IDMT Operating Time Calculator

The IDMT Operating Time Calculator estimates relay tripping time. Simply enter your fault current, pickup setting, and curve type to calculate your operating time. This tool helps engineers and technicians understand how long a protection relay may wait before activating. This calculator also calculates the multiple of pickup current.

Enter the fault current flowing through the relay in Amps
Select the secondary rating of the current transformer
Enter the current at which the relay starts to operate
Enter the time multiplier setting (0.01 to 1)
Select the IEC 60255 curve characteristic

This calculator is for informational purposes only. It is based on standard IEC 60255 formulas. Actual relay operating times may vary by manufacturer. Verify results with appropriate professionals for important decisions.

What Is IDMT Operating Time

IDMT stands for Inverse Definite Minimum Time. This is the time it takes for a protection relay to trip a circuit breaker when there is too much current. The time is "inverse" because higher currents usually make the trip faster. This helps isolate dangerous faults quickly while avoiding trips for small, temporary issues.

How IDMT Operating Time Is Calculated

Formula

t = TMS × k / ((M^α) − 1)

Where:

  • t = Operating time in seconds
  • TMS = Time Multiplier Setting
  • M = Multiple of pickup current (Fault Current / Pickup Current)
  • k and α = Curve constants (Standard, Very, Extremely, or Long Inverse)

The formula compares the fault current to the pickup setting to find the ratio "M". If the fault is much bigger than the setting, the math makes the time smaller. The constants "k" and "alpha" change the shape of the curve to suit different needs, like clearing heavy faults fast or waiting a bit for smaller overloads.

Why IDMT Operating Time Matters

Knowing the tripping time is vital for designing safe electrical systems. It ensures that relays work together correctly so that only the faulty part of the system is switched off.

Why Correct Timing Is Important for Protection Coordination

If the time is wrong, the relay might trip too slowly, damaging equipment. Or, it might trip too fast when it should wait for another device to act. This can cause power outages for more people than necessary. Proper calculation helps prevent widespread blackouts.

For Selecting the Right Curve

Different curves are used for different jobs. Standard Inverse is common for general feeders. Extremely Inverse is often used for motor protection because motors need a lot of current to start but should trip fast if the current stays too high.

For Equipment Safety

The calculation helps ensure that machines are not exposed to high currents for too long. Overheating can destroy insulation and cause fires. By estimating the time correctly, engineers can choose settings that protect expensive assets effectively.

Calculation logic verified using publicly available standards.

View our Accuracy & Reliability Framework →