CPE Electrical Systems & Controls 5 — Questions and Answers
Question 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:
- Floating neutral condition requiring a neutral stabilizer
- Separately derived system requiring a grounding electrode conductor per NEC 250.30 (Correct answer)
- Corner-grounded delta requiring special marking
- Auto-transformer connection for voltage reduction
Correct 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.
Question 2: Which harmonics are typically dominant in the current drawn by a six-pulse VFD from the power line?
- 2nd and 4th (even harmonics)
- 5th and 7th (and higher orders 11th, 13th...) (Correct answer)
- 3rd and 9th (triplen harmonics)
- Fundamental only with negligible harmonics
Correct 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.
Question 3: In an emergency power system, NEC Article 700 requires that the transfer to emergency power be completed within:
- 30 seconds
- 60 seconds
- 10 seconds (Correct answer)
- 2 minutes
Correct answer: 10 seconds
NEC 700.12 requires that emergency systems restore power within 10 seconds of normal power failure.
Question 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:
- Overloaded mechanical load on the motor shaft
- Winding fault or turn-to-turn short in one motor phase (Correct answer)
- Incorrect motor direction of rotation
- Undersized branch circuit conductors
Correct 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.
Question 5: What is the purpose of an interposing relay in a PLC output circuit driving a high-voltage motor starter coil?
- To amplify the PLC output signal to 480V directly
- To isolate the PLC's low-voltage output from the higher-voltage control circuit and provide current amplification (Correct answer)
- To provide time-delay dropout for the motor starter
- To convert the PLC's analog output to a digital switching signal
Correct 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.
Question 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?
- 10,000A IC
- 22,000A IC
- 42,000A IC (Correct answer)
- 65,000A IC
Correct 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.
Question 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?
- 125°F
- 200°F
- 250°F (Correct answer)
- 300°F
Correct 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.
CPE Electrical Systems & Controls 5
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: