preface This is a non-technical article about typical thermal-magnetic circuit breakers found in American homes. After reading this page, you will know what the circuit breakers in your house do, and how to use them. To begin, here is the National Electrical Code (NEC) definition of a circuit breaker: Circuit Breaker: A device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating. The NEC definition points out four important features of a circuit breaker: (1) The circuit can be opened (turned off) or closed (turned on) by hand, typically by using the handle on the circuit breaker. (2) The circuit breaker will open automatically when too much current is drawn through the circuit it protects. (3) It will protect the circuit without destroying itself in the process. (4) Correct choice and installation is important for safe operation. what a circuit breaker does A circuit breaker protects against too much current. Consider for a moment, the amount of power needed to run all the refrigerators, street lights, hot tubs, and beer signs in your city. It's enormous, and available right on the power lines – just hook up to it. The amount or power delivered to any electrical appliance is determined by the design of the appliance. A 1500-watt heater plugs into the same outlet as a night light, yet each draws just the power it needs. Suppose something goes awry with the appliance. What prevents the enormous power available at the power lines from entering the house and creating havoc? The answer is that there is a limit placed by the house circuit breakers upon the amount of power that can be drawn from a circuit. When the current flowing in a circuit exceeds safe limits, the circuit breaker will trip, removing power from the circuit, and preventing disaster. I recommend that you identify all of the circuit breakers in your house and label them. A circuit breaker isolates a circuit fault. When a circuit breaker trips, the entire house usually does not go dark. An electrical short or overload will trip the circuit breaker that protects a single circuit, causing a few outlets, or some of the lights, to shut off, while leaving most of the house unaffected. This property is called "automatic fault isolation". One can also turn off a circuit breaker that protects a circuit until repairs can be made. This is, of course, "manual fault isolation". how circuit breakers are organized The main circuit breaker protects and establishes the rating of the electrical service. Electrical service for a residence usually ranges from 60 amps to 400 amps, with most single-family homes having a service sized from 100 to 200 amps. The entire building's electrical service will be protected by one or more (usually one, but as many as six are allowed by the NEC) main circuit breakers. Because these circuit breakers are the means of disconnecting power to the entire house, they are also known as the service disconnect, and should be labeled "service disconnect". In an emergency, such as a fire or earthquake, you may need to shut off your electricity, gas, and water in a hurry, and under stress. You should find and label the main breaker, main gas valve, and main water valve for your house. Know where they are and how to turn them off. Provide a dedicated tool for turning off each valve, and tie it to the valve with three feet of strong nylon cord. The branch circuit breakers protect the wiring for individual circuits. Smaller circuit breakers distribute power throughout the building. The circuit breakers for these branch circuits are placed in cabinets called distribution panels or load centers. Frequently, the main and branch circuit breakers are located in the same panel. The load center has a maximum total current rating, which will be at least as great as that of the main circuit breaker which protects it. The load center will contain a number of circuit breakers, each of which protects a single circuit, or perhaps a single large appliance. Typical current ratings are 15 or 20 amps for outlets and lights, 30 amps for electric dryers, 40 amps for an electric range or oven, or 50 amps for a hot tub. The circuit breaker will be chosen to carry the current required by the load it supplies, and the associated wiring will be sized to safely carry this load. how a circuit breaker works Because of the slow reaction of the time-current trip element, it is often necessary to allow a circuit breaker to cool before it can be reset. Usually a minute or two is sufficient, but give it an hour or so before calling an electrician. Overheated adjacent circuit breakers can greatly prolong the recovery time, and can also cause tripping at a lower current. A circuit breaker is, fundamentally, a switch; it allows current to flow when "closed", and prevents current flow when "open". The kind of circuit breaker we are discussing here is called a thermal-magnetic circuit breaker. The name comes from the two independent methods of causing the breaker to trip, or change from the closed to the open state. It is also possible to open the breaker manually with the handle. The thermal trip protects wiring against overheating due to overload. A wire carrying current will always generate heat. The design goal of the protection system is to prevent the temperature of wiring from reaching a level that could damage the wiring or start a fire in adjacent flammable material. While it is not practical to directly measure the temperature of the wiring, it is possible to model the heating effects of current flow within the circuit breaker, and operate the safety trip based on the predictions of that model. The modeling is performed by a small coiled spring made of two layers of different metals bonded together. The two metals expand in response to increased temperature, but by different amounts, which causes the spring to bend. The spring is arranged so that the bending applies presure to a trip mechanism. The current flowing in the circuit is passed through the spring, causing the spring to gradually become warmer when high current flows, or to gradually cool as less current flows, following the effects of that current on the rest of the electrical system. A high current, sustained for a long time, will apply the sufficient pressure to the trip mechanism. This type of trip is called an inverse-current-time function trip, because the time it takes to trip the breaker is inversely proportional to the current. The reason for the time function is to allow for brief overloading of the circuit when starting loads that draw a high starting current, such as incandescent lighting or motors. A typical residential circuit breaker will carry 110% of its rated load indefinitely, 150% for one minute, 200% for 20 seconds, and 300% for 5 seconds. These overloads for these short times will not cause wiring temperatures to exceed design limits. The magnetic trip protects wiring against short circuits. A short circuit exists when, through failure or accident, the power-carrying conductors are directly connected to one another. Current during a short circuit condition can be several thousand amps, which will quickly destroy any wiring in its way. For protection against this condition, circuit breakers contain a magnetic trip mechanism in addition to the thermal mechanism. The magnetic trip is a solenoid coil which exerts pressure on a mechanical trip mechanism in proportion to the current. The trip happens within a few milliseconds. The Ground-Fault Interrupter (GFI or GFCI) protects against electrocution. Circuit breakers are available with an optional feature called a Ground Fault Interrupter (GFI) or Ground Fault Circuit Interrupter (GFCI). This trip is operated by an electronic device which continuously monitors the current in all wires going to a circuit, and will trip the breaker if any current leaks out by way of a ground path. The purpose of the GFI is to detect even a partial failure of insulation, or any contact with a person. This trip operates extremely quickly and is sensitive to very small currents. Power tools, appliances, outlets, and wiring must be in good condition to avoid tripping the GFCI. If the GFCI keeps tripping, the fault is rarely with the GFCI, but with something connected to the circuit. A GFCI is required by the NEC in bathrooms, on kitchen counters, outdoors, and around pools and spas, among other places. The Arc-Fault Circuit Interrupter (AFCI) protects against fires. Arc-fault detection has been used to trip circuit breakers in aircraft for a number of years, and is now coming into use in residential wiring. The 2002 edition of the NEC (article 210 and article 550.25) requires the use of arc-fault circuit interrupters for protection of 15a and 20a circuits which serve bedrooms. The reasoning is clear. Most electrical fires are caused not by overheating of wires, but by overheating of bad connections or partial short circuits. A characteristic of both of these conditions is that they usually produce an electrical arc. The arc may be too small to see or hear, and is likely to be hidden inside an electrical box, defective equipment, or electrical plug. An AFCI continuously analyses the characteristics of current and voltage on the circuit, and is capable of recognizing the "signature" of an electrical arc.
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