Published: June 2026
Technical Level: Advanced
Category: Protection and Coordination
Protection coordination is the discipline of setting a power system's protective devices so that, for any fault, the device nearest the fault clears it while every upstream device restrains, isolating the smallest portion of the system and preserving service to the remainder. This paper develops the principles and practice of coordination: the selectivity objective and the consequences of its failure, the inverse-time characteristic that gives overcurrent devices their discriminating behavior, the coordination time interval that separates adjacent devices, the construction and reading of the time-current characteristic plot on which coordination is performed, and the constraints that equipment damage curves and transformer inrush impose on the settings. The objective is to give the engineer a coherent account of how a coordinated protection scheme is designed and verified.
A power system is protected by devices arranged in series from the source toward each load (a main breaker, feeders, branch circuits, and the protection of individual loads), and the purpose of coordination is to ensure that a fault is cleared by the device immediately upstream of it while the devices further upstream hold. When this selectivity is achieved, a fault interrupts service only to the faulted portion of the system, leaving the remainder energized; when it fails, an upstream device operates unnecessarily, extending the interruption to healthy loads that the fault never reached. The cost of miscoordination is this unnecessary loss of service, which in a facility serving critical processes can be severe, and the entire effort of a coordination study is directed at achieving selectivity across the full range of faults the system can experience while still clearing every fault quickly enough to protect equipment and personnel.
The coordination of overcurrent devices is made possible by the inverse-time characteristic, by which a device operates more quickly as the current through it increases. This characteristic allows a device to distinguish a modest overload, which it tolerates or clears slowly, from a severe fault, which it clears rapidly, and it provides the basis on which adjacent devices are separated in time. Because a downstream device sees the same fault current as the upstream device protecting the same path, and because both have characteristics that operate faster at higher current, the engineer can set the downstream device to operate faster than the upstream device at every fault current, so that the downstream device always clears the fault first. The inverse-time relationship between operating time and current, scaled by the device's time-dial or equivalent setting and initiated at its pickup setting, is the fundamental tool of coordination, and the standard curve shapes (moderately, very, and extremely inverse) provide the range of characteristics needed to coordinate against the variety of downstream devices and equipment.
Two devices in series are coordinated when, throughout the range of fault currents they both experience, the upstream device's operating time exceeds the downstream device's total clearing time by an adequate coordination time interval. This interval ensures that the downstream device, together with the operation of its interrupting mechanism, clears the fault before the upstream device begins to operate, and it must account for the downstream device's operating and interrupting times, the overtravel of an electromechanical upstream relay, and a margin for the tolerances of the devices and the uncertainties of the study. The appropriate interval depends on the types of devices being coordinated, differing between two relays and between a relay and a downstream fuse or static device, because the contributing factors differ among device types.
The coordination time interval is the quantitative criterion of selectivity. The engineer verifies, at every fault current that two adjacent devices share, that the margin between their characteristics meets or exceeds the interval, and where the margin is inadequate (where the characteristics approach too closely or cross), the coordination fails at those currents and the settings must be adjusted. The principal adjustment is to the time-dial settings, which shift the characteristics in time to open the margins, constrained by the requirement that the clearing times remain short enough to protect the equipment and to limit the arc-flash hazard.
Coordination is performed on the time-current characteristic plot, a logarithmic graph displaying the operating-time-versus-current characteristic of every device in a coordination path on common axes. A coordinated scheme appears as a family of curves stepping upward from the load toward the source, each device's characteristic lying above the next downstream device's by the coordination time interval, so that for any fault current the lowest curve belongs to the device that should clear the fault. The plot reveals coordination and its failures directly: curves that hold their separation are coordinated, while curves that pinch together or cross expose the currents at which selectivity is lost.
The device characteristics must lie within constraints that the plot also displays. The damage curves of the protected equipment (transformers, cables, motors) must lie above the protective characteristics, so that each device operates before its equipment is harmed. The magnetizing inrush of a transformer must lie below the protective characteristic, so that energizing the transformer does not trip the protection. The maximum load current bounds the pickup below, and the available fault current bounds the range over which coordination must hold. The engineer adjusts the settings until each device protects its equipment, coordinates with its neighbors by the required interval, permits the inrush, and operates within the load and fault-current bounds, all simultaneously, and where these requirements conflict, the plot makes the conflict explicit so that it can be resolved by reselecting devices or accepting a documented compromise.
The most consequential point is that coordination is the deliberate trade of speed for selectivity: the inverse-time characteristic lets the device nearest a fault clear it quickly while upstream devices wait, and the coordination time interval is the margin that guarantees the right device operates first. A scheme tuned for the fastest possible clearing everywhere sacrifices selectivity; a scheme tuned for selectivity accepts slower upstream clearing, and the engineering is in placing that trade correctly for each device pair.
The most common implementation failure is verifying coordination at the maximum bolted fault alone, where curves are well separated, and never checking the minimum credible fault, where they crowd together and selectivity is actually at risk. The interval that protects the system is the one that survives at the low-current end of the range.
The engineer should next carry the coordination onto the time-current plot with the equipment damage curves and transformer inrush points overlaid, and verify the coordination time interval at both ends of the fault-current range, because that explicit verification, not the visual separation of two curves at one current, is what certifies the scheme.
[1] IEEE Standard 242-2001, IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (Buff Book), IEEE, 2001.
[2] IEEE Standard C37.112-2018, IEEE Standard for Inverse-Time Characteristics Equations for Overcurrent Relays, IEEE, 2018.
[3] IEEE Standard 1584-2018, IEEE Guide for Performing Arc-Flash Hazard Calculations, IEEE, 2018.
[4] J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications, 4th ed., CRC Press, 2014.
[5] IEEE Standard C57.12.00-2015, IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers, IEEE, 2015.
[6] NFPA 70-2023, National Electrical Code, National Fire Protection Association, 2023.