What is selective coordination in electrical protection and when is it required by the NEC?

Selective coordination means that when a fault occurs, only the protective device immediately upstream of the fault operates — all other upstream devices remain closed, keeping the rest of the system energized. The NEC mandates selective coordination for emergency systems (NEC 700.32), legally required standby systems (NEC 701.27), and critical operations power systems (NEC 708.54). It also applies to healthcare facilities under NEC 517.26. Selective coordination must be demonstrated over the entire range of fault currents from the minimum to the maximum available, which is a more stringent standard than simple non-simultaneous tripping.

How do I prompt AI to interpret ETAP or SKM protection coordination results?

The most effective approach is to export your bus fault summary and device duty tables from ETAP or SKM as CSV, then paste the data into your AI prompt with a structured request. Include: the system voltage, the number of buses, the pass/fail threshold (device interrupting rating vs available fault current), and the desired output format (executive summary, action item table, or report paragraph). Always ask AI to cite the relevant NEC article — for example, NEC 110.10 for equipment interrupting ratings — in its output so the language is immediately suitable for your engineering report.

What is the IEC 60255 standard inverse curve equation and how is it used in relay coordination?

The IEC 60255-151 standard inverse (SI) overcurrent relay curve is defined by: t = (TMS × 0.14) / ((I/Is)^0.02 − 1), where t is the trip time in seconds, TMS is the time multiplier setting (0.05–1.0), I is the fault current, and Is is the pickup current setting. This curve produces longer trip times at lower fault currents and shorter times at higher currents. Very inverse (VI) and extremely inverse (EI) curves use different constants (K and α) and are selected when faster operation at high fault currents is required. AI can compute operating times for any fault current, helping engineers verify coordination margins between upstream and downstream relays.

What relay setting QA checks should be automated with Python?

The core QA checks that benefit from Python automation are: verifying that phase overcurrent (51) pickup settings are 1.25–1.5× full-load current; confirming that instantaneous (50) settings are at least 1.2× maximum load current and below 80% of available fault current; checking that time dial settings match the study recommendations within ±20%; verifying that ground fault (51G) pickup is set at 50% or less of the phase pickup; and confirming coordination margin between upstream/downstream devices is at least 0.3 seconds for electromechanical relays. A Python script that reads the relay export CSV and flags any item outside these ranges can be run in seconds and generates a report suitable for the project QA file.

What is the coordination margin requirement between protective devices?

The minimum coordination time interval (CTI) between protective devices depends on device technology. For electromechanical relays, IEEE 242 recommends a minimum of 0.3–0.4 seconds to account for relay overtravel, breaker interrupting time, CT accuracy, and relay timing tolerances. For modern digital relays, the minimum CTI can be reduced to 0.1–0.2 seconds due to better timing accuracy and no overtravel. For current-limiting fuses coordinating with downstream breakers, the coordination can be nearly instantaneous because the fuse clears before the breaker can operate. Always verify the CTI at the maximum available fault current, not just at intermediate levels.

AI & Automation·9 min read·June 19, 2026

🔌 AI for Electrical Protection Coordination: Automating Relay Settings and Selectivity Studies

Discover how AI tools accelerate protection coordination studies — from interpreting ETAP and SKM outputs to automating relay setting QA, generating time-current curve reports, and prompting AI for IEC 60255 and IEEE 242 compliant protection calculations.

📖AI Roadmap for Electrical Engineers (Full Access)

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Why Protection Coordination Is Ripe for AI Assistance

Protection coordination is the discipline of ensuring that when a fault occurs in a power system, the protective device closest to the fault clears it first — and only that device. Getting this right requires iterating through time-current curves (TCCs), comparing relay pickup settings against load currents and available fault currents, and documenting every device pair with coordination margins. On a medium-sized industrial project, this can mean dozens of breaker-relay pairs and hours of manual analysis in ETAP or SKM Power*Tools.

Topics covered

protection coordination AIrelay settings automationETAP AISKM Power*Tools AItime-current curvesshort circuit study AIselective coordination NECIEC 60255 relay curvesIEEE 242 protectionovercurrent relay settingsAI electrical protectionrelay coordination Pythonarc flash coordinationOCPD selectivityprotective relay QAelectrical power system AINEC 700 selective coordinationpower systems engineering AI