Key Parameters:

• Trip Curve: Select from the standard IEC and IEEE curves.
• Relay Pickup Current (A): The relay's pickup set point current, denoted as I_s
• Fault Current (A): The anticipated fault current during short circuits.
• TMS (IEC) or TD (IEEE) Setting: Time settings for the relay:
  • IEC: Time Multiplier Setting (TMS).
  • IEEE: Time Dial (TD) or Time Dial Multiplier (TDM).

What is Inverse Time Overcurrent (TOC)?

TOC (or IDMT - Inverse Definite Minimum Time) ensures trip times are inversely proportional to the fault current, providing faster response for higher fault currents.

What is IDMT?

IDMT (Inverse Definite Minimum Time) is a time-current characteristic where higher fault currents result in quicker tripping of the relay.

IEC 60255 IDMT Trip Curves

The trip time for IEC 60255 can be calculated using the formula:

Where:

• \(I_s\): Relay's current setting.
• \(I\): Actual measured current.
• \(k\), \(\alpha\): Curve constants (see table below).

IEC 60255 Equation Constants

IEEE C37.112-1996 IDMT Trip Curves

For IEEE C37.112-1996 standards, the trip time is given by:

Where:

• \(I_s\): Relay's current setting.
• \(I\): Actual measured current.
• \(A\), \(B\), and \(p\): Curve constants (see table below).

IEEE C37.112-1996 Equation Constants