Practice Test Geeks(CPE) Certified Plant Engineer Practice Test
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Electrical Systems & Controls Flashcards

7 cards from real CPE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. A 480V, three-phase transformer bank is connected in delta-wye with a grounded wye secondary. A single-phase load is connected line-to-neutral on the secondary. This is an example of a:

    Answer: Separately derived system requiring a grounding electrode conductor per NEC 250.30

    A delta-wye transformer with a grounded wye secondary is a separately derived system, requiring a grounding electrode and bonding jumper per NEC Article 250.30.

  2. Which harmonics are typically dominant in the current drawn by a six-pulse VFD from the power line?

    Answer: 5th and 7th (and higher orders 11th, 13th...)

    A six-pulse rectifier front-end produces characteristic harmonics at the 6n±1 orders: 5th, 7th, 11th, 13th, etc.

  3. In an emergency power system, NEC Article 700 requires that the transfer to emergency power be completed within:

    Answer: 10 seconds

    NEC 700.12 requires that emergency systems restore power within 10 seconds of normal power failure.

  4. A plant engineer observes that a 3-phase motor draws unequal phase currents of 42A, 38A, and 44A. Voltage unbalance is negligible. The most likely cause is:

    Answer: Winding fault or turn-to-turn short in one motor phase

    With balanced voltages, unequal phase currents indicate an internal motor winding fault, such as a turn-to-turn short or partial winding degradation.

  5. What is the purpose of an interposing relay in a PLC output circuit driving a high-voltage motor starter coil?

    Answer: To isolate the PLC's low-voltage output from the higher-voltage control circuit and provide current amplification

    An interposing relay electrically isolates the PLC's low-power output (typically 24VDC) from higher-voltage motor starter coil circuits and handles the higher coil current.

  6. According to NEC 110.9, overcurrent protective devices must have adequate interrupting capacity. For a 480V industrial panel with an available fault current of 42,000A, which breaker rating is the minimum acceptable?

    Answer: 42,000A IC

    NEC 110.9 requires that overcurrent devices have an interrupting rating equal to or greater than the available fault current at that point — minimum 42,000A IC in this case.

  7. A plant control system uses a 4–20 mA current loop signal to transmit a process temperature of 0–500°F. If the transmitter outputs 12 mA, what is the measured temperature?

    Answer: 250°F

    12 mA corresponds to 50% of the 4–20 mA span: (12−4)/(20−4) = 8/16 = 0.5; 0.5 × 500°F = 250°F.