2-25
Figure 2-21.—Thermal-magnetic element action:
In the thermal-magnetic trip element circuit breaker, a magnetic element (electromagnet) is
connected in series with the circuit load, and a bimetallic element is heated by the load current. With
normal circuit current, the bimetallic element does not bend, and the magnetic element does not attract the
trip bar, as shown in figure 2-21, view A.
If the temperature or current increases over a sustained period of time, the bimetallic element will
bend, push the trip bar and release the latch. The circuit breaker will trip as shown in figure 2-21, view B.
If the current suddenly or rapidly increases enough, the magnetic element will attract the trip bar,
release the latch, and the circuit breaker will trip, as shown in figure 2-21, view C. (This circuit breaker
has tripped even though the thermal element has not had time to react to the increased current.)
Q30. What are the five main components of a circuit breaker?
Q31. What are the three types of circuit breaker trip elements?
Q32. How does each type of trip element react to an overload?
TRIP-FREE/NONTRIP-FREE CIRCUIT BREAKERS
Circuit breakers are classified as being trip free or nontrip free. A trip-free circuit breaker is a circuit
breaker that will trip (open) even if the operating mechanism (ON-OFF switch) is held in the ON
position. A nontrip-free circuit breaker can be reset and/or held ON even if an overload or excessive heat
condition is present. In other words, a nontrip-free circuit breaker can be bypassed by holding the
operating mechanism ON.
Trip-free circuit breakers are used on circuits that cannot tolerate overloads and on nonemergency
circuits. Examples of these are precision or current sensitive circuits, nonemergency lighting circuits, and
nonessential equipment circuits.Nontrip-free circuit breakers are used for circuits that are essential for
operations. Examples of these circuits are emergency lighting, required control circuits, and essential
equipment circuits.
