Facility managers researching electrical safety and system reliability often encounter the term coordination study alongside arc flash studies and short circuit analyses — the three are typically performed together and are closely interdependent. Yet coordination studies are often the least understood of the three. This article explains exactly what a coordination study does, why it directly affects both arc flash incident energy and operational continuity, and when your facility should have one performed.
What Is a Power System Coordination Study?
A power system coordination study — also called a protective device coordination study or a selectivity study — is an engineering analysis that verifies that the protective devices in your electrical distribution system are configured to respond to fault events in the correct sequence: the device electrically closest to the fault trips first, isolating only the smallest possible portion of the system, while upstream devices remain in service.
Every electrical distribution system contains multiple levels of protective devices — fuses, circuit breakers, and relays — each protecting a zone of the system. When a fault occurs, the objective is selective coordination: the device immediately upstream of the fault trips while all other devices stay closed. This limits the impact of the fault to the smallest possible area and keeps the rest of the facility powered.
Without proper coordination, a fault in a branch circuit could trip the main switchgear breaker, taking down the entire facility rather than just the affected circuit. Or, just as dangerously, an upstream device might trip before the downstream device, allowing the fault to persist longer and releasing more energy — increasing arc flash incident energy at the fault location.
What does it mean when protective devices are not coordinated?
When protective devices are not coordinated, the wrong device trips in response to a fault. This creates two categories of problems. Operationally, a fault in a small branch circuit may cause a large upstream breaker to trip, taking down far more equipment than the fault location warrants — a nuisance outage that affects production, safety systems, or critical processes. From an arc flash perspective, a lack of coordination often means that the device clearing a fault is operating with a time delay that allows the arc to persist longer than necessary — directly increasing incident energy at the fault location.
How a Coordination Study Is Performed
A coordination study requires the same system data as a short circuit analysis — one-line diagrams, transformer impedances, conductor data, and protective device specifications — along with detailed information about every protective device in the system: breaker model and trip unit characteristics, relay settings, fuse time-current characteristics, and the interrupting ratings at each device location.
Engineers plot the time-current characteristics (TCC curves) of all devices at each level of the distribution system on a log-log graph. A properly coordinated system shows TCC curves that are clearly separated — the downstream device’s curve lies entirely to the left of the upstream device’s curve across the range of fault currents that could occur at that location. Overlapping curves indicate a coordination failure: a range of fault currents exists for which both devices could trip, with no guarantee of which one responds first.
The study identifies coordination gaps, evaluates the impact of available engineering solutions, and recommends settings or device changes that improve coordination while maintaining adequate equipment protection.
Bowtie Engineering includes protective device coordination study as a standard component of every arc flash and incident energy analysis engagement. Our engineers evaluate coordination at every level of the distribution system and flag gaps that affect both arc flash energy and operational reliability.
How Coordination Directly Affects Arc Flash Incident Energy
The connection between protective device coordination and arc flash incident energy is direct and quantitative. Arc flash incident energy — the heat energy delivered to a worker during an arc flash event — is calculated as a function of arc current, working distance, and arcing duration. Arcing duration is entirely determined by how long it takes the protective device to clear the fault.
A device that clears a fault in four cycles produces four times the arc flash energy of a device that clears the same fault in one cycle. When a coordination study identifies that an upstream device — one with a longer time-delay setting — is likely to clear a fault before the immediately downstream device responds, the arc flash study calculation must use the upstream device’s longer clearing time. The result is a dramatically higher incident energy value at the fault location, requiring heavier PPE and larger arc flash exclusion zones.
Conversely, when a coordination study demonstrates that a downstream device will reliably clear a fault at its instantaneous speed, the arc flash calculation uses the fast clearing time, producing a lower incident energy result. Coordination study findings are therefore directly tied to PPE requirements — improving coordination can reduce incident energy and lighten the PPE burden on workers performing maintenance at the affected equipment.
Can fixing coordination issues reduce arc flash labels without hardware changes?
In many cases, yes. If a coordination problem is caused by misconfigured trip unit settings on existing electronic trip breakers, or relay setpoint errors, the coordination improvement may be achievable through settings adjustments alone — without installing new hardware. This makes coordination study one of the highest-value interventions in arc flash hazard reduction: a relatively low-cost analysis can identify settings changes that reduce incident energy labels across an entire level of the distribution system.
When Coordination and Arc Flash Safety Must Be Balanced
There is a fundamental engineering tension in coordination studies: the settings that produce the best selective coordination — longest time delays on upstream devices — are often the settings that produce the highest arc flash energy at downstream equipment. Minimizing arc flash energy favors the fastest possible clearing times; minimizing nuisance tripping favors selective time delays.
Modern solutions for resolving this tension include:
- Zone-selective interlocking (ZSI). Allows upstream breakers to operate at instantaneous speed when a downstream breaker fails to clear a fault — preserving selectivity while enabling fast clearing under arc flash conditions.
- Differential protection schemes. Provide zone-based protection that can clear faults at instantaneous speed across a defined protection zone without relying on time-overcurrent coordination.
- Arc flash detection relays. Detect the light signature of an arc flash event directly and initiate clearing at speeds far faster than conventional overcurrent protection, dramatically reducing clearing time and incident energy.
The appropriate solution depends on the facility’s system configuration, budget, and operational requirements. A qualified engineer familiar with both coordination analysis and arc flash mitigation strategies must evaluate the options.
Bowtie Engineering’s electrical maintenance and system assessment services include evaluation of protective device condition and settings as part of comprehensive NETA-compliant maintenance programs — ensuring coordination findings are actionable within a complete facility safety strategy.
For the technical standards governing power system protection and coordination, IEEE Standard 242, Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (the IEEE Buff Book) is the primary engineering reference used by protection engineers performing coordination studies.
Frequently Asked Questions
How often should a coordination study be updated?
A coordination study should be reviewed whenever changes are made to the electrical system that could affect the protection scheme — new transformers, breaker replacements, addition of large motors or generators, changes to protective relay settings, or significant changes to the system load profile. As a baseline, coordination study validity should be assessed as part of any arc flash study review, since the two analyses are interdependent. Facilities that have never had a formal coordination study — or cannot locate documentation of one — should commission one as a priority alongside their arc flash study.
What is a time-current characteristic (TCC) curve?
A TCC curve graphically represents how quickly a protective device responds to overcurrent at different fault current magnitudes. The x-axis represents current (in multiples of the device’s rated current) and the y-axis represents time to trip (in seconds). TCC curves for different devices in the same system are plotted together to evaluate their relative response times. A device whose TCC curve lies to the left of an upstream device’s curve at all relevant fault current levels will always trip first — that is coordination. Overlapping curves indicate potential coordination failure.
Does a coordination study apply to both breakers and fuses?
Yes. A coordination study evaluates all types of overcurrent protective devices — circuit breakers (including electronic trip units and thermal-magnetic units), current-limiting fuses, expulsion fuses, and protective relays with associated current transformers. Fuses have fixed time-current characteristics determined by their type and rating; breakers with electronic trip units offer adjustable settings. Mixed breaker-fuse systems require careful coordination analysis because fuse TCC curves are not adjustable and must be accounted for when setting upstream breaker trip units.
Can a coordination study be performed on a single section of the distribution system?
Yes. Coordination studies can be scoped to cover specific portions of the distribution system — for example, a medium-voltage feeder and its associated downstream equipment, or the service entrance and main distribution switchgear. Partial-scope studies are a reasonable approach when the entire system has been previously studied and only a portion has changed. Care must be taken to ensure that the study boundary accurately represents the protective device characteristics at the scope interface, so results within scope are not affected by unmodeled conditions outside the scope boundary.
What happens to arc flash labels if coordination settings are changed?
If coordination settings changes result in different clearing times at any analyzed equipment location, the arc flash calculations for those locations must be re-run with the new device parameters. Updated arc flash labels must be produced for all affected equipment. Workers must be notified of the updated hazard levels and any changes to required PPE. The arc flash study report should be amended to document the settings change and the basis for the revised calculations.
Key Takeaways
- A coordination study verifies that the right protective device trips first during a fault, limiting the impact to the smallest possible system section.
- Coordination directly affects arc flash incident energy: longer device clearing times mean higher energy release and heavier PPE requirements.
- Coordination studies are performed alongside arc flash and short circuit analyses — all three are interdependent.
- Settings optimization identified through a coordination study can reduce arc flash incident energy without hardware replacement.
- Modern solutions including zone-selective interlocking and arc flash detection relays allow fast clearing times while maintaining selective coordination.
Bowtie Engineering performs protective device coordination studies, arc flash analyses, and short circuit studies as integrated engineering engagements for industrial and commercial facilities nationwide. Call 866-730-6620 or visit our website to request a proposal.