NICET Electrical Power Testing

Question 1

What is the primary purpose of an insulation resistance (megger) test on a power transformer winding?

Accepted Answer

To assess the condition of the insulation and detect moisture, contamination, or deterioration

Answer

Insulation resistance testing applies a DC voltage and measures the resulting leakage current. Low or declining values indicate moisture, contamination, or aging insulation. Turns ratio, load capacity, and winding resistance are measured by other tests.

Question 2

A transformer turns ratio (TTR) test is mainly used to verify which of the following?

Accepted Answer

The ratio of primary to secondary winding turns matches the nameplate and tap settings

Answer

A TTR test applies a low AC voltage to one winding and measures the voltage induced on the other. This confirms the turns ratio and tap-changer positions, and it can reveal shorted turns or open windings.

Question 3

Why is the polarization index (PI) often calculated during insulation resistance testing?

Accepted Answer

It is the ratio of the 10-minute reading to the 1-minute reading, and it shows insulation condition independent of temperature and size

Answer

PI is the 10-minute insulation resistance divided by the 1-minute value. Because it is a ratio, it is largely independent of temperature and equipment size. A low PI (near 1) points to contaminated or wet insulation.

Question 4

What is the purpose of primary current injection testing on a low-voltage power circuit breaker?

Accepted Answer

To verify that the breaker's trip unit operates at the specified overcurrent pickup and time-delay settings

Answer

Primary injection passes high current through the breaker's main contacts and current sensors. This tests the whole trip system (sensors, trip unit, and mechanism) against its pickup and time-delay characteristics.

Question 5

Which test is commonly used to evaluate the earth grounding system's electrode resistance in a substation?

Accepted Answer

Fall-of-potential test

Answer

The fall-of-potential method injects current between the grounding electrode and a remote current probe. It measures voltage with a potential probe at various distances, and the readings give the electrode's resistance to earth.

NICET - National Institute for Certification in Engineering Technologies Practice Test

NICET - National Institute for Certification in Engineering Technologies Practice Test

NICET Electrical Power Testing