Trip curve of circuit breaker

Origin of trip curve

The concept of trip curve originated in the IEC world and is used to classify micro-circuit breakers (B, C, D, K and Z) from IEC standards. The standard defines lower and upper limits for trips, but manufacturers have the flexibility to determine the precise specifications within these thresholds that would cause their products to trip. Trip diagrams show the tolerance zones where the manufacturer can set the trip points of its circuit breaker.

Trip curve of circuit breaker

The characteristics and applications of each curve, from most sensitive to least sensitive, are:

Z: Trip at 2 to 3 times rated current, suitable for highly sensitive applications such as semiconductor equipment

B: Trip at 3 to 5 times rated current

C: Trip at 5 to 10 times rated current, suitable for medium inrush current

K: Trip at 10 to 14 times rated current, suitable for loads with high inrush current, mainly used for motors and transformers

D: Trip at 10 to 20 times rated current, suitable for high starting current

Reviewing the “Comparison of all IEC Trip curves” chart, you can see that higher currents trigger faster trips.

The ability to withstand impulse current is an important consideration in the selection of trip curves. Certain loads, especially motors and transformers, experience transient changes in current, known as impulse current, when the contacts are closed. Faster protection devices, such as b-trip curves, would recognize this influx as a failure and turn on the circuit. For these types of loads, trip curves with high magnetic trip points (D or K) can “pass” through the instantaneous current influx, protecting the circuit from false trip