CEA Transformers and Distribution Systems 3

Question 1

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?

Accepted Answer

125% of rated primary current

Answer

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.

Question 2

What physical phenomenon is responsible for the audible hum that a transformer produces during normal operation?

Accepted Answer

Magnetostriction of the core laminations occurring at twice the supply frequency

Answer

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.

Question 3

A transformer nameplate shows insulation class 'F'. What does this classification indicate?

Accepted Answer

The maximum temperature the transformer's insulation can continuously withstand

Answer

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.

Question 4

What is transformer inrush current, and when does it occur?

Accepted Answer

A high initial current (up to 10–12 times rated) that flows when the transformer is first energized

Answer

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.

Question 5

Which instrument is used to test the integrity of a transformer's winding insulation?

Accepted Answer

Megohmmeter (megger)

Answer

A megohmmeter applies a high DC test voltage and measures insulation resistance in megohms, confirming whether the winding insulation is intact or degraded.

(CEA) Certified Electrical Apprentice Practice Test

(CEA) Certified Electrical Apprentice Practice Test

CEA Transformers and Distribution Systems 3