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QOU245

QOU Miniature Circuit Breakers and Switches Unit Mount (Cable-in/Cable-out)

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QOU Miniature Circuit Breakers and Switches

Application Information

Thermal Trip

The thermal trip element of a circuit breaker is a root-mean-squared (rms) current sensing device. The

thermal element, or bimetal, is constructed from metals with dissimilar rates of expansion bonded

together. The thermal element responds to overloads by reacting to the heat generated both by the

current flowing through the circuit breaker and by the heat contribution from ambient conditions. The

bending force of the bimetal causes the circuit breaker to trip (see Figure 1). The deflection of the

bimetal is predictable as a function of current and time. This is the inverse time tripping characteristic of

the thermal element, i.e., the tripping time decreases as the magnitude of the current increases.

Square D calibrates the thermal elements and they are not field-adjustable. The thermal trip elements

are calibrated for 40° C (104° F) ambient temperature, per UL Standard 489 and CSA Standard C22.2

No. 5-02.

Bimetal

Latch

Circuit Breaker

Contacts

Current In

Current Out

Spring

Normal Conditions

Thermal Trip

Figure 1: Thermal Tripping

Magnetic Trip

The magnetic (instantaneous) trip element uses an electromagnetic assembly to trip the circuit breaker

instantaneously (with no intentional delay) at or above a predetermined current value. During a short

circuit of sufficient magnitude, the high-level current passing through the conductor rapidly increases

the magnetic field of the electromagnet that attracts the armature. As the armature is drawn toward the

electromagnet, it initiates an unlatching action and opens the circuit breaker contact (see Figure 2).

Armature

Latch

Spring

Conductor

Circuit Breaker

Contacts

Current Out

Electromagnet

Current In

Normal Conditions

Figure 2: Magnetic Tripping

Trip Indicator

Normal Conditions

When the QOU miniature circuit breaker is tripped, the handle assumes a position between ON (I) and

OFF (O) and the red Visi-Trip® indicator (A) appears in a window in the circuit breaker case. Reset the

circuit breaker and Visi-Trip indicator by pushing the handle to OFF and then to ON.

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© 2005 Schneider Electric All Rights Reserved

09/2005

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