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TM 5-811-14

TECHNICAL MANUAL

COORDINATED POWER SYSTEMS PROTECTION

APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED

HEADQUARTERS, DEPARTMENT OF THE ARMY

FEBRUARY 1991

TM 5-811-14

REPRODUCTION AUTHORIZATION/RESTRICTIONS

This manual has been prepared by or for the Government and,except to the extent indicated below, and is public property andnot subject to copyright.

Copyrighted material included in the manual has been used withthe knowledge and permission of the proprietors and is acknowl-edged as such at point of use. Anyone wishing to make further useof any copyrighted material, by itself and apart from this text,should seek necessary permission directly from the proprietors.

Reprints or republications of this manual should include a creditsubstantially as follows: Department of the Army, TM 5-811-14,Coordinated Power System Protection.

If the reprint or republication included copyrighted material, thecredit should also state: Anyone wishing to make further use ofcopyrighted material, by itself apart from the text, should seeknecessary permission directly from the proprietor.

TM 5-811-14

i

ATECHNICAL MANUAL HEADQUARTERSNo. 5-811-14 DEPARTMENT OF THE ARMY

Washington, DC, 25 February 1991

COORDINATED POWER SYSTEM PROTECTION

Paragraph PageCHAPTER 1. GENERAL REQUIREMENTS

Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1 1-1Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2 1-1References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3 1-1Electrical power systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4 1-1Design procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-5 1-1

CHAPTER 2. ELECTRICAL POWER SYSTEM OVERCURRENTS General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1 2-1Normal current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2 2-1Overload current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3 2-1

APPROVED FOR PUBLIC RELEASE: DISTRIBUTIONS IS UNLIMITEDShort-circuit current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4 2-1Ground-fault current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5 2-1Sources of short-circuit current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6 2-2Time variation of short-circuit current . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7 2-3Symmetrical and asymmetrical short-circuit current . . . . . . . . . . . . . . . . . . 2-8 2-5

CHAPTER 3. OVERCURRENT PROTECTIVE DEVICESGeneral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1 3-1Motor overload relays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2 3-1Fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3 3-1Motor short-circuit protectors (MSCP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4 3-3Circuit breakers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5 3-3Protective relays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6 3-9Automatic reclosing devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7 3-10

CHAPTER 4. PROTECTIVE DEVICES COORDINATIONGeneral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4-1The coordination study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2 4-1Primary and medium-voltage coordination . . . . . . . . . . . . . . . . . . . . . . . . . 4-3 4-3Low-voltage coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4 4-5Ground-fault coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5 4-5Coordination requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6 4-6Maintenance, testing, and calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7 4-7Example of phase coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8 4-7Example of ground-fault protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9 4-23

CHAPTER 5. ELECTRICAL SYSTEM PROTECTION TECHNIQUESGenerator protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1 5-1Transformer protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2 5-6Conductor protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3 5-15Motor protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4 5-20Bus and switchgear protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5 5-29Ground-fault protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6 5-34Miscellaneous equipment protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7 5-44

Appendix A. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1Appendix B. TYPICAL PROTECTIVE DEVICE SETTINGS . . . . . . . . . . . . . . . . . . . B-1Appendix C. TYPICAL TIME-CURRENT CHARACTERISTIC CURVES . . . . . . . . C-1Appendix D. TYPICAL DEVICE AND EQUIPMENT RATINGS . . . . . . . . . . . . . . . . D-1Appendix E. PARTIAL RELAY DEVICE NUMBERS LIST . . . . . . . . . . . . . . . . . . . . E-IAppendix F. REPRESENTATIVE PROTECTIVE DEVICE OPERATING TIMES . . F-1Appendix G. COORDINATION EXAMPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-1Appendix H. COMPUTER SOFTWARE APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . H-1BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bibliography-1GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Glossary-1

TM 5-811-14

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LIST OF FIGURESFigure No. Title Page

2-1. Short-circuit waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-42-2. Symmetrical and asymmetrical current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-63-1. Solid-state circuit breaker characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-43-2. Solid-state ground-fault characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-54-1. Army ammunition plant single-line diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-44-2. Ground-fault protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-64-3. Single-line diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-84-4. Key protection points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-104-5. LC feeder protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-124-6. IC MAIN protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-144-7. Switchgear feeder protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-164-8. Switchgear main protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-184-9. Composite time-current curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-20

4-10. Composite time-current curve using solid-state devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-224-11. Low-voltage ground-fault protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-24

5-1. Single-isolated low-voltage generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-25-2. Multi-isolated medium-voltage generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-35-3. Medium industrial generators (up to 12.5MVA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-45-4. Large industrial generators (greater than 12.5MVA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-55-5. Through-fault protection curve for liquid-immersed category I transformers (5kVA to 500kVA single-

phase, 15kVA to 500kVA three-phase) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-85-6. Through-fault protection curves for liquid-immersed category II transformers (501kVA to 1667kVA

single-phase, 501kVA to 5000kVA three-phase) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-95-7. Through-fault protection curves for liquid-immersed category Ill transformers (1668kVA to 10,000kVA

single-phase, 5001kVA to 10,000kVA single-phase, 5001kVA to 30,000kVA three-phase) . . . . . 5-105-8. Through-fault protection curve for liquid-immersed category IV transformers (above 10,000kVA single-

phase, above 30,000kVA three-phase) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-115-9. Transformer overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-13

5-10. Phase differential relaying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-155-11. Maximum short-circuit current for insulated copper conductors; initial temperature 75 degrees C; final

temperature 200 degrees C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-175-12. Emergency overload current percent of continuous rating EPR-XLP insulated 40 degrees C ambient . 5-195-13. Motor circuit requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-215-14. Minimum protection for induction motor less than 1500 Hp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-235-15. Minimum protection for induction motor 1500 HP and larger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-245-16. Minimum protection for MV, brushless synchronous motor less than 1500 Hp . . . . . . . . . . . . . . . . . . . 5-255-17. Minimum protection for MV, brushless synchronous motor 1500 Hp and larger . . . . . . . . . . . . . . . . . 5-265-18. Motor self-balance differential protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-285-19. Single bus scheme with bus differential relaying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-305-20. Sectionalized bus scheme with bus differential relaying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-315-21. Double bus scheme with bus differential relaying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-325-22. Synchronizing bus scheme with bus differential relaying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-335-23. Residual or common relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-355-24. Zero-sequence relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-365-25. Ground return relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-375-26. Ground differential relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-385-27. Standard overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-405-28. Main-only ground-fault protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-415-29. Improved ground-fault protection and coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-425-30. Main and feeder ground-fault protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-43C-1. Time-current characteristic curves for typical motor overload relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-2C-2. Time-current characteristic curve for typical solid state circuit breaker . . . . . . . . . . . . . . . . . . . . . . . . . C-3C-3. Time-current characteristic curve for typical ground fault attachment . . . . . . . . . . . . . . . . . . . . . . . . . . C-4C-4. Time-current characteristic curve for typical protective relay (50/51) . . . . . . . . . . . . . . . . . . . . . . . . . . C-5C-5. Time-current characteristic curve for typical medium-voltage fuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-6C-6. NEMA damage curves (through 1200A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-7C-7. NEMA damage curves (through 5000A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-SG-1. Example electrical system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-2G-2. 14kV switchgear feeder No.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-3G-3. 15kV switchgear feeder No.7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-4

TM 5-811-14

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LIST OF FIGURES ContinuedFigure No. Title Page

G-4. M1 starting, overload, and CB short-circuit protection curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-7G-5. M2 starting, overload, and CB short-circuit protection curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-8G-6. M3 starting, overload, and CB short-circuit protection curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-9G-7. M4 starting, overload, and CB short-circuit protection curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-10G-8. Composite time-current curves for Example 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-11G-9. M1 starting, overload, and MCP short-circuit protection curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-14

G-10. M2 starting, overload, and MCP short-circuit protection curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-15G-11. MS starting, overload, and MCP short-circuit protection curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-16G-12. M4 starting, overload, and MCP short-circuit protection curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-17G-13. Composite time-current curves for Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-18G-14. M1 starting, overload, and fuse short-circuit protection curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-20G-15. M2 starting, overload, and fuse short-circuit protection curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-21G-16. M3 starting, overload, and fuse short-circuit protection curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-22G-17. M4 starting, overload, and fuse short-circuit protection curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-25G-18. Ground-fault protection at main only (nonselective, single-zone) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-25G-19. Ground-fault protection at main feeder, and branch (selective, time coordinated) . . . . . . . . . . . . . . . . . G-26G-20. Impedance diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-28G-21. Per unit diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-29G-22. M5 staffing, overload and fuse short-circuit curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-31G-23. Protective relays 1, 2, 3, 5, 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-33G-24. Protective relays 1, 2, 3, 4, 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-35

LIST OF TABLESTable No. Title Page

3-1. UL fuse classifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23-2. Circuit breaker trip ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-63-3. Circuit breaker application comparision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-94-1. Maximum overcurrent protection (in percent ) per NFPA 70 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-74-2. Summary of initial protective device settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-234-3. Summary of new settings using solid-state devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-23D-1. Low voltage fuse ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-1D-2. Distribution fuse cutouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-2D-3. Expulsion-type power fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-2D-4. Boric acid type power fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-2D-5. Current-limiting power fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-2D-6. Typical tap ranges and settings of time-overcurrent relays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-2D-7. Typical reactances of three-phases synchronous machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D-2F-1. Relayed circuit breakers, 2.4-13.8kV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F-1F-2. Power air circuit breakers, below 600V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F-1F-3. Molded-case circuit breakers, below 600V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F-1F-4. Medium-and high-voltage fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F-1F-5. Low-voltage fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F-1G-1. Summary of device settings for figure G-23 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-32G-2. Summary of device settings for figure G-24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G-34

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CHAPTER 1

ELECTRICAL POWER SYSTEM OVERCURRENTS

1-1. Purpose 34.5kV to 138kV. Distribution substations furtherThis manual establishes the criteria for design, co-ordination, and construction of power systems formilitary projects. A secondary purpose is to ac-quaint designers with the general applications,philosophies, and purposes for the selection, cali-bration, setting, and testing of protective devices.

1-2. ScopeThis manual describes protection techniques forelectrical power supply and distribution systems.Guidance is included for coordination techniquesand selection of protective devices.

1-3. ReferencesAppendix A contains a list of references used in thisdocument.

1-4. Electrical power systemsElectric power systems consist of four major cate-gories: generating stations, transmission lines, dis-tribution lines, and utilization systems. The electricpower system industry converts and transportsenergy for utilization by numerous industrial,commercial, and residential customers. One of thelargest users of electric energy in the United Statesis the U.S. Government.

a. Generation. The majority of electrical powerproduced in the United States is generated bysteam-turbine plants. Hydro-electric generation ac-counts for only a small percentage because mostavailable water sources have already been placedinto service. Gas-turbines are used primarily forpeaking during short periods of high demand. Fuelfor steam-turbine plants is, for the most part, coalor nuclear.

b. Transmission lines. The voltage rating oflarge generators employed at primary generatingstations ranges from 13.8kV to 24kV. Generatorvoltage is stepped up to transmission voltage levelusing transformers. Transmission voltage levels inthe United States range from 115kV to 765kV.Standard voltages are 115kV, 138kV, 230kv,345kV, 500kV, and 765kV.

c. Distribution lines. Transmission line voltageis stepped down to lower levels at main substations.These lower voltage levels range from about

step the voltage down to distribution voltage levelwhich is in the range of 5kV to 34.5kV. Popularstandard voltages at the distribution level are4.16kV 12.47kV, 13.2kV, 13.8kV, and 34.5kV.

d. Utilization. Distribution transformers areused to step the distribution voltage down to utili-zation levels, usually at 600V. Standard utilizationvoltages include 480Y/277V, 460V, 208Y/120V,240V, and 120V. Higher-level voltages, such as6.9kv and 4.16kV, are popular standard voltagesfor supplying large industrial motor loads.

1-5. Design proceduresUtility features should be designed concurrentlywith the planning of the new installation. The se-lection and design of the power supply and distri-bution systems will depend on the availability, ca-pacity, and reliability of the existing and new sys-tems.

a. Mission. The plant and its mission must begiven priority in establishing coordination require-ments, and in selecting features such as dual feed-ers, or back-up power generation. Where severaldesigns are feasible, the selection will be based onan economic study.

b. New installations. Assistance from local elec-tric utility companies and cooperatives may besought during preliminary design, but no commit-ment should be made to obligate the Governmentto procure electrical power or engage in contractnegotiations. Contact with the local supplier shouldbe limited to obtaining information on sources ofelectricity, their connection point location in regardto the site, conditions of service, utility capacity,and protective device ratings and settings. Theelectric supply and distribution systems shouldconform to prevailing practices of the utility servicearea insofar as they do not conflict with criteria inthis manual.

c. Existing installations. Coordinate planningwith the Directorate of Engineering and Housing,Base Civil Engineer, or official in charge to ensurethat enough power will be available and that thedesign is compatible with the master plan for theinstallation. This may require anticipating what themaster plan should be.

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CHAPTER 2

ELECTRICAL POWER SYSTEM OVERCURRENTS

2-1. General but less than locked-rotor amperes and flows onlyElectrical power systems must be designed to servea variety of loads safely and reliably. Effectivecontrol of short-circuit current, or fault current asit is commonly called, is a major consideration Short-circuit current is greater than locked-rotorwhen designing coordinated power system protec- current and may range upwards of thousands oftion. In order to fully understand the nature of fault amperes. The maximum value is limited by thecurrent as it is applied to electrical power system maximum short-circuit current available on thedesign, it is necessary to make distinctions among system at the fault point. Short-circuit current maythe various types of current available, normal as be further classified as bolted or arcing.well as abnormal. It is also important to a. Bolted short-circuit current. Bolted short-cir-differentiate between the paths which the various cuit current results from phase conductors becom-types of current will take. Both current type and ing solidly connected together. This may occurcurrent path, as well as current magnitude, will from improper connections or metal objects becom-affect the selection and application of overcurrent ing lodged between phases. Obviously, largeprotective devices. amounts of short-circuit current will flow into a

2-2. Normal current b. Arcing short-circuit current. Arcing short-Normal, or load, current may be defined as thecurrent specifically designed to be drawn by a loadunder normal, operating conditions. Dependingupon the nature of the load, the value of normalcurrent may vary from a low level to a full-loadlevel. Motors offer a good example. Normal motorcurrent varies from low values (under light loading)to medium values (under medium loading) tomaximum values (under maximum loading). Maxi-mum load current is called full load current and isincluded on the motor nameplate as FLA (Full-Load Amperes). Normal current, therefore, mayvary from low values to FLA values. Additionally,normal current flows only in the normal circuitpath. The normal circuit path includes the phaseand neutral conductors. It does not include equip-ment grounding conductors.

2-3. Overload currentOverload current is greater in magnitude than full-load current and flows only in the normal circuitpath. It is commonly caused by overloadedequipment, single-phasing, or low line voltage, andthus is considered to be an abnormal current. Someoverload currents, such as motor starting currents,are only temporary, however, and are treated asnormal currents. Motor starting current is afunction of the motor design and may be as much astwenty times full-load current in extreme cases.Motor starting current is called locked-rotorcurrent and is included on the motor nameplate asLRA (Locked-Rotor Amperes). Overload current,then, is greater in magnitude than full-load amperes

in the normal circuit path.

2-4. Short-circuit current

bolted fault.

circuit current results from phase conductorsmaking less than solid contact. This condition mayresult from loose connections or insulation failure.When this happens, an arc is necessary to sustaincurrent flow through the loose connection. Sincethe arc presents an impedance to the flow of cur-rent, smaller amounts of current will flow into anarcing fault than will flow into a bolted fault.

c. Failure classifications. Short-circuit cur-rents, whether bolted or arcing, will involve two ormore phase conductors. Line-to-line faults involvetwo-phase conductors (A-B, B-C, C-A) whilethree-phase faults involve all three phases (A-B-C).Although three-phase bolted short-circuits rarelyoccur in practice, short-circuit studies have tradi-tionally been based upon the calculation of three-phase, bolted short-circuit current. Modern person-al computers and associated software have madethe calculation of all types of fault currents easier toaccomplish.

2-5. Ground-fault currentGround-fault current consists of any current whichflows outside the normal circuit path. A ground-fault condition then, results in current flow in theequipment grounding conductor for low-voltagesystems. In medium- and high-voltage systems,ground-fault current may return to the sourcethrough the earth. Ground-fault protection ofmedium-voltage and high-voltage systems has beenapplied successfully for years using ground currentrelays. Ground-fault protection of low-voltage sys-tems is a considerable problem because of the pres-

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ence and nature of low-level arcing ground faults.Ground-fault current on low-voltage systems maybe classified as leakage, bolted, or arcing.

a. Leakage ground-fault current. Leakageground-fault current is the low magnitude current(milliampere range) associated with portable toolsand appliances. It is caused by insulation failure,and is a serious shock hazard. Personnel protectionis accomplished by using ground-fault circuitinterrupters (GFCI) in the form of GFCI-receptacles or GFCI-circuit-breakers.

b. Bolted ground-fault current. Bolted ground-fault current results when phase conductors becomesolidly connected to ground (i.e., the equipmentgrounding conductor or to a grounded metallicobject). Bolted ground-fault current may equal oreven exceed three-phase, bolted short-circuitcurrent if the system is solidly grounded. Equip-ment protection is accomplished by using standardphase and ground overcurrent devices dependingupon system voltage levels.

c. Arcing ground-fault current. Arcing ground-fault current results from a less than solid connec-tion between phase conductors and ground. Be-cause an arc is necessary to sustain current flowthrough the connection, the magnitude of arcingground-fault current will be less than that of boltedground-fault current. Depending upon the arcimpedance, arcing ground-fault current may be aslow as several amperes (low-level) or as high as20-38 percent of three-phase, bolted short-circuitcurrent (high level) on a 480V system. Consider-able research has been conducted in the area ofarcing ground-fault current magnitudes on lowvoltage systems. Some designers use the 38 percentvalue while others use the 20 percent figure.NEMA PB2.2 applies ground-fault damage curvesinstead of performing a calculation. Equipmentprotection is accomplished by using ground-faultprotective (GFP) devices. Due to ionization of theair, arcing ground faults may escalate into phase-to-phase or three-phase faults.

2-6. Sources of short-circuit currentAll sources of short-circuit current and the impe-dances of these sources must be considered whendesigning coordinated power system protection.

a. Synchronous generators . When a short-cir-cuit occurs downstream of a synchronous genera-tor, the generator may continue to produce outputvoltage and current if the field excitation is main-tained and the prime mover continues turning thegenerator at synchronous speed. The flow of short-circuit current from the generator into the fault islimited only by the generator impedance anddownstream circuit impedances. The magnitude of

generator fault current depends on the armatureand field characteristics, the time duration of thefault, and the load on the generator. The ability ofa generator to supply current during a fault is afunction of the excitation system.

(1) Some generator excitation systems do nothave the ability to sustain short-circuit current. Themagnitude of fault current is determined by thegenerator reactance, and, for such systems, can beessentially zero in 1.0 to 1.5 seconds.

(2) Static exciters derive excitation voltagefrom the generator terminals. Since static excitersdo not sustain short-circuit current, protective de-vices on the system will not operate properly, or atall. Static exciters, therefore, are not recommended.Static exciters with current boost should bespecified for applications requiring static excitation.

(3) Round-rotor generators with brushless ex-citers, typically above 10 MVA, can sustain short-circuit current for several seconds. Salient-polegenerators less than 10 MVA, also with brushlessexciters, will typically sustain short-circuit currentat 300 percent of generator full load amperes.

b. Synchronous motors. When a short-circuitoccurs upstream of a synchronous motor, thesystem voltage goes to zero, and the motor beginslosing speed. As the motor slows down, the inertiaof the load is actually turning the motor and causingit to act like a generator. The synchronous motorhas a dc field winding, like a generator, and actuallydelivers short-circuit current into the fault until themotor completely stops. As with a generator, theshort-circuit current is limited only by thesynchronous motor impedance and the circuitimpedance between the motor and the fault.

c. Induction motors. With one slight difference,a short-circuit upstream of an induction motorproduces the same effect as with a synchronousmotor. Since the induction motor has no dc fieldwinding, there is no sustained field current in therotor to provide flux as is the case with a synchro-nous machine. Consequently, the short-circuit cur-rent decays very quickly.

d. Supply transformers. Supply transformers arenot sources of short-circuit current. Transformersmerely deliver short-circuit current from the utilitygenerators to the fault point. In the process,transformers change the voltage and current mag-nitudes. Transformer impedances will also limit theamount of short-circuit current from the utilitygenerators. Standard tolerance on impedance isplus or minus 7.5 percent for two-winding trans-formers and plus or minus 10 percent for three-winding transformers. The minus tolerance shouldbe used for short circuit studies and the plus toler-ance for load flow and voltage regulation studies.

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2-7. Time variation of short-circuit current current from a rotating machine at any instant inThe short-circuit current from various sources isillustrated in figure 2-1. Since short-circuit currentfrom rotating machines varies with time, it isconvenient to express machine impedance (induc-tive reactance) as a variable value. This variablereactance will allow calculation of short-circuit

time. For the purpose of simplification, three valuesof reactance are assigned to rotating machines forthe purpose of calculating short-circuit current atthree specified times following the occurrence of afault. These three values are called subtransient,transient, and synchronous reactances.

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2-5

a. Subtransient reactance (Xd). Subtransient occurs at some time between the zero and peak ofreactance is a value used to determine the short- the voltage waveform, the resulting short-circuitcircuit current during the first few cycles after a current will be partially asymmetrical. The amountshort-circuit occurs. This is the short-circuit current of offset or asymmetry depends on the point whenvalue to be used in all short-circuit studies. the fault occurs. In circuits containing both

b. Transient reactance (Xd). Transient react- resistance and inductive reactance, the amount ofance is a value used to determine the short-circuit asymmetry will vary between the same limits ascurrent from the first few cycles up to about 30 before. However, the X/R ratio (ratio of inductivecycles after the short-circuit occurs (depending reactance to resistance looking upstream from theupon the design of the machine). This value is often fault point) will determine the rate of decay of theused in voltage regulation studies. DC component. As X/R increases, the rate of decay

c. Synchronous reactance (Xd). Synchronous decreases. Interrupting current ratings may have toreactance is a value used to determine the short-cir- be derated for high X/R values. Practicallycuit current when the steady state condition has speaking, most all short-circuit currents arebeen reached. Steady state is reached several sec- partially asymmetrical during the first few cyclesonds after the short-circuit occurs. This value is after a short-circuit occurs. Modern personaloften used to determine the setting of generator computers can now be used to easily calculatebackup overcurrent relays. symmetrical and asymmetrical current values at

2-8. Symmetrical and asymmetrical short- devices are rated on a symmetrical basis but testedcircuit currents on an asymmetrical basis. Medium-voltageAs shown in figure 2-2, "symmetrical" and "asym-metrical are terms used to describe the symmetryof the short-circuit current waveform around thezero axis. If a short-circuit occurs in an inductivereactive circuit at the peak of the voltage wave-form, the resulting short-circuit current will be to-tally symmetrical. If a short-circuit, in the samecircuit, occurs at the zero of the voltage waveform,the resulting short-circuit current will be totallyasymmetrical. If a short-circuit, in the same circuit,

various times after a fault. Low-voltage protective

switchgear has a momentary and an interruptingrating. The momentary rating is the short-circuitduty during the first cycle after a fault, and de- finesthe equipment's ability to close and latch againstworst-case mechanical stresses. The interruptingrating is the short-circuit duty as the equipmentcontacts part, and is expressed in symmetricalamperes or MVA. Medium-voltage fuses haveinterrupting ratings expressed in symmetricalamperes.

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TM 5-811-14

3-1

CHAPTER 3

OVERCURRENT PROTECTIVE DEVICES

3-1. General units should be used for motor starting times in theDesign of power system protection requires theproper application motor of overload relays, fuses,circuit breakers, protective relays, and other specialpurpose overcurrent protective devices. Thischapter provides detailed information about variousprotective devices, illustrates their time-currentcharacteristics, and identifies information requiredto design coordinated power system protection.

3-2. Motor overload relaysa. Thermal overload relays. The most common

overcurrent protective device is the thermal over-load relay associated with motor starting contac-tors. In both low-voltage and medium-voltagemotor circuits, thermal overload relays detectmotor overcurrents by converting the current toheat via a resistive element. Thermal overloadrelays are simple, rugged, inexpensive, and providevery effective motor running overcurrent protec-tion. Also, if the motor and overload element arelocated in the same ambient, the thermal overloadrelay is responsive to changes in ambient tempera-ture. The relay trip current is reduced in a highambient and increased in a low ambient. Typicaltime-current characteristic curves for thermaloverload relays are shown in appendix C. Thecurves level off at about 10 to 20 times full-loadcurrent, since an upstream short-circuit device,such as a fuse or circuit breaker, will protect themotor circuit above these magnitudes of current.The thermal overload relay, therefore, combineswith the short-circuit device to provide total over-current protection (overload and short-circuit) forthe motor circuit.

(1) Melting alloy type overload relays, as thename implies, upon the circuit when heat is suffi-cient to melt a metallic alloy. These devices may bereset manually after a few minutes is allowed forthe motor to cool and the alloy to solidify.

(2) Bimetallic type overload relays open thecircuit when heat is sufficient to cause a bimetallicelement to bend out of shape, thus parting a set ofcontacts. Bimetallic relays are normally used onautomatic reset, although they can be used eithermanually or automatically.

(3) Standard, slow, and quick-trip (fast) relaysare available. Standard units should be used formotor starting times up to about 7 seconds. Slow

8-12 second range, and fast units should be used onspecial-purpose motors, such as hermetically sealedand submersible pump motors which have very faststarting times.

(4) Ambient temperature compensated over-load relays should be used when the motor is lo-cated in a nearly-constant ambient and the thermaloverload device is located in a varying ambient.

b. Magnetic current overload relays. Basically,magnetic current relays are solenoids. These relaysoperate magnetically in response to an over-current. When the relay operates, a plunger ispulled upward into the coil until it is stopped by aninsulated trip pin which operates a set of contacts.Magnetic relays are unaffected by changes inambient temperature. Magnetic current relays maybe used to protect motors with long starting timesor unusual duty cycles, but are not an alternativefor thermal relays.

c. Information required for coordination. Thefollowing motor and relay information is requiredfor a coordination study.

(1) Motor full-load ampers rating from themotor nameplate.

(2) Overload relay ampere rating selected inaccordance with NFPA 70.

(3) Overload relay time-current characteristiccurves.

(4) Motor locked rotor amperes and startingtime.

(5) Locked rotor ampere damage time formedium-voltage motors.

3-3. FusesA fuse is a non-adjustable, direct acting, single-phase device that responds to both the magnitudeand duration of current flowing through it. Fusesmay be time delay or non-time delay, current-lim-iting or non-current-limiting, low-voltage or high-voltage. Fuse terminology and definitions are listedin the glossary. Underwriters Laboratories (UL)further classifies low-voltage fuses as shown intable 3-1.

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a. Nontime delay fuses. The Nontime delay fuse associated with medium-voltage and high-voltageconsists of a single type of fusible element, called a fuses graphically displays minimum melting timeshort-circuit element. Normal overloads and cur- and total (or maximum) clearing time as a functionrent surges often cause nuisance openings of this of current magnitude.type of fuse. For this reason, substantial oversizing e. Current-limitation. Current-limiting fusesof these fuses is required when used in motor cir- are so fast acting that they are able to open thecuits. Therefore, Nontime delay fuses should be circuit and remove the short-circuit current wellused only in circuits with noninductive loads such before it reaches peak value. Current-limiting fusesas service entrances and circuit breaker back-up limit the peak short-circuit current to a value lessprotection. than that available at the fault point and open in less

b. Time delay fuses. The time delay fuse is con- than one-half cycle. To be effective, however, suchstructed with two different types of fusible ele- fuses must be operated in their current-limitingments: overload and short-circuit. These elements range. Peak let-through charts, also called current-are somewhat similar in operation to the thermal limiting effect curves, should be used to determineand magnetic elements of an inverse-time circuit the effectiveness or degree of protection offered bybreaker. The overload element will interrupt all current-limiting fuses. These curves plotoverload currents, and the short-circuit element will instantaneous peak let-through current as a functionopen in response to short-circuit currents. The time of available RMS symmetrical short- circuit current.delay fuse can be applied in circuits subject to f. Medium-voltage fuses. Medium-voltage fusesnormal overloads and current surges (e.g., motors, are either (1) distribution fuse cutouts or (2) powertransformers, solenoids, etc.) without nuisance fuses. Distribution fuse cutouts are designed foropening. Significant oversizing is not necessary. pole or crossarm mounting and should be used pri-

c. UL classification. As shown in table 3-1, UL marily on distribution feeders and circuits. Powerhas established distinct classifications for low-volt- fuses have a higher dielectric strength than distri-age fuses. These classifications define certain oper- bution fuse cutouts and should be used primarily inating characteristics associated with a particular substations. The majority of medium-voltage fusesfuse class. However, the fact that a fuse is classi- are used for applications within buildings, vaults, orfied, for example, as UL RK-5 does not mean that enclosures. They are boric acid type fuses ratedall of its operating characteristics are identical with 416034.5kV or current-limiting fuses ratedthose of other manufacturers Class RK-5 fuses. 2400V34.5kV.Both time delay and Nontime delay fuses are g. High-voltage fuses. Some medium-voltageclassified as RK-5. Therefore, each type of RK-5 fuses and all high-voltage fuses are rated for out-fuse will require different application procedures. door use only. These devices are boric acid typeUL classifications and time delay characteristics fuses rated 4160V138kV, fiberlined expulsionshould always be specified along with current and fuses rated 7200Vl6lkV, or distribution fuse cut-voltage ratings for low-voltage fuses. This will outs rated 4800V138kV.eliminate any confusion on the Contractor's part h. Current-limiting power fuses. Current-limit-and insure that the correct fuse is always provided. ing power fuses include E-rated, C-rated, and R-UL Class H fuses are tested for short-circuit ratings rated fuses. E-rated, current-limiting, power fusesat 10,000 amperes symmetrical, and are, therefore, rated 100E and below open in 300 seconds at cur-not current-limiting. UL Class K fuses are tested at rents between 200 percent and 240 percent of their50,000, 100,000, or 200,000 amperes symmetrical, E-rating. Fuses rated greater than 100E open in 600

and are, therefore, current-limiting. However, ClassK fuses are not labeled as current-limiting, becauseClass K fuses are interchangeable with UL Class Hand NEMA Class H fuses. Therefore, Class H andClass K fuses should be avoided in favor of ClassRK and Class L fuses. Rejection-type fuses andfuse holders prevent underrated fuses frominadvertently being installed.

d. Time-current characteristic curves. Fusecurves are available from various manufacturers.'Typical fuse time-current characteristic curves areshown in appendix C. Medium-voltage and high-voltage fuses show an operating "band" while low-voltage fuses show an operating line. The band

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seconds at currents between 220 percent and breakers have, for years, been equipped with elec-264 percent of their E-rating. C-rated, current-lim- tromechanical trip devices. Modern, solid-state de-iting power fuses open in 1000 seconds at currents vices, however, are rapidly replacing electrome-between 170 percent and 240 percent of their C- chanical trips. Solid-state trips are more accessible,rating. R-rated, current-limiting power fuses are easier to calibrate, and are virtually unaffected bysuitable for use on medium-voltage motor control- vibration, temperature, altitude, and duty-cycle.lers only. Generally, R-rated fuses open in 20 sec- Furthermore, solid-state devices are easy toonds at 100 times the R-rating. coordinate, and provide closer, more improved pro-

I. Information required for coordination. The tection over electromechanical units. Still, electro-following fuse information is required for a coordi- mechanical units have their applications. Industrialnation study: plants with harsh environments, such as steel mills

(1) Fuse continuous current rating. and ammunition plants, may demand the more(2) Fuse time-current characteristic curves. rugged electromechanical devices. Today, molded-(3) Fuse interrupting-current rating. case circuit breakers are being equipped with solid-(4) UL classification and time delay character- state trip units to obtain more complex tripping

istics. characteristics. Surface-mount, or integrated-j. Fuse ratings. Standard voltage and current circuit, technology is allowing very sophisticated

ratings for fuses can be found in appendix D. molded-case circuit breakers to be constructed in

3-4. Motor short-circuit protectors (MSCP) breakers are also being equipped with solid-stateMotor short-circuit protectors are current-limiting,fuse-like devices designed specifically for use inswitch-type, combination motor controllers. ULconsiders MSCPs to be components of motor con-trollers rather than fuses. Therefore, MSCPs aremarked by letter designations (A-Y) instead ofampere ratings and may not be used as fuses.MSCPs may be used in motor circuits provided theMSCP is part of a combination motor controllerwith overload relays and is sized not greater than1,300 percent of motor FLA (NFPA 70). This rela-tively new arrangement (first recognized by NFPA70-1971), provides short-circuit protection, over-load protection, motor control, and disconnectingmeans all in one assembly. MSCPs provide excel-lent short-circuit protection for motor circuits aswell as ease of selection. However, the limitednumber of manufacturers that can supply MSCPshas so far prohibited their use by the Governmentexcept for sole-source applications.

3-5. Circuit breakersA circuit breaker is a device that allows automaticopening of a circuit in response to overcurrent, andalso manual opening and closing of a circuit.Circuit breaker terminology and definitions arelisted in the glossary. Low-voltage power circuit

small frame sizes. Most low-voltage power circuit

trip units. New microprocessor-based circuitbreakers are now available that offer true RMScurrent sensing. The increased use of switching-mode power supplies for computer systems andother harmonic-generating, non-linear loads createdthe need for true RMS sensing, which is a majoradvantage over peak-sensing trip units.

a. Low-voltage circuit breakers. Low-voltagecircuit breakers are classified as molded-case circuitbreakers or power circuit breakers. A molded-casecircuit breaker is an integral unit enclosed in aninsulated housing. A power circuit breaker isdesigned for use on circuits rated 1000 Vac and3000 Vdc and below, excluding molded-case circuitbreakers.

(1) Low-voltage circuit breaker trip units maybe of the electromechanical (thermal-magnetic ormechanical dashpot) or solid-state electronic type.Low-voltage circuit breakers may include a numberof trip unit characteristics. These characteristics arelisted below and illustrated in figures 3-1 and 3-2.Typical circuit breaker time-current characteristiccurves may be found in appendix C. Circuit breakercurves are represented as "bands." The bandsindicate minimum and maximum operating times forspecific overcurrents.

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3-5

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(a) Long-time pick-up allows fine tuning ofthe continuous current rating. Typical settingsrange from 50 percent-100 percent of circuit break-er sensor current rating.

(b) Long-time delay varies the tripping timeunder sustained overcurrent and allows momentaryoverloads. Three to six bands are typicallyavailable.

(c) Short-time pick-up controls the amountof high-level current that can be carried for shortperiods of time without tripping and allows down-stream devices to clear faults without tripping up-stream devices. Typical settings range from 1.5 to9 times long-time pick-up setting.

(d) Short-time delay is used with short-timepick-up to improve selectivity. It provides timedelay to allow the circuit breaker to trip at the se-lected short-time pick-up current. Three bands(minimum, intermediate, and maximum) are typi-cally available.

(e) Short-time I t switch introduces a ramp2

function into the short-time characteristic curve toimprove coordination with downstream deviceswhose characteristic curves overlap the circuitbreaker characteristic curve.

(f) Instantaneous pick-up establishes thetripping current level with no intentional time delay.Typical settings range from 1.5 to 9 times Long-time pick-up setting.

(g) Ground-fault pick-up establishes groundfault tripping current level and may incorporate theI t function. Ground-fault pick-up is typically2

adjustable from 20 percent to 100 percent of sensorrating. Ground-fault pick-up should never be setabove 1200 A in accordance with NFPA 70.

(h) Ground-fault delay incorporates timedelay for coordination. Three to six time delaybands are typically available. Ground-fault delayshould not exceed one second for ground-fault cur-rents greater than 3000 A in accordance withNFPA 70.

(2) Specifications should detail only those func-tions that are necessary on a particular project.

(a) The continuous current rating may befixed or adjustable.

(b) Molded-case breakers with solid-statetrips and power breakers normally have adjustablelong-time and short-time functions.

(c) Power breakers may or may not have theinstantaneous function.

(d) Most molded-case circuit breakers, espe-cially in the smaller sizes, are not provided withlong-time adjustments, short-time functions, orground-fault functions.

(3) The inverse-time (or thermal-magnetic) cir-cuit breaker contains a thermal and a magneticelement in series and is similar in operation to timedelay fuses. This circuit breaker will trip thermallyin response to overload currents and magneticallyin response to short-circuit currents. Magnetictripping is instantaneous while thermal trippingexhibits an inverse-time characteristic (i.e., thecircuit breaker operating characteristics of time andcurrent are inversely proportional). Inverse-timecircuit breakers have three basic current ratings:trip rating, frame rating, and interrupting rating.Trip rating is the minimum continuous currentmagnitude required to trip the circuit breakerthermally. The frame rating identifies a particulargroup of circuit breakers and corresponds to thelargest trip rating within the group. Each groupconsists of physically interchangeable circuitbreakers with different trip ratings, as shown intable 3-2. Although NEMA recognizes other frameratings in addition to those listed in table 3-2, theseare the most common ones supplied bymanufacturers. The interrupting rating describes theshort-circuit withstand capability of a circuitbreaker.

(4) The instantaneous-trip circuit breaker isnothing more than an inverse-time circuit breakerwith the thermal element removed and is similar inoperation to the non-time delay fuse. This circuitbreaker is often referred to by other names, such as,magnetic circuit breaker, magnetic-only circuitbreaker, or motor circuit breaker. Instantaneous-trip circuit breakers may be used in motor circuits,but only if adjustable, and if part of a circuit breakertype, combination motor controller with overloadrelays. Such an arrangement is called a MotorCircuit Protector (MCP) and provides short-circuitprotection (circuit breaker magnetic element),overload protection (overload relays), motorcontrol, and disconnecting means all in oneassembly. Instantaneous-trip circuit breakers haveframe and interrupting ratings but do not have tripratings. They do have an instantaneous currentrating which, for motor circuits, must be adjustable

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and not exceed 1,300 percent of the motor FLA breaker will always open. Since molded-case(NFPA 70). MCPs provide excellent motor circuit circuit breaker contacts are designed to "blowprotection and ease of specification, and should be open" on high short-circuit currents, failure of theconsidered for installations with numerous motors upstream breaker to operate is not a concern. Sincewhere MCCs would be specified. low-voltage power breakers are not designed to

(5) A current-limiting circuit breaker does not "blow open," power breakers should not be appliedemploy a fusible element. When operating within its in cascade. Individual components within a cascadecurrent-limiting range, a current-limiting circuit system should not be replaced since the entirebreaker limits the let-through I t to a value less than system is UL approved. Individual components are2

the I t of the quarter cycle of the symmetrical not UL approved. Additionally, individual2

current. Current-limiting circuit breakers employ components should be from the same manufacturersingle and double break contact arrangements as as the cascade system. By virtue of the design, thiswell as commutation systems to limit the let- approach does not pro- vide a coordinated system.through current to satisfy the fundamental def- b. Medium-voltage circuit breakers. ANSI de-inition of current-limitation without the use of the fines medium-voltage as 1000V or more, but lessfuses. Current-limiting circuit breakers can be reset than 100kV. Switching a medium-voltage circuitand service restored in the same manner as involves either opening or closing a set of contactsconventional circuit breakers even after clearing mechanically. When closing the contacts, the ap-maximum level fault currents. Manufacturers of plied mechanical force must be greater than thecurrent-limiting circuit breakers publish peak let- forces which oppose the closing action. An arc isthrough current (I ) and energy (I t curves. The created when the contacts are opened, which mustp

2

manufacturer should be contacted for specific ap- be extinguished. Medium-voltage circuit breakersplication information. are classified according to the medium (oil, air,

(6) Integrally fused circuit breakers employ vacuum, or SF ) in which their contacts are im-current limiters which are similar to conventional mersed. Normally, metal clad, drawout switchgearcurrent-limiting fuses but are designed for specific is used at medium-voltages up to 15kV. Air-mag-performance with the circuit breaker. Integrally netic, vacuum, and SF -filled-interrupter circuitfused circuit breakers also include overload and low breakers are available in drawout switchgear. Oillevel fault protection. This protection is coordi- circuit breakers are used outdoors, as individualnated so that, unless a severe fault occurs, the cur- units, and thus are not available in drawoutrent limiter is not affected and replacement is not switchgear mounting.required. Current limiters are generally located (1) Medium-voltage air circuit breakers arewithin the molded case circuit breaker frame. An either of the air-magnetic type or of the air-blastinterlock is provided which ensures the opening of type. Due to cost and size restrictions, air-blastthe circuit breaker contacts before the limiter cover breakers are not normally used in medium-voltagecan be removed. Single phasing is eliminated by the drawout switchgear construction. In recent years,simultaneous opening of all circuit breaker poles. most medium-voltage drawout switchgear em-Many circuit breakers employ mechanical interlocks ployed air-magnetic breakers. However, due toto prohibit the circuit breaker from closing with a cost, size, and noise limitations, vacuum and SFmissing current limiter. The continuous ampere circuit breakers are replacing air circuit breakers inrating of integrally fused circuit breakers is selected medium-voltage drawout switchgear.in the same manner as for conventional circuit (2) The contacts of vacuum circuit breakers arebreakers. The selection of the individual limiters hermetically-sealed in a vacuum chamber orshould be made in strict accordance with the bottle. The vacuum in a new bottle should bemanufacturer's published literature to achieve the about 10 Torr, and should be at least 10 Torr fordesired level of circuit protection. proper operation. One Torr equals 760 millimeters

(7) A molded-case circuit breaker can be ap- of mercury. Vacuum interrupters are much smallerplied in a system where fault current may exceed its and quieter than air circuit breakers, and require norating if it is connected in series on the load side of arc chutes. Vacuum circuit breakers in drawoutan acceptable molded-case circuit breaker. Such an switchgear mounting are available in a variety ofapplication is called cascade system operation. The continuous current and MVA ratings at 5kV toupstream breaker must be rated for maximum 15kV.available fault current and both breakers must be (3) Sulfur hexaflouride, SF , is a nonflammable,tested and UL certified for a series rating. Cascade nontoxic, colorless, and odorless gas, which hasoperation depends upon both breakers opening at long been used in high-voltage circuit breakers.the same time, and upon the fact that the upstream Now, SF -filled-interrupter circuit breakers are

6

6

6

-8 -4

6

6

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available in drawout switchgear for 5kV and 15kVapplications. Like vacuum interrupters, the circuitbreaker contacts are immersed in a hermetically-sealed bottle filled with SF gas. SF circuit breakers6 6 in drawout switchgear mounting are available in avariety of continuous current and MVA ratings.

c. EMI/RFI considerations. With today's in-creasing use of sensitive, solid-state devices, the ef-fects of Electro-Magnetic Interference (EMI) andRadio-Frequency Interference (RFI) must be con-sidered. Solid-state devices, due to their many ad-vantages, are rapidly replacing the rugged electro-mechanical devices previously used. One disadvan-tage of solid-state devices, however, is their sensi-tivity to power source anomalies and electrostaticand electromagnetic fields. Recent developments inthe design and packaging of solid-state deviceshave incorporated effective shielding techniques.However, the designer must still evaluate the envi-ronment and ensure that additional shielding is notrequired. Equipment and devices must comply withMIL-STD-461.

d. In formation needed for coordination. Thefollowing circuit breaker information is required fora coordination study:

(1) Circuit breaker continuous current andframe rating.

(2) Circuit breaker interrupting rating.(3) Circuit breaker time-current characteristic

curves.e. Circuit breaker ratings. Standard voltage and

current ratings for circuit breakers may be found inappendix D. To meet UL requirements, moldedcase circuit breakers are designed, built andcalibrated for use in a 40 degrees C (104 degrees F)ambient temperature. Time-current characteristictrip curves are drawn from actual test data. Whenapplied at ambient temperatures other than 40degrees C, frequencies other than 60 Hz, or otherextreme conditions, the circuit performancecharacteristics of the breaker may be affected. Inthese cases, the current carrying capacity and/ortrip characteristics of the breaker may vary.Therefore, the breaker must be rerated.

(1) Since thermal-magnetic circuit breakers aretemperature sensitive devices, their rated con-tinuous current carrying capacity is based on a ULspecified 40 degrees C (104 degrees F) calibrationtemperature. When applied at temperatures otherthan 40 degrees C it is necessary to determine thebreaker's actual current carrying capacity underthose conditions. By properly applying manufac-turer' s ambient rerating curves, a circuit breakers

current carrying capacity at various temperaturescan be predicted.

(2) Application of thermal-magnetic circuitbreakers at frequencies above 60 Hz requires thatspecial consideration be given to the effects of highfrequency on the circuit breaker characteristics.Thermal and magnetic operation must be treatedseparately.

(a) At frequencies below 60 Hz the thermalrerating of thermal-magnetic circuit breakers isnegligible. However, at frequencies above 60 Hz,thermal rerating may be required. One of the mostcommon higher frequency applications is at 400 Hz.Manufacturer's rerating curves are available.

(b) At frequencies above 60 Hz, tests indicatethat it takes more current to magnetically trip acircuit breaker than is required at 60 Hz. At fre-quencies above 60 Hz, the interrupting capacity ofthermal-magnetic breakers is less than the 60 Hzinterrupting capacity.

(3) When applying thermal-magnetic circuitbreakers at high altitudes, both current and voltageadjustments are required. Current rerating isrequired because of the reduced cooling effects ofthe thinner air present in high altitude applications.Voltage rerating is necessary because of thereduced dielectric strength of the air. Refer toANSI C37-13 and ANSI C37-14 for specificrerating factors to be applied at various altitudes.

(4) Trip curves provide complete time-currentcharacteristics of circuit breakers when applied onan ac systems only. When applying thermal-mag-netic circuit breakers on dc systems, the circuitbreaker's thermal characteristics normally remainunchanged, but the manufacturer should be con-sulted to be certain. The magnetic portion of thecurve, on the other hand, requires a multiplier todetermine an equivalent dc trip range. This is nec-essary because time-current curves are drawn usingRMS values of ac current, while dc current ismeasured in peak amperes. Additionally, the X/Rratio of the system as seen by the circuit breakerwill affect its dc rating. When a circuit breakeropens a dc circuit, the inductance in the system willtry to make the current continue to flow across theopen circuit breaker contacts. This action results inthe circuit breaker having to be derated.Furthermore, some circuit breakers require the acwaveform to pass through a current zero to openthe circuit. Since dc does not have current zeros,the circuit breaker must be derated. For dcapplications the manufacturer should be contactedfor derating requirements.

f. System X/R ratio. Normally, the system X/Rratio need not be considered when applying circuit

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breakers. Circuit breakers are tested to cover mostapplications. There are several specific applications,however, where high system X/R ratios may pushshort-circuit currents to 80 percent of the short-circuit current rating of standard circuit breakers.These applications are listed below.

(1) Local generation greater than 500kVA atcircuit breaker voltage.

(2) Dry-type transformers, 1.0 MVA andabove.

(3) All transformer types, 2.5 MVA and above.(4) Network systems.(5) Transformers with impedances greater than

values listed in the ANSI C57 series.(6) Current-limiting reactors in source circuits

at circuit breaker voltage.(7) Current-limiting busway in source circuits

at circuit breaker voltage.If the system X/R ratio is known, multiplying fac-tors from various references can be used to deter-mine the circuit breaker short-circuit current rating.If the system X/R ratio is unknown, the maximumX/R ratio of 20 may be assumed and theappropriate multiplying factor used.

g. Circuit breaker application. Molded-case cir-cuit breakers, power circuit breakers, and insulated-case circuit breakers should be applied as follows:

(1) Molded-case circuit breakers have tradi-tionally been used in panelboards or loadcenterswhere they were fixed-mounted and accessible.Low-voltage power circuit breakers, on the otherhand, were traditionally used in industrial plants andinstalled in metal-enclosed assemblies. All powercircuit breakers are now of the drawout-typeconstruction, mounted in metalclad switch-gear.Therefore, molded-case breakers should be used infixed mountings, and power breakers should beused where drawout mountings are employed.

(2) Since power breakers were traditionallyused in metal-enclosed assemblies, they were ratedfor 100 percent continuous duty within the assem-bly. On the other hand, molded case breakers weretraditionally used in open air. When used in a metalenclosure, molded-case breakers had to be deratedto 80 percent of continuous rating. Molded-casebreakers are now available at 100 percent ratingwhen installed in an enclosure.

(3) Power breakers have traditionally been ap-plied where selectivity was very important, thusrequiring high short-time ratings to allow down-stream devices to clear the fault. Molded-casebreakers were, instead, designed for very fast oper-ations. Fast opening contacts under high short-cir-

cuit current conditions resulted in molded-casebreakers having higher interrupting ratings thanpower breakers.

(4) An insulated-case circuit breaker is some-what of a hybrid circuit breaker which incorpo-rates advantages of both the molded-case andpower circuit breaker. However, an insulated-casebreaker is not a power breaker, and should not beapplied as such. Insulated-case breakers are notdesigned and tested to the same standards as powerbreakers. An insulated-case breaker is essentially ahigher capability molded-case breaker. Allcommercially available insulated-case breakers are100 percent rated.

(5) Molded-case or insulated-case breakersshould be used in noncritical, small load applica-tions with high interrupting requirements. Powerbreakers should be used in critical applicationswhere continuity of service is a requirement. Foroverlapping applications, designer judgmentshould be based on factors discussed in this TM.Refer to table 3-3 * for circuit breaker applicationcomparisons.

3-6. Protective relaysProtective relays are classified according to theirfunction, and there are a wide variety of protectiverelays available. The overcurrent relay, for example,monitors current and operates when the currentmagnitude exceeds a preset value.

__________*Adapted from Application Considerations from Circuit

Breakers-Choosing the Right Type for Specific Applications byS.H. Telander, Consulting-Specifying Engineer Magazine, July,1987.

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a. Overcurrent relay. The most common relay (2) Residually-connected CTs and core-bal-for short-circuit protection is the overcurrent relay. anced CTs are illustrated in chapter 5. Residually-These relays are much more sophisticated than the connected CTs are widely used in medium-voltagesimple thermal overload relays discussed previously systems, while core-balanced CT's form the basis offor motor applications, and have a wide range of several low-voltage ground-fault protectiveadjustments available. Electromagnetic attraction schemes. Relays connected to core-balance CTsrelays may be ac or dc devices and are used for can be made very sensitive. However, core-instantaneous tripping. Electromagnetic induction balanced CTs are subject to saturation fromrelays are ac only devices. Electromagnetic unbalanced inrush currents or through faults notattraction and induction relays, like all elec- involving ground. High magnitude short-circuittromechanical devices, are simple, rugged, reliable, currents may also saturate core-balance CTs thusand have been used successfully for years. How- preventing relay operation.ever, solid-state electronic relays are rapidly re- d. EMI/RFI With today's increasing use ofplacing the electromechanical types. Solid-state sensitive, solid-state devices, the effects of Electro-relays require less panel space and exhibit better Magnetic Interference (EMI) and Radio-Frequencydynamic performance and seismic-withstand capa- Interference (RFI) must be considered. Solid-statebility. Additionally, solid-state overcurrent relays devices, due to their many advantages, are rapidlyare faster, have more precisely-defined operating replacing the rugged electromechanical devicescharacteristics, and exhibit no significant overtravel. previously used. One disadvantage of solid-stateAs in the case of circuit breakers, electromechanical devices, however, is their sensitivity to powerrelays will continue to find applications in harsh source anomalies and electrostatic and electromag-environments. Overcurrent relays have a variety of netic fields. Recent developments in the design andtap and time dial settings. Typical relay ratings are packaging of solid-state devices have incorporatedshown in appendix D, and typical over-current relay effective shielding techniques. However, thetime-current characteristic curves are shown in designer must still evaluate the environment andappendix C. ensure that additional shielding is not required.

b. Relay device function numbers. Protective Equipment and devices must comply with MIL-relays have been assigned function numbers by STD-461.IEEE that are used extensively to specify protective e. New developments. Microprocessor-based re-relays. A partial list of relay function numbers is lays are also becoming available which provideincluded in appendix E. multiple relay functions as well as metering, fault

c. Instrument transformers. Protective relays event recording, and self-testing in a single enclo-will always be associated with medium-voltage and sure. This system requires fewer connections andhigh-voltage circuits, involving large current mag- less panel space than individual relays and associ-nitudes. Therefore, current transformers (CT) are ated peripherals.required to isolate the relay from line voltages andto transform the line current to a level matching the 3-7 Automatic reclosing devicesrelay rating. CTs are normally rated 5A on thesecondary with a primary rating corresponding tothe requirements of the system. Potential or voltagetransformers (VT) are single-phase devices, usuallyrated 120V on the secondary with primary ratingmatched to the system voltage.

(1) CT burden is the load connected to the sec-ondary terminals. Burden may be expressed as volt-amperes and power factor at a specified current, orit may be expressed as impedance. The burdendifferentiates the CT load from the primary circuitload.

Automatic reclosing schemes should not be appliedwhere the load being protected is a transformer orcable, since faults in these types of loads are usuallynot transient in nature. Automatic reclosingschemes applied to permanent faults in transformeror cable loads may result in equipment damage andpersonnel hazards. Additionally, automatic re-closing schemes should be guarded against in motorcircuits. If the system voltage is restored out ofphase, the motor windings, shaft, and drivecouplings may be damaged. Furthermore, reclosersshould be applied only on aerial distribution systems.

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CHAPTER 4

PROTECTIVE DEVICES COORDINATION

4-1. General of series devices from the load to the source areWhere there are two or more series protective de-vices between the fault point and the power supply,these devices must be coordinated to insure that thedevice nearest the fault point will operate first. Theother upstream devices must be designed to operatein sequence to provide back-up protection, if anydevice fails to respond. This is called selectivecoordination. To meet this requirement, protectivedevices must be rated or set to operate on minimumovercurrent, in minimum time, and still be selectivewith other devices on the system. When the aboveobjectives are fulfilled, maximum protection toequipment, production, and personnel will beaccomplished. As will be seen later in this chapter,protection and coordination are often in directopposition with each other. Protection may have tobe sacrificed for coordination, and vice versa. It isthe responsibility of the electrical engineer to designfor optimum coordination and protection. This issometimes more art than science.

4-2. The coordination studyA coordination study consists of the selection orsetting of all series protective devices from the loadupstream to the power supply. In selecting orsetting these protective devices, a comparison ismade of the operating times of all the devices inresponse to various levels of overcurrent. The ob-jective, of course, is to design a selectively coordi-nated electrical power system. A new or revisedcoordination study should be made when the avail-able short-circuit current from the power supply isincreased; when new large loads are added or ex-isting equipment is replaced with larger equipment;when a fault shuts down a large part of the system;or when protective devices are upgraded.

a. Time-current characteristic curves. Time isplotted on the vertical axis and current is plotted onthe horizontal axis of all time-current characteristiccurves. Log-log type graph paper is used to covera wide range of times and currents. Characteristiccurves are arranged so that the area below and tothe left of the curves indicate points of "nooperation, and the area above and to the right ofthe curves indicate points of "operation." The pro-cedure involved in applying characteristic curves toa coordination study is to select or set the variousprotective devices so that the characteristic curves

located on a composite time-current graph from leftto right with no overlapping of curves. The result isa set of coordinated curves on one composite time-current graph.

b. Data required for the coordination study.The following data is required for a coordinationstudy.

(1) Single-line diagram of the system understudy.

(2) System voltage levels.(3) Incoming power supply data.

(a) Impedance and MVA data.(b) X/R ratio.(c) Existing protection including relay device

numbers and settings, CT ratios, and time-currentcharacteristic curves.

(d) Generator ratings and impedance data.(e) Transformer ratings and impedance data.

(4) Data on system under study.(a) Transformer ratings and impedance data.(b) Motor ratings and impedance data.(c) Protective devices ratings including mo-

mentary and interrupting duty as applicable.(d) Time-current characteristic curves for

protective devices.(e) CT ratios, excitation curves, and winding

resistance.(f) Thermal (I t) curves for cables and rotat-2

ing machines.(g) Conductor sizes and approximate lengths.

(5) Short-circuit and load current data.(a) Maximum and minimum momentary (first

cycle) short-circuit currents at major buses.(b) Maximum and minimum interrupting duty

(5 cycles and above) short-circuit currents at majorbuses. The exact value of ground-fault current(especially arcing ground-fault current) is im-possible to calculate. Methods are available for es-timating ground-fault current. The application ofNEMA damage curves for ground-fault current isillustrated in appendix G.

(c) Estimated maximum and minimum arcingand bolted ground-fault currents at major buses.

(d) Maximum load currents.

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(e) Motor starting currents and starting times. (2) When coordinating inverse time overcurrent(f) Transformer protection points.

c. Coordination procedure. The followingprocedure should be followed when conducting acoordination study:

(1) Select a convenient voltage base and con-vert all ampere values to this common base. Nor-mally, the lowest system voltage will be chosen, butthis may not always be the case.

(2) Indicate short-circuit currents on the hori-zontal axis of the log-log graph.

(3) Indicate largest (or worst case) load ampa-ities on the horizontal axis. This is usually a motorand should include FLA and LRA values. (c) Safety factor for CT satu- 0.22 seconds

(4) Specify protection points. These include ration, setting errors, con-magnetizing inrush point and NFPA 70 limits forcertain large transformers.

(5) Indicate protective relay pick-up ranges.(6) Starting with the largest (or worst case)

load at the lowest voltage level, plot the curve forthis device on the extreme left side of the log-loggraph. Although the maximum short-circuit currenton the system will establish the upper limit ofcurves plotted to the right of the first and succeed-ing devices, the number of curves plotted on asingle sheet should be limited to about five to avoidconfusion.

(7) Using the overlay principle, trace the curvesfor all protective devices on a composite graph, se-lecting ratings or settings that will provide over-current protection and ensure no overlapping ofcurves.

d. Coordination time intervals. * When plotting main relay contact. This eliminates over-travel incoordination curves, certain time intervals must be the relay so equipped. The time interval often usedmaintained between the curves of various protec- on carefully calibrated systems with high-dropouttive devices in order to ensure correct sequential instantaneous relays is 0.25 seconds.operation of the devices. These intervals are re-quired because relays have overtravel and curvetolerances, certain fuses have damage characteris-tics, and circuit breakers have certain speeds ofoperation. Sometimes these intervals are calledmargins.

(1) Coordination can be easily achieved with stream relay curve could be as low as 0.1 secondlow voltage current-limiting fuses that have fast where clearing times below 1 second are involved.response times. Manufacturer's time current curvesand selectivity ratio guides are used for bothoverload and short-circuit conditions, precludingthe need for calculating time intervals._____________ terval may be decreased to a shorter time as ex-* Reprinted with permission from ANSI/IEEE Standard 242-1986, IEEE Recommended Practice for Protection and Coordi- (7) When coordinating circuit breakersnation of Industrial and Commercial Power Systems, copyright equipped with direct-acting trip units, the charac-1986 by IEEE. teristics curves should not overlap. In general only

relays, the time interval is usually 0.3-0.4 seconds.This interval is measured between relays in serieseither at the instantaneous setting of the load sidefeeder circuit breaker relay or the maximum short-circuit current, which can flow through bothdevices simultaneously, whichever is the lowervalue of current. The interval consists of thefollowing components:

(a) Circuit breaker opening 0.08 secondstime (5 cycles).

(b) Relay overtravel . . . . . . . 0.10 seconds

tact gap, etc.

(3) This safety factor may be decreased by fieldtesting relays to eliminate setting errors. This in-volves calibrating the relays to the coordinationcurves and adjusting time dials to achieve specificoperating times. A 0.355 margin is widely used infield-tested systems employing very inverse andextremely inverse time overcurrent relays.

(4) When solid-state relays are used, overtravelis eliminated and the time may be reduced by theamount included for overtravel. For systems usinginduction disk relays, a decrease of the time intervalmay be made by employing an overcurrent relaywith a special high-dropout instantaneous elementset at approximately the same pickup as the timeelement with its contact wired in series with the

(5) When coordinating relays with downstreamfuses, the circuit opening time does not exist for thefuse and the interval may be reduced accordingly.The total clearing time of the fuse should be usedfor coordination purposes. The time marginbetween the fuse total clearing curve and the up-

(6) When low-voltage circuit breakers equippedwith direct-acting trip units are coordinated withrelayed circuit breakers, the coordination time in-terval is usually regarded as 0.3 seconds. This in-

plained previously for relay-to-relay coordination.

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4-3

a slight separation is planned between the different 4-3. Primary and medium-voltage coordinationcharacteristics curves. This lack of a specified timemargin is explained by the incorporation of all thevariables plus the circuit breaker operating times forthese devices within the band of the devicecharacteristic curve.

(8) Delta-wye transformers. When protecting adelta-wye transformer, an additional 16 percentcurrent margin over margins mentioned previouslyshould be used between the primary and secondaryprotective device characteristic curves. This helpsmaintain selectivity for secondary phase-to-phasefaults since the per-unit primary current in onephase for this type of fault is 16 percent greaterthan the per-unit secondary current which flows fora secondary three-phase fault.

Figure 4-1 shows a single-line diagram (modifiedfor simplicity) of the electrical distribution systemat an Army Ammunition Plant. Two 115kV utilitylines supply the double-ended, main substation,which transforms the voltage down to 13.8kV fordistribution throughout the facility. The utilitycompany should participate in the selection of relayprotection for the incoming 115kV lines in theevent of a fault in the 115kV bus or the maintransformers. The 15kV switchgear is designed pri-marily for supplying the 480V substations, althoughmedium and large-size motors could also be served.Primary and medium-voltage protection is coveredin detail by the coordination examples in appendix G.

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4-4

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4-5

4-4. Low-voltage coordination downstream standard overcurrent de- vices, andLow-voltage coordination involves selectingfeeder-breaker, tie-breaker, main-breaker, andtransformer fuse ratings and settings that provideoptimum protection of equipment while maintainingselective coordination among the low-voltage,protective devices. Total system coordination withupstream medium-voltage and primary protectivedevices must also be incorporated. Low-voltageprotection is covered in detail by the coordinationexamples in appendix G.

4-5. Ground-fault coordinationMost of the concern about ground-fault protectionand coordination, today, centers on low-voltagesystems where low-level arcing faults are a consid-erable problem. The phenomena of arcing faultsbegan in the 1950's with the advent of large capac-ity 480Y/277V solidly-grounded systems. Medium-and high-voltage grounded systems don't experi-ence the arcing ground fault problem common tolow-voltage systems, and have employed groundcurrent relays for years. Currently, there are threemethods for achieving low-voltage arcing ground-fault protection.

a. Method 1. The non-selective, single-zonemethod, shown in figure 4-2a, applies ground-faultprotection only at the main service disconnect. Thisis minimum protection as required by NFPA 70,and is required only on 480Y/277V services rated1000A or more. Non-selective, single-zone ground-fault protection may be difficult to coordinate with

additional ground-fault protection at downstreamlevels may have to be considered even though notrequired by NFPA 70.

b. Method 2. The selective, time-coordinatedmethod, shown in figure 4-2b, applies ground-faultprotection at additional levels downstream of themain service disconnect. Coordination is achievedby intentional time-delays to separate the variouslevels. This method achieves the coordination thatMethod 1 does not, but protection is sacrificed byinclusion of the time-delays. Additionally, Method2 costs more than Method 1.

c. Method 3. The selective, zone-coordinatedmethod, shown in figure 4-2c, applies ground-faultprotection at downstream levels like Method 2does, but includes a restraining signal which canoverride the time-delay. Coordination and protec-tion are both maximized by the application of thissystem of restraining signals by allowing each levelto communicate with other levels. This method, ofcourse, costs more than the other methods, andshould be considered only for special purposeapplications.

d. Government facilities. Except for special in-stallations requiring precise ground-fault protectionand coordination, government facilities shouldincorporate ground-fault protection in accordancewith NFPA 70 only. Where coordination is not pos-sible with downstream feeders, main GFP devicesand feeder protective devices should he set to opti-mize coordination as much as possible.

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4-6

4-6. Coordination requirements knowledge of ANSI C57.12 requirements for trans-The primary purpose of the coordination procedureis to select the proper ratings and settings for theprotective devices on an electrical distributions