Appendix A: Trip Curves & Equations
A Trip Curves & Equations
This section provides equations and curve characteristics for current and voltage to show the relationship between trip time and threshold levels. Determine which curve closely fol- lows the requirements of your system and select this curve in the applicable protective functions.
A.1 Instantaneous Curve
The Instantaneous response characteristics can be used with protection functions 50, 50N, 50HS, and 50HSN.
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TriptoTime | 0.1 |
0.01 
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Multiples of Pickup
Figure A.1 Instantaneous Curve
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T = time to trip, in seconds |
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D = time dial setting, 0.1 to 9.9 in steps of 0.1
A, B, N, tr = constants
Equation A.1 Standard Inverse Curves Equation
A
A.2 Standard Time Overcurrent Equation
The ISGS comes with nine standard overcurrent characteris- tic curves that can be adjusted with a time dial parameter. Seven of the nine curves are based on suggested IEEE stan- dards for approximation of electromechanical relays.
Table A.1 describes the first seven curves (SEA1 to SEA7) listed below.
Standard Overcurrent Coefficients
Curve Type | Des. | A1 | B1 | N1 | tr |
Inverse | SEA 1 | 8.9341 | 0.17966 | 2.0938 | 8.8 |
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Short Inverse | SEA 2 | 0.2663 | 0.03393 | 1.2969 | 0.831 |
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Long Inverse | SEA 3 | 5.6143 | 2.18592 | 1.0000 | 12.9 |
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Moderately | SEA 4 | 0.3022 | 0.12840 | 0.5000 | 1.07 |
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Very Inverse | SEA 5 | 5.4678 | 0.10814 | 2.0469 | 5.741 |
Extremely | SEA 6 | 7.7624 | 0.02758 | 2.0938 | 7.432 |
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Slightly | SEA 7 | 0.4797 | 0.21359 | 1.5625 | 1.5625 |
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The A, B, and N coefficients are for the standard relay formula
Table A.1 Standard Overcurrent Coefficients
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Trip(Seconds) | 10 |
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0.01 |
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Multiples of Pickup
Figure A.2 Inverse Curve (SEA1)
Siemens Energy & Automation, Inc. | 53 |
