Protective Relaying: Principles and Applications,
4th Edition
Original Exam Mastery & Engineering Practice Workbook — 2026
Based on the subject coverage of J. Lewis Blackburn and Thomas J. Domin, 4th Edition.
This workbook is an independently authored study resource. It reinforces power-system protection concepts, calculations,
application judgment, and troubleshooting. It does not reproduce the Stuvia solutions manual, publisher solution sets, or
copyrighted end-of-chapter answers.
Source and scope
The 4th edition is a 695-page CRC Press text by J. Lewis Blackburn and Thomas J. Domin. Its published contents cover
protection philosophy, per-unit values, phasors, symmetrical components, relay inputs, grounding,
generator/transformer/bus/motor/line/pilot protection, stability and reclosing, microprocessor applications, and improving
protection performance. The linked Stuvia listing is a paid 220-page solutions-manual document; only public bibliographic/topic
information was used for this workbook.
Study method
Attempt each problem first. For calculations, write the governing equation and identify units/base values. For application
problems, identify the protected zone, measured quantities, desired reliability attribute, and credible failure mode before
selecting an answer.
Concept Useful relationship / reminder
Three-phase base current I_base = S_base / (√3 V_base)
Per-unit impedance Z_pu = Z_actual / Z_base
Base impedance Z_base = V_base² / S_base (three-phase base convention)
Apparent impedance Z_app ≈ V / I for the measured fault loop
I_fault,pu ≈ V_prefault,pu / X_th,pu
Three-phase fault, simple model
Symmetrical components Positive = normal sequence; negative = reverse sequence; zero = in-phase components
Protection reliability Dependability = operate when required; Security = avoid unwanted operation
Protective Relaying — Original Exam Mastery 2026 Page 1
, Chapter 1: Introduction and General Philosophies
1. Protection objectives — Which combination best describes the core objectives of a power-system protection scheme?
A. Maximum current during every disturbance
B. Reliability, selectivity, speed, and appropriate simplicity/economics
C. Zero operation for internal faults
D. Operation of every relay for every fault
Answer: B. Reliability, selectivity, speed, and appropriate simplicity/economics
Rationale: A protection system should detect relevant faults reliably, isolate only the affected portion when practical, act quickly
enough to limit damage, and balance simplicity and cost against required performance.
2. Selectivity — A downstream feeder fault occurs on a radial system. What is the preferred protection response?
A. Trip the entire transmission system
B. Trip the nearest appropriate interrupting device first, with backup available
C. Trip every feeder simultaneously
D. Block all overcurrent protection
Answer: B. Trip the nearest appropriate interrupting device first, with backup available
Rationale: Selectivity means clearing the smallest practical portion of the system. Primary protection should clear the fault first,
while upstream devices provide coordinated backup.
3. Relay classification — A relay whose principal purpose is to detect abnormal system conditions and initiate
circuit-breaker operation is best classified as a:
A. Protective relay
B. Metering relay
C. Regulating transformer
D. Synchronizing bus
Answer: A. Protective relay
Rationale: Protective relays detect specified abnormal conditions and issue control actions, typically through trip logic and
circuit breakers.
4. Performance — A relay operates for a fault inside its intended zone and the associated breaker clears the fault. This is
best described as:
A. Correct operation
B. Incorrect operation
C. No conclusion
D. A communication failure
Answer: A. Correct operation
Rationale: Correct operation means the protection responded as intended for the system condition and cleared the fault within
the intended protection design.
Protective Relaying — Original Exam Mastery 2026 Page 2
, 5. Application study — Before applying a protective relay, which information is especially important?
A. Only the relay brand
B. System configuration, equipment impedances, CT/VT information, fault studies, loading, and existing protection
C. Only the nominal voltage
D. Only the breaker interrupting rating
Answer: B. System configuration, equipment impedances, CT/VT information, fault studies, loading, and existing
protection
Rationale: Protection application depends on system topology, impedances, fault levels, instrument-transformer
connections/ratios, loading, and coordination with existing devices.
Chapter 2: Fundamental Units: Per-Unit and Percent Values
6. Per-unit basics — A 100 MVA, 230 kV three-phase base is selected. What is the corresponding three-phase base
current?
A. 251 A
B. 1,000 A
C. 2,510 A
D. 25,100 A
Answer: A. 251 A
Rationale: For a three-phase system, I_base = S_base/(√3 V_base). Thus 100,000,000/(1.732×230,000) ≈ 251 A.
7. Base conversion — When changing the MVA base for a per-unit impedance while voltage base is unchanged, the
impedance on the new base is obtained by multiplying the old per-unit impedance by:
A. S_old/S_new
B. S_new/S_old
C. V_new/V_old
D. The square of frequency ratio
Answer: B. S_new/S_old
Rationale: With the same voltage base, Z_pu,new = Z_pu,old × (S_new/S_old).
8. Percent impedance — An impedance of 0.08 per unit is equivalent to:
A. 0.8%
B. 8%
C. 80%
D. 800%
Answer: B. 8%
Rationale: Percent impedance equals per-unit impedance multiplied by 100, so 0.08 pu = 8%.
Protective Relaying — Original Exam Mastery 2026 Page 3
4th Edition
Original Exam Mastery & Engineering Practice Workbook — 2026
Based on the subject coverage of J. Lewis Blackburn and Thomas J. Domin, 4th Edition.
This workbook is an independently authored study resource. It reinforces power-system protection concepts, calculations,
application judgment, and troubleshooting. It does not reproduce the Stuvia solutions manual, publisher solution sets, or
copyrighted end-of-chapter answers.
Source and scope
The 4th edition is a 695-page CRC Press text by J. Lewis Blackburn and Thomas J. Domin. Its published contents cover
protection philosophy, per-unit values, phasors, symmetrical components, relay inputs, grounding,
generator/transformer/bus/motor/line/pilot protection, stability and reclosing, microprocessor applications, and improving
protection performance. The linked Stuvia listing is a paid 220-page solutions-manual document; only public bibliographic/topic
information was used for this workbook.
Study method
Attempt each problem first. For calculations, write the governing equation and identify units/base values. For application
problems, identify the protected zone, measured quantities, desired reliability attribute, and credible failure mode before
selecting an answer.
Concept Useful relationship / reminder
Three-phase base current I_base = S_base / (√3 V_base)
Per-unit impedance Z_pu = Z_actual / Z_base
Base impedance Z_base = V_base² / S_base (three-phase base convention)
Apparent impedance Z_app ≈ V / I for the measured fault loop
I_fault,pu ≈ V_prefault,pu / X_th,pu
Three-phase fault, simple model
Symmetrical components Positive = normal sequence; negative = reverse sequence; zero = in-phase components
Protection reliability Dependability = operate when required; Security = avoid unwanted operation
Protective Relaying — Original Exam Mastery 2026 Page 1
, Chapter 1: Introduction and General Philosophies
1. Protection objectives — Which combination best describes the core objectives of a power-system protection scheme?
A. Maximum current during every disturbance
B. Reliability, selectivity, speed, and appropriate simplicity/economics
C. Zero operation for internal faults
D. Operation of every relay for every fault
Answer: B. Reliability, selectivity, speed, and appropriate simplicity/economics
Rationale: A protection system should detect relevant faults reliably, isolate only the affected portion when practical, act quickly
enough to limit damage, and balance simplicity and cost against required performance.
2. Selectivity — A downstream feeder fault occurs on a radial system. What is the preferred protection response?
A. Trip the entire transmission system
B. Trip the nearest appropriate interrupting device first, with backup available
C. Trip every feeder simultaneously
D. Block all overcurrent protection
Answer: B. Trip the nearest appropriate interrupting device first, with backup available
Rationale: Selectivity means clearing the smallest practical portion of the system. Primary protection should clear the fault first,
while upstream devices provide coordinated backup.
3. Relay classification — A relay whose principal purpose is to detect abnormal system conditions and initiate
circuit-breaker operation is best classified as a:
A. Protective relay
B. Metering relay
C. Regulating transformer
D. Synchronizing bus
Answer: A. Protective relay
Rationale: Protective relays detect specified abnormal conditions and issue control actions, typically through trip logic and
circuit breakers.
4. Performance — A relay operates for a fault inside its intended zone and the associated breaker clears the fault. This is
best described as:
A. Correct operation
B. Incorrect operation
C. No conclusion
D. A communication failure
Answer: A. Correct operation
Rationale: Correct operation means the protection responded as intended for the system condition and cleared the fault within
the intended protection design.
Protective Relaying — Original Exam Mastery 2026 Page 2
, 5. Application study — Before applying a protective relay, which information is especially important?
A. Only the relay brand
B. System configuration, equipment impedances, CT/VT information, fault studies, loading, and existing protection
C. Only the nominal voltage
D. Only the breaker interrupting rating
Answer: B. System configuration, equipment impedances, CT/VT information, fault studies, loading, and existing
protection
Rationale: Protection application depends on system topology, impedances, fault levels, instrument-transformer
connections/ratios, loading, and coordination with existing devices.
Chapter 2: Fundamental Units: Per-Unit and Percent Values
6. Per-unit basics — A 100 MVA, 230 kV three-phase base is selected. What is the corresponding three-phase base
current?
A. 251 A
B. 1,000 A
C. 2,510 A
D. 25,100 A
Answer: A. 251 A
Rationale: For a three-phase system, I_base = S_base/(√3 V_base). Thus 100,000,000/(1.732×230,000) ≈ 251 A.
7. Base conversion — When changing the MVA base for a per-unit impedance while voltage base is unchanged, the
impedance on the new base is obtained by multiplying the old per-unit impedance by:
A. S_old/S_new
B. S_new/S_old
C. V_new/V_old
D. The square of frequency ratio
Answer: B. S_new/S_old
Rationale: With the same voltage base, Z_pu,new = Z_pu,old × (S_new/S_old).
8. Percent impedance — An impedance of 0.08 per unit is equivalent to:
A. 0.8%
B. 8%
C. 80%
D. 800%
Answer: B. 8%
Rationale: Percent impedance equals per-unit impedance multiplied by 100, so 0.08 pu = 8%.
Protective Relaying — Original Exam Mastery 2026 Page 3
