QOU Miniature Circuit Breakers and Switches

Circuit Breaker Tripping Characteristics (Trip Curves)

19

09/2005

© 2005 Schneider Electric All Rights Reserved

CIRCUIT BREAKER TRIPPING CHARACTERISTICS (TRIP CURVES)

The tripping characteristics of thermal-magnetic circuit breakers are represented by a characteristic

tripping curve that plots tripping time versus current level. See page 20. The curve shows the amount

of time required for a circuit breaker to trip for overcurrent through the entire tripping range of the circuit

breaker. Manufacturing tolerances result in a curve that is a band bound by minimum and maximum

values of total 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 percent

of the circuit breaker rating at ambient temperature (40°C/104°F), the circuit breaker will automatically

open the circuit within limits specified by the band.

Thermal Tripping Characteristics

The upper left portion of the characteristic tripping curve displays the thermal response of the circuit

breaker. On overcurrent levels, up to the instantaneous tripping level, thermal tripping occurs when the

bimetal in the circuit breaker responds to the heat associated with the overcurrent. The bimetal

deflects, unlatching the mechanism and mechanically causing the circuit breaker to trip and open the

circuit. The larger the overcurrent, the faster the circuit breaker operates to open the circuit (inverse

time).

Magnetic (Instantaneous) Tripping Characteristics

The lower right portion of the characteristic tripping curve displays the magnetic (instantaneous)

tripping response of the circuit breaker. This takes place when overcurrents of sufficient magnitude

operate the magnetic tripping mechanism. Magnetic tripping occurs with no intentional delay.