Electrical Calculators

This calculator computes the protection trip time according to IEC 60255 and IEEE C37.112-1996 protection curves.

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:

\[t(I) = TMS \left(\frac{k}{\left(\left(\frac{I}{I_s}\right)^\alpha - 1\right)}\right)\]

Where:

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

IEC 60255 Equation Constants

Curve Type	k	α
Standard Inverse	0.140	0.020
Very Inverse	13.5	1
Extremely Inverse	80	2
Long Time Standard Inverse	120	1

IEEE C37.112-1996 IDMT Trip Curves

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

\[t(I) = TD \left(\frac{A}{\left(\left(\frac{I}{I_s}\right)^p - 1\right)} + B\right)\]