Short-Circuit and Protection Coordination Studies

How 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. Problem areas are flagged.
• Final Report — A detailed report including recommendations for corrective action is published.

How is a Protection-Coordination Study Done? 

• Short-circuit study — An analysis of available fault currents in the power system is performed.
• Data collection — In addition to the data collected for the short-circuit study, additional information is gathered on present settings and ratings of all protective devices.
• Manufacturer’s data — Each protective device has unique response characteristics, documented on manufacturer’s “time-current curves” that are needed for the study.
• Computer analysis — Allows the engineer to determine the optimum settings that will provide the best protection for the system.
• Tabulate results — The settings and ratings of each protective device are put in a table for comparison with present field settings and ratings.
• Final Report — A detailed report with recommendations is published.

How Often Should These Studies Be Done?

The initial short-circuit/protection coordination study should be done when a power system is first designed. However, for many systems, this is mistakenly the last time it is performed. These studies should be done on a regular basis — every five to six years in more static situations. As often as needed when change occurs due to power company energy changes or facility expansion and addition of electrical equipment, such as circuit breakers, motor control centers, transformers and cables.

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