CPE Electrical Systems & Controls 5

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:

Accepted Answer

Separately derived system requiring a grounding electrode conductor per NEC 250.30

Answer

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?

Accepted Answer

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

Answer

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:

Accepted Answer

10 seconds

Answer

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:

Accepted Answer

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

Answer

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?

Accepted Answer

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

Answer

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.

(CPE) Certified Plant Engineer Practice Test

(CPE) Certified Plant Engineer Practice Test

CPE Electrical Systems & Controls 5