Introduction to trip curves for UL 489 Miniature Circuit Breakers and UL 1077 Supplementary Protectors
Trip Curves, aka Time Current Curves, can be an intimidating topic. The goal of this short paper is to introduce you to the concept of trip curves and explain how to read and understand them.
- The X axis represents a multiple of the operating current of the circuit breaker.
- The Y axis represents the tripping time. A logarithmic scale is used in order to show times from .001 seconds up to 10,000 seconds (2.77 hours) at multiples of the operating current.
- Thermal Trip Curve. This is the trip curve for the bi-metallic strip, which is designed for slower overcurrents to allow for in rush/startup, as described above.
- Magnetic Trip Curve. This is the trip curve for the coil or solenoid. It is designed to react quickly to large overcurrents, such as a short circuit condition.
- The Ideal Trip Curve. This curve shows what the desired trip curve for the bi-metallic strip is. Because of the organic nature of the bi-metallic strip, and changing ambient conditions, it is difficult to precisely predict the exact tripping point.
- Circuit protection devices include fuses, miniature circuit breakers, molded case circuit breakers, supplementary protectors, motor protection circuit breakers, overload relays, electronic fuses and air circuit breakers.
- Trip Curves predict the behavior of circuit protection devices in both slower, smaller overcurrent conditions, and larger, faster over current conditions.
- Choosing the correct trip curve for your application provides reliable circuit protection, while limiting nuisance or false trips.
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