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IEC Protection Curves
“IEC (International Electro-technical Commission) 60255-3, Electrical relays - Part 3: Single input energizing
quantity measuring relays with dependent or independent time” standard describes the protection curves
named as below.
1. IEC STI: IEC Short Time Inverse Curve.
2. IEC SI: IEC Standard Inverse Curve.
3. IEC VI: IEC Very Inverse Curve.
4. IEC EI: IEC Extremely Inverse Curve.
5. IEC LTI: IEC Long Time Inverse Curve.
6. IEC Thermal Overload Curve.
Universal trip time delay formula is given above, while IEC 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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