Page 175 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 8 Circuit Protection 137

Circuit Breakers trips, the button extends out and provides a visual indicator. A
panel mount breaker should always have the current rating
Circuit breakers are functionally resettable fuses. Most of us clearly printed on the face of the button and the side of the
are familiar with the circuit breakers in our homes and have body.
reset a few over the years. Circuit breakers are also found in Multipole, panel mount circuit breakers are typically
virtually every type of electrical circuit made. Circuit break- ganged, single-pole units with flip-type actuators. The actua-
ers can be found in the smallest appliances, with ratings as tors are pinned together so that if one breaker trips it will trip
low as a few milliamps, all the way through to huge power the other two at the same time, thus protecting the entire cir-
distribution systems with ratings in the million amp range. cuit. Figure 8-8 shows a multipole, panel mount circuit
Circuit breakers provide a level of convenience that a fuse breaker assembly.
cannot match; however, this convenience comes at a cost.
Circuit breakers are typically much more expensive than the
equivalent fuse.
Figure 8-6 shows a stylized schematic of a thermal circuit
Clamp
breaker. Power is fed through a bimetal strip, flexible cable,
Screws Circuit Breakers
flat spring, and a set of contacts. If the current exceeds the rat-
Serrated
ing of the breaker, the bimetal strip heats and curves up,
End Terminals
releasing the latch and allowing the contacts to open. After the Connector Pin
bimetal strip cools the contacts can be reset.
Flip Levers
For panel mount applications, small circuit breakers are
available that have the same general appearance as a push but- Spacers
ton. Figure 8-7 shows a panel mount unit. When the breaker
Washers
Nuts
Figure 8-8 Multipole, Panel Mount Circuit Breaker
Contacts
Bimetal Strip Latch Assembly

Terminal

Terminal Flexible Cable Older light bulb base fuses are available in circuit breaker
versions. Although these units are not very common anymore,
Spring they are convenient units for bench work. A panel of light bulb
Closed
sockets can be set up and an assortment of breakers can be kept
on hand. Simply selecting the appropriate rating and screwing
it into a socket provides ready protection for any given project.
Figure 8-9 shows a light bulb base circuit breaker.

Open Screw Terminal
Reset Button
Figure 8-6 Thermal Circuit Breaker Schematic
Button Terminal Tripped Position

Mounting Nut
Body
Reset Button
Figure 8-9 Light Bulb Base Circuit Breaker
Body
Tripped Position
Terminals
The power centers or breaker boxes in homes and offices
10 AMP
10 use specially designed circuit breakers that are easily inserted
240 VAC
and removed. Figure 8-10 shows a typical power center cir-
cuit breaker. The mounting socket is clipped onto a rail and
Current Rating then the top of the breaker is rotated in until the input termi-
Figure 8-7 Push Button, Panel Mount Circuit Breaker nal engages the power buss. The output is generally a screw