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

100% of rated primary current

Answer

150% of rated primary current

Answer

200% of rated primary current

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

Overloading causing excess current in the secondary

Answer

High ambient temperature causing winding expansion

Answer

Improper primary voltage causing core saturation

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

The transformer's maximum rated voltage

Answer

The transformer's full-load efficiency percentage

Answer

The transformer's short-circuit current withstand rating

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

The steady-state full-load secondary current during normal operation

Answer

The maximum current drawn during a bolted secondary short circuit

Answer

The small magnetizing current drawn under no-load conditions

Question 5

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

Accepted Answer

Megohmmeter (megger)

Answer

Clamp-on ammeter

Answer

Oscilloscope

Answer

Wattmeter

Question 6

What happens to a transformer's overall efficiency when it operates significantly below its rated KVA load?

Accepted Answer

Efficiency decreases because constant core losses become a larger fraction of output power

Answer

Efficiency increases proportionally as load decreases

Answer

Efficiency remains constant regardless of load level

Answer

Efficiency increases because copper losses fall to zero at light load

Question 7

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?

Accepted Answer

57.7%

Answer

50%

Answer

66.7%

Answer

86.6%