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Purpose
The purpose of this guide is to provide a basic overcurrent protection philosophy for conductors.
Cable Overcurrent Protection
Time-current curve (TCC) landmarks (figure 1)
- Ampacity – located in the upper decade
- Emergency overload curve – located in the upper 2 decades, typically not shown
- Short circuit damage curve – located in the bottom 3 decades
TCC regions (figure 2)
- Equipment operating region – located at and to the left of the ampacity
- Equipment damage region – located to the right and above the damage curves
- Protective device operating region – located between the equipment operating and damage regions
Suggested overcurrent protection (figure 3)
- Set device long time pickup (LTPU) function at or below the ampacity
- Set all other device functions at or below the damage curves
Comments
- If current penetrates the limits of the thermal overload curve, cable insulation life is reduced.
- If the maximum through-fault current penetrates the limits of the short circuit damage curve, cable insulation will be damaged. The through-fault current is defined as the maximum current that can flow for a fault at the load-side terminals of the feeder.
Fig. 1 Cable TCC landmarks
Fig. 2 Cable TCC regions
Fig. 3 Cable overcurrent protection
OH Conductor Overcurrent Protection
TCC landmarks (figure 4)
- Ampacity – located in the upper decade
- Short Circuit Damage Curve – located in the bottom 3 decades
TCC regions (figure 5)
- Equipment operating region – located at and to the left of the ampacity
- Equipment damage region – located to the right and above the damage curve
- Protective device operating region – located between the equipment operating and damage regions
Suggested overcurrent protection (figure 6)
- Set device long time pickup (LTPU) function at or below the ampacity
- Set all other device functions below the damage region
Comments
- If the maximum through-fault current penetrates the limits of the short circuit damage curve, conductor damage will occur. The through-fault current is defined as the maximum current that can flow for a fault at the load-side terminals of the conductor.
Fig. 4 OH conductor TCC landmarks
Fig. 5 OH conductor TCC regions
Fig. 6 OH conductor overcurrent protection
References
- Other Application Guides offered by SKM Systems Analysis at www.skm.com
- Aluminum Electrical Conductor Handbook, The Aluminum Association Inc., Washington, D.C., 3rd edition, 1989
- Electrical Transmission and Distribution Reference Book, ABB Power T&D Company, Raleigh, North Carolina, 1997
- Protective Relaying Theory and Applications, 2nd Edition, Marcel Dekker, New York, 2004
The latest revision of:
- IEEE Std 242, IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book)
- IEEE Std C37.113, IEEE Guide for Protective Relay Applications to Transmission Lines
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