IST Electrical and Electronic Knowledge 4

Question 1

In a 3-phase wye-connected system, the phase-to-neutral voltage is 277 V. What is the line-to-line voltage?

Accepted Answer

480 V

Answer

Line voltage in a wye system equals phase voltage times the square root of 3 (277 x 1.732 ≈ 480 V).

Question 2

What is the approximate forward voltage drop across a conducting silicon diode?

Accepted Answer

0.7 V

Answer

Silicon diodes typically drop about 0.6 to 0.7 V when forward biased.

Question 3

A 10 kΩ resistor is in series with a 100 µF capacitor. What is the RC time constant?

Accepted Answer

1 second

Answer

τ = R x C = 10,000 Ω x 0.0001 F = 1 second.

Question 4

A megohmmeter (megger) is primarily used to test which property of a motor or cable?

Accepted Answer

Insulation resistance

Answer

A megger applies high DC voltage to measure insulation resistance in megohms.

Question 5

A transformer with a 4:1 turns ratio has 480 V applied to its primary. What is the secondary voltage?

Accepted Answer

120 V

Answer

Voltage divides by the turns ratio in a step-down transformer: 480 / 4 = 120 V.

IST - Industrial Skills Test Practice Test