🔧 Engineering Decision Tool D3

Protection Coordination

Interactive calculator

Evaluate and set protective device coordination for overcurrent and ground fault protection in power systems.

📐 Calculation Steps

1

Identify fault levels

From short-circuit study

Maximum and minimum fault current at each bus.

2

Plot TCC curves

Per manufacturer curves

Time-current characteristics on log-log scale.

3

Check selectivity

t_feeder + 0.1s < t_main for all I_f

Verify no time overlap at any fault level.

4

Arc flash review

E proportional to t_clear

Evaluate if decreased trip delay reduces arc flash energy.

🧮 Formula

Selectivity & TCC Analysis

t_feeder + margin < t_main at all fault levels; selectivity ratio = t_main/t_feeder > 1.5

Coordinated device operation ensures selective fault isolation.

Symbol	Variable	Unit	Description
I_f	Fault current	kA
t_main	Main device trip time	s
t_feeder	Feeder device trip time	s
margin	Selectivity margin	s

📋 Worked Example: 480V Switchboard Selectivity

Result:

🔍 Result Interpretation

✅ Margin > 0.2s

✅ 0.1-0.2s

✅ < 0.1s

🔥 Guidance

Coordination failure Margin < 0.1s at critical fault

Device miscoordination can cause wide-area outages.

💡 Engineering Recommendations

🔧

Arc flash > 8 cal/cm2 at downstream gear

Implement maintenance switch or zone interlocking.

Maintenance switch reduces main breaker delay to instantaneous during energized work.

⚡ Quick Facts

Current-based, time-based, energy-based, zone interlocking

Selectivity types

Distribution required selective per 240.12

NEC requirement

Faster trip = less energy, but may lose selectivity

Arc flash trade-off

🔬 Engineering Insight

"The designer's challenge is balancing selectivity with arc flash energy. Modern approaches use zone-selective interlocking (ZSI): fast trip for downstream faults (lower arc flash) while maintaining selectivity through signaling between devices."

📜 Standards & References

IEEE 242-2001 — IEEE

Protection and coordination of industrial power systems.

↗

NEC 240.12 — NFPA

Electrical system coordination requirements.

↗

❓ Frequently Asked Questions

What is the difference between total and partial selectivity? ▼

Total selectivity means full coordination through entire fault range. Partial means coordinated up to specific level, above which both devices trip.

🔧 Related Calculators

→

Short Circuit Calculator

→

Arc Flash Calculator

📖 Related Knowledge

→ Protective Device Coordination Fundamentals

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