Electrical Coordination Study: Why It Matters for Arc Flash

An electrical coordination study verifies that protective devices in a power system are set to trip in the right order. When a fault occurs, the device closest to the fault should open first. Everything upstream stays energized. That is selective coordination, and it is the goal of every coordination study.

When devices are not coordinated, a fault on a branch circuit can trip the main breaker and take down the whole building. Beyond the operational disruption, poor coordination has a direct impact on arc flash hazard levels. Fault clearing time is one of the primary variables in the IEEE 1584 incident energy calculation. A device that takes too long to open releases more energy at the fault point.

Coordination studies are almost always performed alongside arc flash studies and short circuit studies. The data you collect for one feeds all three. Running them separately is unusual and usually means paying for field data collection twice.

What selective coordination means in practice

Every electrical system has layers of protection. Power comes in from the utility, passes through a service entrance disconnect, flows through a main breaker or fused switch, down through feeders to distribution panels and MCCs, and out through branch circuits to loads. Each layer has a protective device. Those devices need to be coordinated with each other.

For selective coordination to work, the feeder device must clear a feeder fault before the main has time to open. The branch device must clear a branch fault before the feeder opens. The time-current characteristic curves of each device have to be plotted to confirm there is enough time separation at every protection level.

How coordination affects arc flash incident energy

The arc flash analysis calculates incident energy at each bus using the IEEE 1584 methodology. Three variables drive the result: arcing fault current, working distance, and the time the protective device takes to clear the fault.

Of those three, clearing time is the one an engineer can most directly influence through the coordination study. A breaker that takes 0.5 seconds to open releases far more energy than one that clears in 0.05 seconds. A legacy breaker setting that is longer than necessary for coordination purposes can drive up incident energy at every location that device protects.

This is why the coordination study and the arc flash study have to be done together. Using a default clearing time or a conservative estimate in the arc flash calculation produces results that may overstate the hazard at some locations and understate it at others. You need the actual device settings, confirmed by the coordination study, to get numbers you can trust.

Types of protective devices in a coordination study

Circuit breakers

Thermal-magnetic breakers have a fixed time-current characteristic determined by the frame and trip rating. Electronic trip breakers have adjustable long-time, short-time, and instantaneous settings. Getting the actual settings from the field matters because trip units are frequently adjusted without documentation. The drawings may show factory defaults. The device may be set differently.

Fuses

Fuses have fixed time-current characteristics determined by type and ampere rating. Current-limiting fuses can clear faults in less than a half-cycle, which dramatically reduces incident energy at the fuse location. The type classification matters: Class J, Class RK1, Class RK5, Class L, and Class T fuses each have different time-current curves that must be plotted accurately. Substituting a different class changes the coordination picture.

Relays

Electromechanical and microprocessor-based relays are used primarily in medium-voltage systems with current transformers. Overcurrent relays have pickup and time dial settings that control when and how fast they trip the associated breaker. Relay settings are more adjustable than breakers or fuses, which gives engineers more flexibility in achieving coordination at the medium-voltage level. They also require more careful verification in the field because they are not always accessible and settings may not be current.

Time-current curves explained

A time-current curve (TCC) plots how long a protective device takes to open as a function of the fault current flowing through it. The Y-axis is time in seconds, usually on a logarithmic scale. The X-axis is current in amperes or multiples of full-load current, also on a logarithmic scale.

The coordination study overlays TCC curves for all devices in a protection zone on the same plot. For selective coordination, the curves cannot overlap in the region where fault current is expected. If curves cross, the upstream device may open first under certain fault magnitudes. That is a coordination failure.

Power system analysis software generates these plots automatically once you enter device data. The engineer reviews them to confirm coordination margins are adequate across the full range of possible fault currents, from minimum fault current at the end of a long cable run to maximum bolted fault current at the transformer secondary.

Data needed for a coordination study

Every piece of equipment with a protective device needs to be documented. The minimum data set includes:

• Circuit breakers: manufacturer, frame size, trip rating, and all trip unit settings (long-time pickup and delay, short-time pickup and delay, instantaneous pickup)
• Fuses: manufacturer, type classification, ampere rating, voltage rating, and interrupting rating
• Relays: type, model, CT ratio, pickup setting, time dial setting, and any instantaneous or directional elements
• Transformers: kVA rating, primary and secondary voltage, impedance percentage, connection type
• Cables: conductor size, length, material, and number of conductors per phase
• Utility: available fault current at the service entrance and clearing time for utility protection

Missing or inaccurate settings are the most common source of coordination study errors. A breaker may be labeled with factory default settings but have been adjusted years ago. The coordination study reflects what is actually installed and set, not what the one-line diagram shows.

Download the free arc flash field data collection checklist for a complete list of data points organized by equipment type.

Who performs a coordination study

Coordination studies require a licensed electrical engineer with experience in power system protection. They require power system analysis software to generate the TCC plots and run the underlying fault current calculations.

The same engineer who performs the arc flash study typically runs the coordination and short circuit studies. They are not separate projects. They share the same system model, the same field data, and the same final report. Treating them as one engagement is more efficient and less expensive than commissioning them separately.

Engineering firms that specialize in power systems or protection engineering are the most common source. Some electrical contractors with in-house engineering staff offer coordination studies as part of a broader arc flash and electrical safety package.

When a coordination study is required

A coordination study should be performed whenever a new electrical system is commissioned. It should be updated any time the arc flash study is updated. The NFPA 70E 2027 requirement to review arc flash studies at intervals not exceeding five years applies to the coordination data underlying those studies as well.

The National Electrical Code requires selective coordination for specific system types regardless of arc flash requirements. NEC 700 (emergency systems), NEC 701 (legally required standby), and NEC 708 (critical operations power systems) all mandate selective coordination for the entire path from service entrance to load.

Any change to the electrical system that affects fault current levels or protective device settings requires a review: a new transformer, a different breaker, changed relay settings. Any of these can break coordination margins that existed before the change.

How 70Ez supports coordination study data collection

The bottleneck in most coordination studies is the same as in arc flash studies: getting accurate data from the field. Trip unit settings have to be read from the physical device, not assumed from drawings. Fuse types have to be verified by reading the cartridge or checking the disconnect nameplate. Relay settings require accessing relay menus or testing records.

70Ez gives field technicians a structured way to photograph and document equipment in the field. The AI reads nameplates for transformers, breakers, and other equipment. The data is organized by project and exports into SKM PowerTools, ETAP, or EasyPower so the engineer can build the system model without re-entering every data point from field notes.

Trip unit settings and relay settings still require direct field verification. But the nameplate and rating data that feeds the rest of the model is captured faster and with fewer transcription errors. See how arc flash data collection works for a walkthrough of the field process from first scan to software export.