After a verified custom model replaces an unverified library file, the coordination study reflects the published Federal Pioneer USD relay behavior and exposes every modeling compromise for review. The number that matters is clearing time at each prospective current: current produces thermal and mechanical stress, while relay and breaker timing determines how long that stress remains on the protected equipment.
Current, thermal load, and clearing time
A protection model converts current into operating time. On a time-current coordination plot, the selected current passes vertically through the relay curve; the intersection gives the modeled operating time. That result must then be interpreted with the associated interrupting device because relay operation and current interruption are separate events.
Higher current generally demands faster clearing because conductor heating rises with current squared. The resulting thermal duty depends on both current and duration, commonly evaluated through an I²t relationship. This is heat, not logic: an apparently small curve-entry error can change the modeled exposure substantially when it moves the clearing time at high fault current.
| Quantity or limit | Why it matters | Where to read it |
|---|---|---|
| Prospective current | Defines the point evaluated on the time-current curve | SKM short-circuit or coordination study result |
| Pickup current | Establishes where the relay begins its operating region | Relay settings and published curve |
| Operating-time band | Represents the published minimum and maximum response where provided | Manufacturer time-current curve |
| Clearing time | Determines thermal and mechanical exposure duration | Combined relay and interrupting-device model |
| Curve endpoints | Prevent unsupported extrapolation beyond published behavior | Axes and boundaries on the source curve |
Library-file and custom-model comparison
Two approaches are practical: import an existing Federal Pioneer USD library record or construct the relay model from authoritative device information. An imported model saves entry time only when its provenance, curve basis, settings, and compromises are known.
| Criterion | Existing library file | Engineer-built model |
|---|---|---|
| Setup effort | Lower if the file imports cleanly | Higher because curve data and settings must be entered |
| Traceability | Depends on documentation supplied with the file | Directly tied to the curve and settings used for entry |
| Model compromises | May be hidden or undocumented | Can be recorded with the study |
| Error detection | Requires an independent comparison after import | Occurs during entry and again during validation |
| Legacy-device compatibility | May depend on how an older record was represented or converted | Can use the capabilities of the current SKM model |
| Reviewability | Weak when the origin and revision are unknown | Strong when source documents and check points are retained |
Use a verified library file only when it can be traced to the correct device and checked against the installed settings. When no validated Federal Pioneer USD record is available, build the model locally. That choice makes curve approximations visible and avoids inheriting an unknown interpretation from another study.
Model boundary and device identity
Confirm the relay identity before entering curve points. The evidence identifies the USD as an older solid-state Federal Pioneer relay and separately identifies a Federal Pioneer SS relay. Treat USD and SS as different device identifiers unless manufacturer documentation explicitly maps their curves and settings.
Record the complete nameplate description, installed tap or pickup selection, time setting, enabled elements, sensing ratio, and associated breaker or trip device. Read missing values from the equipment, approved settings record, commissioning documentation, or the applicable manufacturer curve. A model with the correct family name but the wrong setting is not a valid representation.
Define what the SKM object represents. If it represents relay operating time only, keep breaker opening or interrupting time in the breaker model. Combining those times without documenting the boundary can count the same delay twice or omit it entirely.
Curve reconstruction and data control
Start from the clearest authoritative time-current curve available for the exact relay and selected settings. Preserve the source document with the study package and identify the page, curve designation, and setting basis in the model notes. If two published curves disagree, compare revision information and installed hardware identity; resolve the discrepancy before treating either curve as definitive.
Enter enough points to preserve knees, slope changes, definite-time regions, and published endpoints. Sparse points can turn a curved response into straight segments that materially change coordination margins. Where the publication shows a tolerance band, represent both boundaries if the software supports them; otherwise document whether the entered curve follows the minimum, maximum, or another defined boundary.
Check axis interpretation before copying any values. Verify whether current is expressed in primary amperes, secondary amperes, pickup multiples, or another stated basis. Verify that operating time is read from the correct scale. Never extend the model outside the published current and time range merely to make the plot look complete.
SKM model-entry procedure
- Create a new relay record using an unambiguous name containing
Federal PioneerandUSD. Add a local revision or study note rather than implying an unsupported manufacturer revision. - Select the SKM relay representation that matches the shape and functions shown on the source curve. Record any feature that the software cannot reproduce exactly.
- Set the current basis from the installed sensing ratio and relay setting records. Keep primary and secondary quantities distinguishable throughout entry.
- Enter the pickup and time-current data from the applicable published curve. Preserve knees, limits, and tolerance boundaries rather than smoothing them for appearance.
- Model the associated breaker or interrupting-device time separately when the relay curve represents operating time alone.
- Save the record in a controlled project library. Attach or reference the curve used, the installed settings, the entry date, and the engineer's assumptions.
- Plot the new curve beside the source document and perform independent point checks before using it in a coordination decision.
Verification and recurring modeling errors
Validate the model at several currents spanning pickup, intermediate inverse-time behavior, every curve knee, and the highest published current. For each check point, read the time from the source curve and from SKM, then record the comparison. Investigate visible deviations rather than relying on a similar plot shape.
Run the coordination study using the actual installed settings and examine the complete protection chain. Confirm that relay operation, breaker opening, and any fixed delay appear once. Review coordination margins at relevant load, starting, inrush, overload, and fault-current points using the requirements established for the project.
Common errors include importing a file with unknown provenance, selecting the wrong device variant, confusing the USD and SS records, mixing primary amperes with secondary amperes, reading the wrong logarithmic decade, entering only a centerline when the publication provides a band, and extrapolating past a curve endpoint. Manufacturer publications can also contain inconsistent curves, so the drawing itself requires an engineering cross-check against settings and device identity.
Frequently asked questions
Why does a Federal Pioneer USD relay need a custom SKM model?
A custom model is appropriate when no traceable, validated library record is available. It ties the SKM curve directly to the installed settings and applicable manufacturer time-current data.
Why does an imported USD relay file still need validation?
The file may contain undocumented approximations, a different current basis, or settings from another installation. Compare its pickup, knees, endpoints, and operating times with the applicable source curve before use.
Why does the SKM curve differ from the published relay curve?
Check current units, sensing ratio, pickup setting, logarithmic-decade placement, interpolation points, and tolerance-band selection. Also verify whether breaker opening time has been included in one model, both models, or neither.
Why does the Federal Pioneer SS relay require a separate record?
SS and USD are separate identifiers in the available device information. Use one model for both only when authoritative documentation explicitly establishes identical curves and setting behavior.
When should I stop building the USD model and contact support?
Stop when the device identity, setting basis, sensing ratio, curve revision, or model boundary cannot be resolved from controlled records and equipment inspection. Escalate to the equipment manufacturer's official technical support and SKM's official support channel with the nameplate data, curve documents, settings, and point-by-point discrepancy; keep the model out of design decisions until the ambiguity is closed.