Relay Coordination Tool

IEC 60255 IDMT times · CTI coordination check · Time-current curves

Relay Settings

Primary (Downstream) Relay

Curve (IEC 60255)

CT Primary (A)

Pickup (× In)

TMS

Set Time (s)

Backup (Upstream) Relay

Curve (IEC 60255)

CT Primary (A)

Pickup (× In)

TMS

Set Time (s)

Fault Current (A, primary)

Min. CTI (s)

Result

Enter relay settings and fault current, then calculate to see operating times and the time-current curve

IEC 60255 formula results for planning and study purposes. Verify against relay manufacturer curves and a full protection coordination study before commissioning.

Relay Coordination Tool: IDMT Operating Time and Two-Relay CTI Coordination Check

Protection relays need to trip in the right order – the relay closest to a fault should operate first, with the backup relay upstream waiting long enough (the CTI or Coordination Time Interval) to act only if the first one fails. This tool calculates exact IDMT operating time per IEC 60255 curves, checks two relays together for coordination and if they’re not coordinated – tells you exactly what TMS to set on the backup relay to fix it.

How to Use

Choose Single Relay mode to check one relay or Two-Relay Coordination mode to check primary and backup together.
For each relay, select the curve – Standard Inverse, Very Inverse, Extremely Inverse or Definite Time.
Enter CT Primary (A) and Pickup (× In) – together these give the actual pickup current.
Enter the TMS (or Set Time for Definite Time curve).
Enter the Fault Current (A, primary) at the relay location.
In coordination mode, select the Min. CTI (0.30s, 0.35s, or 0.40s).
Click Calculate Operating Time to see the result, coordination status and the time-current curve.

Key Features

All four IEC 60255 curves: Standard Inverse, Very Inverse, Extremely Inverse, Definite Time
Single-relay mode for a quick operating time check
Two-relay coordination mode checked against the same fault current
Automatic pass/fail status based on your selected minimum CTI
When not coordinated, calculates the required backup TMS (rounded to the nearest standard step) automatically
Visual time-current characteristic curve (log-log) for both relays
Flags the “relay does not pick up” case separately
Export as PDF (including the curve) and copy result option

Formula / Logic Used

IDMT Operating Time (IEC 60255) $t = \frac{TMS \times K}{(M)^{\alpha} – 1}$

Where $M$ = fault current ÷ pickup current and $K, \alpha$ are curve constants: Standard Inverse (K=0.14, α=0.02), Very Inverse (K=13.5, α=1), Extremely Inverse (K=80, α=2). For Definite Time, operating time is simply the fixed “Set Time,” regardless of fault current.

Two-Relay Coordination Check $CTI = t_{backup} – t_{primary}$

Relays are coordinated if CTI ≥ your selected minimum (typically 0.3s). If not:

$Required\ TMS_{backup} = \frac{TMS_{backup} \times (t_{primary} + CTI_{min})}{t_{backup}}$

This is rounded up to the nearest standard relay step (0.025) for a practical, settable value.

Who Should Use This Tool

Protection engineers setting or verifying overcurrent relay grading on radial feeders. Also useful for diploma and B.Tech Electrical students learning IDMT curves, TMS and CTI concepts.

Frequently Asked Questions (FAQs)

1. How do I calculate the operating time of an IDMT relay?

Divide fault current by pickup current to get the multiple of pickup (M), then apply the IEC 60255 formula with the K and α constants for your curve. This tool does it instantly from your CT rating, pickup, TMS and fault current.

2. What is CTI in relay coordination, and why does it matter?

CTI is the minimum time gap needed between a primary relay and its backup so the backup only acts if the primary fails. Without enough CTI, both relays could trip together , cutting off more of the network than necessary.

3. What TMS should I set on my backup relay if it’s not coordinated?

It depends on how much extra time the backup needs above the primary’s operating time to meet your minimum CTI. This tool calculates the exact required TMS and rounds it to the nearest standard step (0.025).

4. Standard Inverse vs Very Inverse vs Extremely Inverse – what’s the difference?

These curves differ in how fast operating time drops as fault current rises – Extremely Inverse drops fastest, suited to circuits with high inrush, while Standard Inverse is the general-purpose default. Compare all three for the same settings here.

5. Why would a relay not operate even during a fault?

If fault current is less than or equal to the pickup current, the relay never trips, however long you wait. This tool flags that case separately so you know the pickup setting may need lowering.

Related Tools

Transformer Calculator – get the transformer’s secondary fault current for use here
Cable Size Calculator – check short-circuit withstand of the protected cable
Voltage Drop Calculator – verify drop on the same protected feeder

Relay Coordination Tool: IDMT Time & CTI Check (IEC 60255)