Power System Protection Coordination Calculator
Multi-tab calculator for Power System Protection Coordination
Protective device coordination calculator for overcurrent protection of electrical distribution systems per IEEE 242 and NEC 240.
Plot and compare two device TCC curves
๐ฅ Input Parameters
๐ค Results
๐ Calculation Steps
Plot TCC curves
Log-log plot of both devices.
Check at max fault
Verify no overlap at maximum fault current.
Check at min fault
Verify at low-end selectivity.
Arc flash check
Evaluate if coordination delays increase arc flash energy.
๐งฎ Formula
Selectivity and Coordination
t_up > t_down + margin for all I_f; selectivity ratio = t_up/t_down > 1.5
Time-based selectivity - upstream device must be slower at all fault levels.
| Symbol | Variable | Unit | Description |
|---|---|---|---|
| I_f | Fault current | kA | |
| t_up | Upstream trip time | s | |
| t_down | Downstream trip time | s | |
| margin | Coordination margin | s |
๐ Worked Example: 480V Switchboard-Feeder Selectivity
๐ Result Interpretation
๐ฅ Guidance
Wide-area outage possible during fault.
๐ก Engineering Recommendations
Arc flash > 12 cal/cm2 with coordination
Implement ZSI or maintenance switch.
ZSI reduces clearing time from 0.3s to 0.05s during downstream faults.
โก Quick Facts
๐ฌ Engineering Insight
"The trade-off between selectivity and arc flash is a fundamental design challenge. Zone-selective interlocking (ZSI) solves this by providing fast fault clearing (for arc flash) while maintaining selectivity through signaling - the downstream device signals the upstream to stay open when it sees the fault."
๐ Standards & References
Protection and coordination of industrial/commercial power systems.
โ Frequently Asked Questions
No - at very high fault currents, instantaneous elements may overlap. Total selectivity is only possible within a defined fault current range.
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