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IDMT Relay Coordination Calculator

Enter settings for two or more relays (downstream to source) to compute trip times and check grading margin across the full fault-current range. Curves per IEC 60255-151 and IEEE C37.112.

AAJappsEngineering Calculation Report

IDMT Relay Coordination Calculator

IEC 60255-151 / IEEE C37.112

Generated

For preliminary engineering use only — always verify results against project-specific standards and a qualified engineer's review before use in a real design.

Relay 1 (Downstream)

Curve standard

Curve type

Pickup current, Is (A)

TMS

CT ratio (primary : 1) — use 1 if fault current entered is already relay-side

Relay 2 (Upstream)

Curve standard

Curve type

Pickup current, Is (A)

TMS

CT ratio (primary : 1) — use 1 if fault current entered is already relay-side

Fault current to evaluate (A)

Max fault current for graph & sweep (A)

Time-current characteristic (TCC)

Results

Relay 1 (Downstream) trip time

0.227 s

at 2,000 A

Relay 2 (Upstream) trip time

0.658 s

at 2,000 A

Required grading margin (CTI)

0.4 s typical for electromechanical relays, 0.3 s for numerical/digital

Relay 1 (Downstream) → Relay 2 (Upstream) margin at 2,000 A

+0.432 s

Full-range chain coordination check

Grading margin swept across the entire fault current range shown on the graph for every successive relay pair, not just the single point above.

Relay 1 (Downstream) → Relay 2 (Upstream)

Minimum margin 0.254 s at 10,000 A

FAIL