A circuit breaker's function is, like a fuse, to break a circuit path when a predetermined amount of current is passed. In my opinion, circuit breakers should never be used to protect electronic devices such as radios, amplifiers or crossovers. Most common circuit breakers (thermal snap action) take far too long to open the circuit path. This does not mean that they are not useful. When they are properly selected they do a good job of protecting wiring and devices such as electric motors. Some breakers are self resetting. Others require manual resetting. I strongly recommend using a manual reset type. This will allow you to watch for any problems when the circuit path is restored.

Thermal Circuit Breakers:
The diagram below shows the simplified version of a self resetting circuit breaker. In this device, the current flows from the battery terminal, through the bi-metal strip and then to the other terminal. The bi-metal strip is made of two different types of metal which have different coefficients of expansion. This means that one will expand more than the other when the rise in temperature is the same for both pieces. In this case, the two metals are bonded to each other. (now keep in mind that this is a simplified diagram) When the strip heats up from the current flow through it, one type of metal expands more than the other. In this case, the black metal expands more than the red and the strip tends to bend upward and disconnect the contacts. You can see that the metal starts to bend as the current increases. When the temperature reaches a given point, the piece will snap into the open position and the current flow will stop. The bi-metal strip is stamped into a special shape which causes the 'snap' action. This will assure that there is EITHER a solid connection OR a complete disconnect. You can see a similar snap action in the top of some soda cans. If you push down on the top it starts to bend downward. After the pressure reaches a certain point, the top will snap down. If you release the pressure slowly, the top will snap into it's original position. This is what happens when the bi-metal strip cools in the breaker.

The two devices below are self-resetting circuit breakers. The top one is used to replace the glass fuses in older vehicles. The second one is a universal type that can be used in place of a fuse.

Earlier, a thermal snap breaker was mentioned. They use the shape of the diaphragm (made of bi-metal material) to prevent the breaker from opening and closing too often/quickly when there is an overload. The breaker below is the same breaker that's at the top of the page and again above (the one used to replace glass fuses). This breaker uses a different method to slow down the action of the breaker. The wires wrapped around the bi-metal strip form what is essentially a heater. When the circuit opens because the bi-metal strip has overheated, current will begin to flow through the coil of wire (which is connected to both terminals of the breaker). If there is a short circuit that has not cleared, current will flow through the wire and will keep the bi-metal strip hot. Since the bi-metal strip cannot cool, it cannot straighten out and close the contacts. This breaker won't open the circuit completely but the current flow through the tiny wire is limited so the circuit (which can safely carry the rated current for the breaker -- 30 amps here) is in no danger of being damaged.

The circuit breaker below doubles as a switch. When working on the wiring for your system, you should disconnect the main power cable (going to your amps) from the battery. The problem is... too many people are too lazy to do it. With this, all you have to do is push the button and it breaks the circuit. When you're done, close the red lever and it's reconnected. If this breaker is tripped, it must be manually reset.

Magnetic Circuit Breakers:
Some circuit breakers use a magnetic actuator to trip the circuit. In this type of breaker, the current flow through the electrical device (amp, fog lights...) passes through an electromagnetic actuator. When the current flow exceeds the current rating of the breaker, the magnetic field in the electromagnet is strong enough to trip the breaker and allow the contacts to open. This type of breaker generally has to be manually reset. A well designed 'magnetically' actuated circuit breaker can operate very quickly (possibly as fast as or faster than a fuse of equal current rating).

In the following diagram you can push the 'overcurrent trip' button to simulate too much current flow and trip the breaker. Then press the 'reset' button and watch the breaker reset. The 'show legend' button will show you a legend of the parts. Keep in mind that this is just a generic diagram and doesn't depict any particular breaker.

Thermal/Magnetic Breakers:
Some breakers use both thermal and magnetic trip functions. The magnetic function works the same as the previous explanation. The thermal part functions a little differently than the previous example. In the combination breaker, the bi-metal strip is more likely to be used to trip the breaker internally (by tripping the latch) instead of pulling the contacts apart when heated.

Reliability:
In my opinion and from my experience, circuit breakers are less reliable than fuses (especially when the breaker is mounted in the harsh environment under the hood). Quality fuses like ANL and Maxi fuses have a solid element (no solder connections) and will almost never have an intermittent or poor electrical connection. A circuit breaker will eventually have higher contact resistance than when it was new. This is especially true if the breaker has been tripped (by overcurrent) more than a few times. If you're going to compete and can't take a chance of having a problem like a bad connection in the power line, you should use a good quality fuse. Now I know that people have had fuses blow in competition but it was because the fuse was not properly rated, not because the fuse was defective. For those who have had trouble with glass fuses, read the fuses page of this site.

You should remember:
1.Do not use a circuit breaker in place of a fuse to protect electronic components such as amplifiers.