Protection and coordination studies that hold up under utility review.
Selectivity you can defend on paper: fault current analysis, time-current curve development, device coordination across the facility and up to the utility, and a study report written to be checked rather than filed.
What a protection study establishes
Power system protection has one job that everything else serves: isolate a fault as quickly as possible while removing as little of the system as possible. Those two goals pull against each other, and a protection coordination study is where the trade-off gets resolved deliberately instead of by accident.
Selectivity is the principle that the protective device electrically nearest a fault should operate first. Achieving it means every device upstream must be slower, at every level of fault current, by a margin large enough to survive real-world tolerances in device operating time, current transformer error, and breaker interrupting time.
That margin is what a coordination study proves. Plot the time-current characteristics of every device in a chain on the same axes, and the study shows whether the curves are properly separated across the full range of fault current the system can actually produce — or whether they cross somewhere, which is where the nuisance trips and the failures to clear come from.
Where coordination goes wrong
- Curves cross at high fault current. Devices coordinate at moderate fault levels but lose selectivity near the maximum available fault current at the bus.
- Stale source data. Coordination is based on a utility available-fault-current figure that is years old and no longer reflects the utility system.
- Added generation changes direction. Non-directional overcurrent devices set for a radial system misbehave once a distributed generation source can feed a fault from the other side.
- Undocumented field changes. Settings were adjusted during commissioning or troubleshooting and never reflected back into the study or the settings record.
- Coordination stops at the fence. The facility coordinates internally but was never checked against the utility's upstream feeder protection.
How the study is performed
- Data collection. One-line diagram, utility source fault data, transformer and conductor data, motor contribution, and the make, model, and existing settings of every protective device in scope.
- System model and verification. Building the model and reconciling it against what is actually installed — the step that most often surfaces surprises on existing facilities.
- Short-circuit analysis. Maximum and minimum fault current at each bus, which bounds both the pickup settings and the coordination margins.
- Time-current curve development. Plotting device characteristics together and setting the separation between them across the fault current range.
- Device settings. Selecting pickup, time dial, and instantaneous settings that achieve selectivity while still protecting equipment within its damage curve.
- Report. Documented assumptions, source data, calculations, curves, and a settings table — written so a utility reviewer can follow the reasoning rather than take it on faith.
When you need one
A coordination study is typically required when adding distributed generation to an existing service, when a utility asks for one as part of interconnection review, when adding or replacing major equipment, after a misoperation where a fault took out more of the system than it should have, or when an existing study no longer matches the installed system.
What the study delivers.
Short-circuit study
Maximum and minimum available fault current at each bus in scope, from the utility source through the facility, with the assumptions and source data stated.
Time-current coordination curves
Device characteristics plotted together across the fault current range, showing the separation achieved at each level of the protection chain.
Recommended device settings
Pickup, time dial, and instantaneous settings for each device in scope, tabulated for implementation. Relay settings scope →
Coordination study report
Assumptions, source data, methodology, results, curves, and settings in one document, formatted for submittal to a utility or authority having jurisdiction.
Protection and coordination study questions.
What is a protection coordination study?
A protection coordination study establishes that the protective devices on a power system operate selectively: when a fault occurs, the device closest to that fault clears it, and every device upstream stays closed. Without coordination, a fault on one branch trips a main breaker and takes down everything behind it — or worse, nothing trips at the right time and equipment damage follows.
The study works from the system one-line and equipment data through fault current calculation, time-current curve development, and device setting selection, and ends in a written report documenting the coordination basis.
What is the difference between a coordination study and a relay coordination study?
They describe the same objective at different scopes. A protective device coordination study covers all protective devices in the zone of interest — fuses, low-voltage breaker trip units, and relays. A relay coordination study focuses specifically on the protective relays and their settings, which is typically what matters at a utility interconnection or on a medium-voltage system.
In practice most interconnection work needs both views: the relay elements coordinated with each other, and the whole chain coordinated down through facility devices and up toward the utility.
What information do you need to perform a coordination study?
At minimum: the system one-line diagram, utility source data (available fault current and X/R ratio at the service point, which the utility supplies on request), transformer nameplate data including impedance, conductor and cable data, and the make, model, and current settings of the existing protective devices. Motor data matters where motor contribution to fault current is significant.
Where documentation is incomplete or out of date — common on older industrial facilities — part of the engagement is establishing what is actually installed before anything can be coordinated.
Why does the utility want to see a coordination study for my interconnection?
Because your protection has to coordinate with theirs. The utility's feeder protection was set without your generation on the circuit; adding a source changes fault current magnitude and direction, and can cause the utility's devices to misoperate or to fail to see faults they previously cleared. A coordination study demonstrates that your interconnection protection operates before, and in a defined relationship to, their upstream devices.
Does a coordination study require a PE stamp?
Often, yes — when the study is submitted as engineering record to a utility or an authority having jurisdiction. Requirements vary by utility and jurisdiction. Studies submitted as engineering record are sealed by our licensed Professional Engineer, and we confirm the requirement during the scope call so there is no surprise at submittal.
How often should an existing coordination study be revisited?
Whenever the system it describes changes. Adding generation, changing the utility service, replacing a transformer, adding a large motor load, or replacing protective devices all invalidate parts of an existing study. Utility source fault current also changes over time as the utility system evolves, which is why the available fault current figure used in a study should be current rather than carried forward from an older report.
Where coordination fits in the wider scope.
Need a coordination study a utility will accept?
Send us your one-line and the utility source data, and we'll tell you what the study needs to cover.