A time-current curve (TCC) for circuit breakers is a graphical representation that shows how long a circuit breaker will take to trip at various levels of overcurrent. It illustrates the relationship between fault current magnitude and tripping time, helping engineers design protection systems that are both safe and selective.
What the Time-Current Curve Shows
- X-axis: Current (as a multiple of the breaker’s rated current, In)
- Y-axis: Time to trip (typically on a logarithmic scale, in seconds or milliseconds)
Key Curve Regions
- Long-Time Region (Overload Protection)
- Protects against modest overcurrents (e.g., 1.2–6× In)
- Trip times range from seconds to minutes
- Allows temporary inrush currents (like motor starts) without nuisance tripping
- Short-Time Region
- Handles higher overcurrents (e.g., 6–10× In)
- Trips in tens to hundreds of milliseconds
- May include adjustable delay for coordination with downstream devices
- Instantaneous Region (Short-Circuit Protection)
- Responds to very high fault currents (e.g., 10× or more)
- Trips almost immediately—within a few milliseconds
Why Time-Current Curves Matter
- Selectivity/Coordination: Ensure only the breaker closest to the fault trips, preventing wider outages.
- System Protection: Match breaker performance to equipment thermal and mechanical limits.
- Safety and Compliance: Validate system design against NEC, IEC, or UL standards.
Example
A 100A circuit breaker might have a TCC like this:
|
Current |
Trip Time |
|
120A (1.2×) |
20 seconds |
|
300A (3×) |
2 seconds |
|
1,000A (10×) |
< 0.1 seconds |
Summary
A time-current curve is an essential tool for:
- Sizing breakers
- Protecting equipment
- Coordinating multiple protection devices
- Ensuring system reliability and safety
