Basics of Circuit Breakers in Layperson TermsEquipment that needs electricity to operate doesn’t know the difference between normal electrical current and dangerous electrical current. Normal electrical current would be the normal load amperes the equipment needs to run. If there is a problem, the equipment might use too much current, which could result in over heating and even melting of the wires to the equipment. When current moves through wires, it causes the wires to get warm, depending on the size of the wires compared to the amount of current. Too much current and things get very unhappy, melt, and even burn down buildings. So a device called a circuit breaker is placed in the circuit such that the current has to go through the breaker. The breaker is selected by its voltage rating and current rating so that it protects the wires and equipment connected to it. It will “trip open” if the current is too much and lasts too long. In addition, if the electrical wires actually touch each other, a short circuit occurs and a massive amount of current can flow through the wires. An electrical arc can take place, causing an explosion and burning of anything nearby, including people. So the breaker also has a current rating at which it will “trip instantly” if there is a short circuit…an electrical arc. This assumes the breaker is sized and rated properly.
How do they actually work? The most basic breakers have two protective functions. The first is “overload protection.” In this case, especially in older breakers, an element inside is heat sensitive. As the current increases, this element heats up and eventually will cause the breaker to trip open, opening the circuit and stopping the flow of current. This internal “thermal overload” device is sized properly by the manufacturer. The second protective function is “short circuit protection,” also called fault current protection. An internal magnetic pickup device, a coil, operates instantly when there is a sudden very high current above a fixed level. So overload and fault protection is provided by a properly voltage rated, current rated, and install breaker.
Far more complicated breakers are available today, using electronic protective components, with many more protective functions, ratings, installation configurations, and interrupting designs. Using these requires professional help. Here is a list to give you some idea of the complexity. Do not use any of this information to make a decision on breaker selection. A lot of very important information is not included here.
Examples of Specific Breakers
• Square D – KAB36225 - 225 amp and 600 volt
• GE General Electric - TKMA31200 - 1200 amp and 600 volt
• Westinghouse - HLA3400 - 400 amp and 600 volt
• Cutler Hammer - MA36800 - 800 amp and 600 volt
General Types of breakers
• Molded case breakers rated 120 volts to 600 volts and 15 amps to 2000 amps
• Insulated case breakers rated up to 600 volts and 3000 amps
• Low voltage air breakers rated up to 600 volts and 4000 amps
• Medium voltage air breakers
• Medium voltage vacuum breakers
• Medium voltage and high voltage oil breakers
• Medium voltage and high voltage SF6 breakers
Basic Protective functions
• Ground fault pickup and delay
• Overload pickup and delay
• Short time pickup and delay
• Instantaneous pickup and trip.
• There are modern breakers that have many times the number of functions shown here.
Buzz Words explained
• Thermo Magnetic – Having overload and short circuit protection
• Current Limiting – Stopping current extremely fast, possibly within 1/240 second (1/4 cycle)
• Arc Chute – Part of a breaker that extinguishes the electrical arc caused when current is interrupted
• Interrupting Rating – The maximum amount of current a device can interrupt without failing
More obscure words
• Roller Smith – Very old, heavy breakers that used a tank filled with oil to interrupt current
• Tri-Pac – A basic older Westinghouse breaker with fuses inside to interrupt very high current
This is basic information about circuit breakers. Is should not be used for decision making.