Transformers and Distribution Systems Flashcards
7 cards from real CEA practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Transformers and Distribution Systems flashcards as text
According to NEC Article 450.3(B), what is the maximum primary overcurrent protection allowed for a transformer whose rated primary current is 9A or more, with no secondary protection?
Answer: 125% of rated primary current
NEC 450.3(B) limits primary overcurrent protection to a maximum of 125% of the transformer's rated primary current when the primary current is 9A or more.
What physical phenomenon is responsible for the audible hum that a transformer produces during normal operation?
Answer: Magnetostriction of the core laminations occurring at twice the supply frequency
Transformer hum is caused by magnetostriction—the slight dimensional changes in core laminations as they magnetize and demagnetize at 60 Hz, producing vibration at 120 Hz.
A transformer nameplate shows insulation class 'F'. What does this classification indicate?
Answer: The maximum temperature the transformer's insulation can continuously withstand
Insulation class designations (A, B, F, H, etc.) indicate the maximum temperature the insulation system can continuously withstand; Class F is rated for 155°C.
What is transformer inrush current, and when does it occur?
Answer: A high initial current (up to 10–12 times rated) that flows when the transformer is first energized
Inrush current is a high transient current that occurs the moment a transformer is energized due to core saturation, and it typically decays within a few cycles.
Which instrument is used to test the integrity of a transformer's winding insulation?
Answer: Megohmmeter (megger)
A megohmmeter applies a high DC test voltage and measures insulation resistance in megohms, confirming whether the winding insulation is intact or degraded.
What happens to a transformer's overall efficiency when it operates significantly below its rated KVA load?
Answer: Efficiency decreases because constant core losses become a larger fraction of output power
Core (iron) losses are constant regardless of load, so at light load they represent a larger percentage of output, reducing overall efficiency even though copper losses are lower.
When two single-phase transformers are connected in an open-delta (V-V) bank, what percentage of the capacity of a full closed-delta bank can the open-delta bank deliver?
Answer: 57.7%
An open-delta bank using two transformers can deliver approximately 57.7% (1/√3) of the capacity of a full three-transformer closed-delta bank.