CPE Electrical Systems & Controls 3

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?

Accepted Answer

Approximately 233 kVAR

Answer

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:

Accepted Answer

De-energizes because the NC contact opens, breaking continuity

Answer

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?

Accepted Answer

ANSI 40 — Loss of Field Relay

Answer

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?

Accepted Answer

304A

Answer

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?

Accepted Answer

PPE Category 3 (min 25 cal/cm²)

Answer

12 cal/cm² exceeds the Category 2 threshold (8 cal/cm²) and falls within Category 3 (minimum 25 cal/cm² arc rating required).

(CPE) Certified Plant Engineer Practice Test

(CPE) Certified Plant Engineer Practice Test

CPE Electrical Systems & Controls 3