Typically 30-70% of new pricing depending on age and condition
- Factory reconditioned with warranty: 60-80% of new pricing
- Field reconditioned: 40-60% of new pricing
- As-is condition: 20-40% of new pricing
- Testing and certification costs: $50-$500 per breaker depending on size
- Obsolete or hard-to-find models may command premium prices
Cost Factors to Consider
- Age: Newer breakers typically command higher prices
- Manufacturer: Premium brands maintain higher resale value
- Condition: Evidence of light use and good maintenance increases value
- Testing: Certification of proper operation adds value
- Obsolescence: Discontinued models may be more expensive due to limited availability
- Special features: Ground fault protection, zone selective interlocking, etc.
- Accessories: Included auxiliary devices increase value
- Documentation: Original manuals and test reports add value
When purchasing used LV circuit breakers, thorough testing is essential to ensure safety and reliability. Circuit breakers are mechanical devices subject to wear, and their ability to properly interrupt fault currents may be compromised if they've been improperly maintained or have interrupted significant faults in the past. Reputable suppliers will provide comprehensive testing documentation including contact resistance measurements, insulation resistance tests, and operational verification. For critical applications, factory reconditioned breakers with warranty coverage may offer the best balance of cost savings and reliability. Always ensure that used breakers meet current safety standards and are compatible with your specific application requirements.