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Executive summaryMatthew T. Glennon, P.E. AVP, Electrical Loss Control, Hartford Steam BoilerHow do you know if a circuit breaker will work when called upon to interrupt an electrical fault or short-circuit? The answer is, you probably don’t know for sure.
If unusually high currents exceed the capability of protective devices in the power system, a short-circuit can cause the devices to explode like a bomb. What is needed is a short-circuit/protection coordination study done on your electrical system.Analyzing the Currents A short-circuit study is an analysis of an electrical system that determines the magnitude of the currents that flow during an electrical fault.
Comparing these calculated values against the equipment ratings is the first step to ensuring that the power system is safely protected. Once the expected short-circuit currents are known, a protection coordination study is performed to determine the optimum characteristics, ratings and settings of the power system protective devices.How is a Short-Circuit Study Done?. Data collection — Information on all the components is obtained during a field visit and from electrical utility and manufacturers, and then tabulated. One-line diagram — A power system diagram that shows how all components are electrically connected is created, or, if one already exists, updated. Computer analysis — Using computer software, the system data is input and the short-circuit currents at various points in the system are calculated. Tabulate results — The results are typically put into a table for comparison with ratings of equipment in the system.
Matthew Glennon is a registered Professional Engineer in New Jersey with over 12 years of experience in electric power engineering and construction. He is a magna cum laude graduate of Manhattan College where he earned a Bachelor of Engineering degree in electrical engineering. In addition, he holds a Master of Engineering degree in electric power engineering from Rensselaer Polytechnic Institute (RPI) in Troy, N.Y.
And an MBA in Finance from Rutgers University. He is a member of IEEE, NFPA, and the National Society of Professional Engineers (NSPE), as well as a Correspondent to the National Academy of Forensic Engineers (NAFE). The full article Short-Circuit and Protection Coordination Studies IntroductionHow do you know if a circuit breaker will work when it is called upon to interrupt an electrical fault or short-circuit? If you haven’t had a short-circuit/protection coordination study done on your electrical system recently, the answer is: you don’t know for sure.Even the best designed and maintained power systems occasionally suffer short-circuits that draw unusually high currents. If these currents exceed the capability of the protective devices in the power system, a short-circuit event will cause the devices to explode like a bomb. The First Step A short-circuit study is an analysis of an electrical system that determines the magnitude of the currents that flow during an electrical fault. Comparing these calculated values against the equipment ratings is the first step to answering the question above and ensuring that the power system is safely protected.
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Once the expected short-circuit currents are known, a protection coordination study is performed to determine the optimum characteristics, ratings, and settings of the power system protective devices. How is a Short-Circuit Study Done?A short-circuit study is comprised of the following steps:. Data collection — Information on all the components is obtained during a field visit and then tabulated. Additional data is obtained from the electric utility, manufacturers, or calculated from field data.
One-line diagram — A power system diagram that shows how all components are electrically connected is created, or, if one already exists, updated. Additional data needed for the study, such as cable impedances, can be obtained with information from this diagram.
Computer analysis — Using one of the many computer software programs available, the system data is input and the short-circuit currents at various points in the system are calculated. Tabulate results — The output of the computer program is typically put into a table for comparison with the ratings of equipment in the system.
Matthew Glennon is a registered Professional Engineer in New Jersey with over 12 years of experience in electric power engineering and construction. He is a magna cum laude graduate of Manhattan College where he earned a Bachelor of Engineering degree in electrical engineering. In addition, he holds a Master of Engineering degree in electric power engineering from Rensselaer Polytechnic Institute (RPI) in Troy, N.Y.
And an MBA in Finance from Rutgers University. He is a member of IEEE, NFPA, and the National Society of Professional Engineers (NSPE), as well as a Correspondent to the National Academy of Forensic Engineers (NAFE).
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To determine how long a breaker will take to trip at a given current, find the level of current on the bottom of the graph. Draw a vertical line to the point where it intersects the curve. Then draw a horizontal line to the left side of the graph and find the time to trip. For example, in this illustration a circuit breaker will trip when current remains at 6 amps for 0.6 seconds.It can be seen that the higher the current, the shorter the time the circuit breaker will remain closed. “If the trip point adjustment is set to minimum (900 A), and a fault current of 900 amps or greater occurs, the breaker will trip within 1 cycle (16.8 ms).” Could you please elaborate further on that because I didn’t get why would it take 1 cycle for the breaker to trip if it is set to a minimum trip point adjustment and a fault of 900A occurs.
In the last figure, a vertical line is drawn from the 450% point up till it reaches 1.68 ms (1 cycle). However, the vertical line hits the curve at infinite points and can trip anywhere between 1 ms and 55 seconds. Appreciate your help.