Overcurrent Relay Curve Constants
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Below is a table of curve constants used for different overcurrent relay characteristics.
These constants
are aligned with industry standards like IEEE, IEC, and SEL relay documentation.
| Curve Type |
A |
B |
P |
L |
Description |
| SEL U1 |
1.5 |
1 |
2 |
0 |
Standard SEL curve with moderate inverse characteristics. |
| SEL U2 |
1.8 |
1 |
2 |
0 |
Moderately faster SEL curve for quicker operation. |
| SEL U3 |
2.0 |
1 |
2 |
0 |
SEL curve with faster response to high fault currents. |
| SEL U4 |
2.5 |
1 |
2 |
0 |
SEL curve for systems requiring faster high fault response. |
| SEL U5 |
3.0 |
1 |
2 |
0 |
Highly sensitive SEL curve for maximum fault current response. |
| IEEE Very Inverse (VI) |
19.61 |
0.491 |
2 |
0 |
Defined by IEEE C37.112, suitable for heavily loaded systems. |
| IEEE Extremely Inverse (EI) |
28.2 |
0.1217 |
2 |
0 |
Steeper curve for systems requiring highly sensitive protection. |
| IEEE Moderately Inverse (MI) |
0.0515 |
0.114 |
0.02 |
0 |
Standard IEEE curve for balanced inverse fault response. |
| IEC Standard Inverse (C1) |
0.14 |
0 |
0.02 |
0 |
Standard IEC curve for typical inverse-time characteristics. |
| IEC Very Inverse (C2) |
13.5 |
0 |
1 |
0 |
IEC curve for faster fault current response. |
| IEC Extremely Inverse (C3) |
80 |
0 |
2 |
0 |
Highly steep IEC curve for sensitive fault conditions. |
| IEC Long-Time Inverse (C4) |
120 |
0 |
1 |
0 |
Gradual IEC curve for overload conditions. |
| IEC Short-Time Inverse (C5) |
0.05 |
0 |
0.04 |
0 |
Rapid IEC curve for high transient fault conditions. |
| US U1 (Moderately Inverse) |
0.0226 |
0.0104 |
0.02 |
0 |
Moderate response curve for typical inverse-time applications. |
| US U2 (Inverse) |
0.180 |
5.95 |
2 |
0 |
Inverse curve with faster operation at higher fault currents. |
| US U3 (Very Inverse) |
0.0963 |
3.88 |
2 |
0 |
Very steep curve for heavy fault current applications. |
| US U4 (Extremely Inverse) |
0.02434 |
5.64 |
2 |
0 |
Extremely steep curve for sensitive fault current response. |
| US U5 (Short-Time Inverse) |
0.00262 |
0.00342 |
0.02 |
0 |
Short-time inverse curve for rapid fault clearance. |
| UK Long-Time Inverse (LTI) |
120 |
0 |
1 |
0 |
Used in UK systems for gradual overload protection. |
| UK Reset (R) |
45900 |
0 |
5.6 |
0 |
UK-specific curve for highly steep reset conditions. |
For more details, refer to IEEE C37.112
and IEC 60255.