Variables
Description
What is this formula?
This formula calculates the operating time of an IEC Standard Inverse overcurrent relay based on its Time Multiplier Setting (TMS) and Plug Setting Multiplier (PSM).
When to use it
Use this formula when coordinating protection systems, adjusting overcurrent relays, verifying selectivity, and performing fault-clearing studies.
Example
An overcurrent relay has:
TMS = 0.20
PSM = 10
Formula:
t = 0.14TMS / (PSM^0.02 − 1)
Substitution:
t = (0.14 × 0.20) / (10^0.02 − 1)
Result:
t = 0.595 s
Applications
Protection coordination studies
Overcurrent relay settings
Distribution system protection
Substation protection design
Fault-clearing analysis
Note
This equation corresponds specifically to the IEC Standard Inverse characteristic. Other widely used relay curves such as IEC Very Inverse, IEC Extremely Inverse, IEEE Moderately Inverse, and IEEE Very Inverse use different coefficients and exponents. Therefore, the operating time obtained is valid only for the selected relay characteristic.