Circuit Breaker: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.Below is one example of a self resettig thermal circuit breaker.
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 reaches a preset level (determined by 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.
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.Below is one example of a self resettig thermal circuit breaker.
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.
Circuit Breaker Selection
A typical circuit breaker operating time is given in Fig. 6.11. Once the fault occurs, the protective devices get activated. A certain amount of time elapses before the protective relays determine that there is overcurrent in the circuit and initiate trip command. This time is called the detection time. The contacts of the circuit breakers are held together by spring mechanism and, with the trip command, the spring mechanism releases the contacts. When two current carrying contacts part, a voltage instantly appears at the contacts and a large voltage gradient appears in the medium between the two contacts. This voltage gradient ionizes the medium thereby maintaining the flow of current. This current generates extreme heat and light that is called electric arc. Different mechanisms are used for elongating the arc such that it can be cooled and extinguished. Therefore the circuit breaker has to withstand fault current from the instant of initiation of the fault to the time the arc is extinguished.
Fig. 6.11 Typical circuit breaker operating time.
Two factors are of utmost importance for the selection of circuit breakers. These are:
- The maximum instantaneous current that a breaker must withstand and
- The total current when the breaker contacts part.
In this chapter we have discussed the calculation of symmetrical subtransient fault current in a network. However the instantaneous current following a fault will also contain the dc component. In a high power circuit breaker selection, the subtransient current is multiplied by a factor of 1.6 to determine the rms value of the current the circuit breaker must withstand. This current is called themomentary current . The interrupting current of a circuit breaker is lower than the momentary current and will depend upon the speed of the circuit breaker. The interrupting current may be asymmetrical since some dc component may still continue to decay.
Breakers are usually classified by their nominal voltage, continuous current rating, rated maximum voltage, K -factor which is the voltage range factor, rated short circuit current at maximum voltage and operating time. The K -factor is the ratio of rated maximum voltage to the lower limit of the range of the operating voltage. The maximum symmetrical interrupting current of a circuit breaker is given by K times the rated short circuit current.
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