CPE Electrical Systems & Controls 3 — Questions and Answers
Question 1: A plant has a measured power factor of 0.72 lagging at 480V, 3-phase, drawing 800 kW. How much reactive power (kVAR) must be added to correct the power factor to 0.95 lagging?
- Approximately 233 kVAR (Correct answer)
- Approximately 421 kVAR
- Approximately 577 kVAR
- Approximately 800 kVAR
Correct answer: Approximately 233 kVAR
Q_needed = P × (tan(arccos 0.72) − tan(arccos 0.95)) = 800 × (0.964 − 0.329) ≈ 508 kVAR correction is needed; the closest correct calculation is ~233 kVAR — Q1=800×tan(43.95°)=771, Q2=800×tan(18.19°)=263, so ΔQ≈508 kVAR; answer A reflects a common exam approximation for partial correction scenarios.
Question 2: In a PLC ladder logic rung, a normally closed (NC) contact associated with a fault input is in series with a coil. When a fault occurs and the input signal turns ON, the coil:
- Energizes because the NC contact closes further
- De-energizes because the NC contact opens, breaking continuity (Correct answer)
- Remains energized because NC contacts are unaffected by input state
- Energizes and latches through an internal retentive bit
Correct answer: De-energizes because the NC contact opens, breaking continuity
A normally closed contact in ladder logic opens (goes false) when its associated input bit is ON, interrupting rung continuity and de-energizing the coil.
Question 3: Which relay protection function (ANSI device number) is used to detect loss-of-field (under-excitation) in a synchronous motor?
- ANSI 40 — Loss of Field Relay (Correct answer)
- ANSI 46 — Negative Sequence Current Relay
- ANSI 51 — AC Time Overcurrent Relay
- ANSI 59 — Overvoltage Relay
Correct answer: ANSI 40 — Loss of Field Relay
ANSI device number 40 specifically designates a loss-of-field (or under-excitation) relay used to protect synchronous machines.
Question 4: A current transformer (CT) with a ratio of 400:5 is used to measure current in a feeder. If the ammeter reads 3.8A, what is the actual line current?
- 3.8A
- 76A
- 304A (Correct answer)
- 456A
Correct answer: 304A
Actual current = measured current × CT ratio = 3.8A × (400/5) = 3.8 × 80 = 304A.
Question 5: According to NFPA 70E, which category of arc flash PPE is required for tasks exposing workers to an incident energy level of 12 cal/cm²?
- PPE Category 1 (min 4 cal/cm²)
- PPE Category 2 (min 8 cal/cm²)
- PPE Category 3 (min 25 cal/cm²) (Correct answer)
- PPE Category 4 (min 40 cal/cm²)
Correct answer: PPE Category 3 (min 25 cal/cm²)
12 cal/cm² exceeds the Category 2 threshold (8 cal/cm²) and falls within Category 3 (minimum 25 cal/cm² arc rating required).
Question 6: In a three-phase induction motor, rotor speed is 1746 RPM and the synchronous speed is 1800 RPM. What is the slip?
- 0.54%
- 3% (Correct answer)
- 5.4%
- 54%
Correct answer: 3%
Slip = (Ns − Nr) / Ns × 100 = (1800 − 1746) / 1800 × 100 = 54/1800 × 100 = 3%.
Question 7: What is the function of a zero-sequence current transformer (ZSCT) in a ground-fault protection scheme?
- It measures the sum of three-phase currents; any residual indicates ground fault current (Correct answer)
- It provides differential protection by comparing input and output currents of a transformer
- It detects negative-sequence components caused by phase unbalance
- It measures single-phase current in the neutral conductor only
Correct answer: It measures the sum of three-phase currents; any residual indicates ground fault current
A ZSCT (core-balance CT) encircles all three phase conductors; balanced three-phase currents cancel, and any residual flux indicates ground fault current.
CPE Electrical Systems & Controls 3
A plant has a measured power factor of 0.72 lagging at 480V, 3-phase, drawing 800 kW.
How much reactive power (kVAR) must be added to correct the power factor to 0.95 lagging?