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ANSI / IEEE Protection Curves
“IEEE (The Institute of Electrical and Electronics Engineers, Inc.) C37.112-2006: IEEE Standard Inverse-Time
Characteristic Equations for Overcurrent Relays - Description” standard describes the protection curves named
as below.
1. IEEE MI: IEEE Moderately Inverse Curve.
2. IEEE VI: IEEE Very Inverse Curve.
3. IEEE EI: IEEE Extremely Inverse Curve.
Universal trip time delay formula is given above, while ANSI / IEEE constants for various curves are given on the
below table.
t Trip delay (s).
A A constant for the characteristic (s).
I Momentary current (A).
Is Set current threshold (A).
α A constant for the characteristic (-).
B A constant for the characteristic (s).
TMS Time Multiplier Setting (-). □
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