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Protective device coordination — also called selectivity — is the practice of sizing and setting overcurrent protection devices so that only the device nearest a fault operates, leaving the rest of the system energized. IEEE 242 (the Red Book, "IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems") is the primary engineering reference for coordination studies in North American power systems.
NEC 240.87 addresses arc energy reduction for circuit breakers rated 1200A or more. Zone-selective interlocking (ZSI), differential protection, energy-reducing maintenance switch, and other arc flash mitigation schemes are all techniques evaluated during coordination studies. The NEC requirement has driven renewed attention to coordination at the distribution level in commercial and industrial facilities.
A coordination study involves analyzing time-current characteristic (TCC) curves for each protective device in the system on a log-log plot of current vs. time. The goal is to achieve a minimum coordination time interval (CTI) between each device pair — typically 0.2 to 0.4 seconds for thermal-magnetic breakers and 0.05 to 0.1 seconds for electronic trip units — while keeping clearing times short enough to protect conductors and equipment.
SparkShift's short-circuit and arc flash calculators provide the fault current data needed as input to a coordination study. Enter the available fault current at each bus, the transformer impedance, and the connected feeder impedances to get the maximum and minimum fault currents at each protective device location. This data feeds directly into coordination software or manual TCC overlay work.
For IEEE 242 compliant coordination studies on systems above 600V, engage a licensed professional engineer. SparkShift is a calculation tool for engineers and is not a substitute for a full power system engineering study.
Open the short circuit calculator. Results include NEC citations and can be saved to a project for PDF export.
A coordination study analyzes protective device time-current characteristics to ensure that only the device nearest a fault operates, leaving the rest of the system energized. IEEE 242 (the Red Book) is the primary engineering reference for coordination studies.
NEC 240.87 requires arc energy reduction methods for circuit breakers rated 1200A or more. Acceptable methods include zone-selective interlocking (ZSI), differential protection, arc flash detection relays, energy-reducing maintenance switches, and active arc flash mitigation systems.
The minimum CTI between protective device pairs is typically 0.2 to 0.4 seconds for thermal-magnetic circuit breakers and 0.05 to 0.1 seconds for modern electronic trip units with ZSI. IEEE 242 recommends evaluating CTI at the maximum available fault current.
SparkShift calculates fault currents and arc flash incident energy, which are inputs to a coordination study. Full coordination studies — including TCC overlay plots — require dedicated power system software. SparkShift is a calculation tool, not a substitute for a professional power system engineering study.
Disclaimer: SparkShift is a calculation tool for electricians and engineers. It is not a substitute for a professional power systems engineering study. SparkShift is not approved by NCEES, IEEE, NFPA, or any professional organization. For stamped engineering documents, engage a licensed Professional Engineer in your jurisdiction.