Specification

and QOB Miniature Circuit Breakers

Trip Curves

07/2008

Trip Curves

The tripping characteristics of QO and QOB circuit breakers can be represented by a characteristic

tripping curve that plots tripping time versus current level. The curve shows the amount of time

required by a circuit breaker to trip at a given overcurrent level. The curve has a performance band that

is bound by a minimum and a maximum value of clearing time. Total clearing time is the sum of the

sensing time, unlatching time, mechanical operating time and arcing time of the circuit breaker. For

currents in excess of 135% of the circuit breaker rating at rated ambient temperature (40°C), the circuit

breaker will automatically open the circuit within limits specified by the band.

Thermal Tripping Characteristics

The upper left portion of each trip curve displays the thermal response of the circuit breaker. On low-

fault current levels, up to the magnetic tripping level, thermal tripping occurs when a bimetal in the

circuit breaker responds to heat associated with the overcurrent. The bimetal deflects, unlatching the

mechanism and mechanically causing the circuit breaker to trip and open the circuit. The greater the

overcurrent, the faster the circuit breaker will operate to clear the circuit.

Magnetic Tripping Characteristics

The lower right portion of each trip curve displays the magnetic tripping response of the circuit breaker.

This takes place when overcurrents of sufficient magnitude operate in an internal magnetic armature

which unlatches the mechanism. Magnetic tripping occurs with no intentional time delay.

Figure 9: Typical QO Trip Curve

Thermal Tripping

Magnetic Tripping