???????????????????????????????????????????????????????????????? CIRCUIT BREAKER
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.
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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Electrical Circuit Breakers
In this the third part of the article covering circuit breakers, the following topics are covered:
Methods Of Mounting Circuit Breakers
For our study purposes we will divide the methods used to mount circuit breakers into three general groups: Fixed, Removable, and Drawout. A review of these mounting methods follows.
Fixed Mounted Circuit Breakers
A circuit breaker that is bolted in its enclosure and wired to the load frame, is called call a fixed mounted circuit breaker. These units are typically rated 600 volts or less and are front mountable. Power is provided to the breaker typically by wires or sectional type busbars. Power feeding the circuit breaker must be turned off in order to physically remove the fixed mounted breaker.
Removable Mounted Circuit Breakers
A removable circuit breaker has two parts, a base, which is bolted to and wired to the frame, and the actual breaker, which has insulated parts that electrically mate with the base. This means of mounting allows the unit to be replaced with out re-wiring the unit on the line side of the breaker. This type of mounting is typically used for breakers rated 600 volts or less.
Drawout Mounted Circuit Breakers
A drawout circuit breaker also has two parts, the base, which is bolted and wired to the frame and the actual breaker, which slides into and electrically mates with the base. This allows the unit to be replaced without having to turn off the power feeding the breaker. The load must be turned off in order to test or remove the unit.
As a safety feature these units are interlocked to automatically turn the power off just before removal of the breaker begins. By design, only the circuit breaker?s load must be turned off to remove the breaker. This method of mounting allows for a single breaker to be disconnected from the power supply. That is to say that it does not require that all of the power be disconnected from all of the breakers installed in the larger enclosure such as a motor control center.
There are various designs used to facilitate the ?racking-in?(installation) and ?racking out?(withdrawal) of the drawout type circuit breakers. Some utilize some form of jacking screw to initially move and thus electrically disengage the breaker, then a traveling trolley type of hoist (somewhat like a small boat winch) supports the breaker during removal and re-installation. A transient supporting device is necessary as these large (physical size) breakers are too heavy and too bulky to be safely moved into and out of position by one person.
Methods Of Securing Circuit Breakers
Circuit breakers are typically secured in place by one of the following methods:
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?Stab-Lock? Type Breakers
This method of circuit breaker mounting utilizes a male-female type of plug and receptacle connection to a metal busbar on one end. The opposite end of the breaker is mated to the enclosure housing and does not make electrical contact with the busbar and is not part of the electrical circuit. These types of breakers are found in homes and light commercial applications installed in loadcenters. With this method of mounting, some movement of the circuit breaker case is normal. This small amount of breaker case movement is typically 1/8 of an inch or less on the busbar end. Single pole breakers tend to exhibit the greatest amount of movement due to their greater aspect (width to height) ratio. The circuit conductor termination lug may (but not necessarily) also exhibit some minor movement of the termination lug; again this movement is less than about 1/8 of an inch.
On occasion the line connection(s) may lose some gripping force overtime; when this occurs one may be tempted to re-pinch the connection closed just a bit. This pinching should be avoided, as repeated bending results in the metal losing its strength. When a loose connection has become evident, the breaker should be replaced. The busbar and its insulation should also be inspected for indications of overheating related damage. With this mounting method, voltage drop test should be done from the loadcenter or panelboard?s main lugs to the breaker?s load terminal(s).
Bolted Type Breakers
When a longer service life breaker is wanted, a bolted type is typically used. These types have a metal tab (one for each phase) sticking out from one end that is bolted to the busbar with a machine screw (bolt type fine threads and not sheet metal screw type steep pitch threads). When replacing these types of breakers, the retaining bolts or machine screws will have power on them unless power to the entire panelboard has been removed.
It is not uncommon for some individuals to determine that it is necessary to replace these types of breakers with power still applied to the busbars. I am not a big fan of working any thing above 12 volts hot, for I have witnessed too many good folks get hurt doing what was initially anticipated as being a quick and simply task. When this type of breaker must be replaced with power still applied to the busbars, it should be done only under strict safety procedures; using proper personnel protective equipment and double insulated tools (everyday plastic handle screwdrivers must not be used). A detailed job safety analysis should be conducted before any hot work is undertaken. On more than one occasion I have witnessed some highly competent professionals conducting rehearsals of this type of activity several times until each safety step was done correctly. Take the time you need to be safe.
Din Rail Mounted Breakers
With this method, a mounting rail is secured to the enclosure and the breaker is snapped onto the mounting rail. This allows replacement to be done quickly as the device can be unclipped and a new one clipped on to the DIN rail. Conductors for the supply and load are typically secured to the breaker using pressure connectors that are tightened by some type of threaded fastener. While not as easy to replace as a stab-lock type breaker, this method does allow for some saving of time, both during panel building and individual breaker replacement later on.
The letters DIN stand for German Industry Standards. DIN rails are available in more than one physical size. The DIN rail mounting method is increasingly replacing thru-bolt, foot, and plate mounting methods once more commonly used.
In the next part of this article the following topics will be covered:
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