Circuit Breaker Abstract

A circuit breaker was examined to determine how it works. Basically, it is designed to disconnect the current if the current passing through the breaker is higher than the allowable current. Its main parts consist of a switch, connecting pads, and the disconnect device. Since the breaker consists of many individual parts, the only improvement found was to combine some of the parts into a single part. Introduction Our group dissected a thirty-amp household Circuit-breaker. Its purpose is to protect electrical appliances from being damaged though excessive currents. A circuit breaker limits the amount of current that may safely enter a household electrical system via a predetermined amperage rating. It is placed in series between your house and the provided electricity. Any current in excess of the rated current amount will trip the circuit breaker in to a non-conducting or open path condition. The closed path is maintained until either magnetism or thermal expansion causes the circuit to trip. Device Description The circuit breaker has several features that are worth noting in the design discussion.

   First is its ability to detect various types of loading situations. The breaker can not only open a circuit in response to a current spike, but can also react to a sustained moderate current draw, just above its rated current. The circuit breaker also has an easily resetable 3-position switch, and various internal safety features such as spark arrestors and cavity vents. Operation of the circuit breaker is simple, but utilizes complex mechanisms. Essentially there are to main internal mechanisms; the trigger and the switch. The trigger is the device that senses the abnormal current load. A sharp spike in current will cause a magnetic field to form in the trigger, releasing the switch. A slightly elevated, but more constant current draw through the breaker will cause the bi-metal composition of the trigger mechanism to deflect in an arc like manner, which is also capable of releasing the switch. This reaction may take a longer amount of time to open the circuit, but is designed to provide the user with a short time of extended current draw through the circuit before the breaker opens. The switch, which is activated internally by the trigger or externally by the user, simply opens or closes a set of contacts which complete the circuit. The switch can be set to open or closed from the outside of the breaker's case, but can only be set to the tripped position internally, as a results of the trigger mechanism. Once the breaker has been tripped internally, it must be reset externally by switching it off, and then back on.

   The features enable the breaker to do several jobs at once, eliminating the need for multiple elements in the circuit. For example, it provides the user with an easily assessable on/off switch, fault protection against current spike, and fault protection against heavy current draw. The design of the trigger mechanism allows one internal part to the job of two, as does the switch, which by design can be shut off either internally or externally. Some problems that come to light with these features include a limited service life and expensive manufacturing. Limited service life is a function of build quality. Many internal parts move without precise surface preparation or lubrication, indicating that repetitive motion could wear out the assemblies. Several internal springs may also wear out over time. And, because there are may small parts that fit integrally with each other, manufacturing costs may be inflated. Discussion It has been discussed that the circuit breaker has many good design points, such as double fault protection, a manually switchable mode, and built in safety features to prevent spark ignition and meltdown. And it seems that there aren't many undesirable features of the unit, possibly with the exception of cost.

   Because of the complexity of the inner workings, manufacturability is more difficult, thus raising costs. A small inspection was done to determine if any parts could be eliminated or combined to reduce the build cost, but no immediate solutions were identified. It appears that all attempts to improve or simplify the device have been implemented and maximized, leaving the only avenue for complexity reduction to be a change in application for the part. Conclusion A common household thirty amp circuit breaker was dissected in lab this week. It consisted of hard plastic, moving metal parts, and two springs (Figure 1). The circuit is placed in series between an electrical power source such as Georgia Power and the fuse box or beginning of a home's electrical system. The circuit breaker operates via two mechanisms. The circuit is tripped or opened when either a high peak current exceeds the rated value of the circuit as when under constant thermal expansion the circuit experiences small spikes in the current that exceeds the amperage rating. The mechanism for tripping the circuit during a high peak value is magnetism. A metal sleeve when subjected to high peak voltages acts much as a solenoid, which magnetically attracts the triggering device allowing the tension in the spring to disconnect the contact, points and hence open the circuit.

   Similarly for the tripping mechanism under constant thermal expansion the expansion of the top of the bimetallic strip causes the trigger to be pulled down which has the same effect. The circuit breaker's best point is that the device is effective and compact. On the other had it is not 100% effective and must also be manually reset which can be problematic if it is dark and no lights are available with which to see the fuse box. Another bad point is that the circuit breaker has many moving parts, which complicates construction. Simpler or fewer parts might drive cost down and make manufacturing more efficient.

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