Power Systems Lab · 18 modules · all computed in your browser
Overcurrent coordination
IEEE C37.112 / IEC 60255-151 curves, CTI check.
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Power Systems Lab · 18 modules · all computed in your browser
IEEE C37.112 / IEC 60255-151 curves, CTI check.
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Time-current coordination (log-log TCC)
Downstream · feeder relay (e.g. SEL-351)
Upstream · transformer/bus backup
Downstream t
0.689 s
Upstream t
2.827 s
Margin
2.138 s
Coordinated: margin ≥ 0.3 s CTI.
Test points in multiples of pickup (Test Universe-style): 2× → 7.028 s · 5× → 1.308 s · 10× → 0.689 s · 20× → 0.030 s
Learn it
An overcurrent relay can’t tell where a fault is, only how big it is. Inverse curves make relays trip faster for bigger currents. Grading them, with each upstream relay slower than the one below by a coordinating time interval, means the closest relay clears the fault and the others ride through it as backup.
Quiz · 0/3
1. Doubling TD on an IEEE curve…
2. Which curve best coordinates with fuses?
3. With numerical relays and a 3-cycle breaker, a typical CTI is about…