NETA Instrument Transformer Testing (CTs and PTs)

Question 1

What is the primary purpose of performing a ratio test on a current transformer (CT)?

Accepted Answer

To verify that the CT accurately steps down current by its nameplate ratio

Answer

To measure the insulation resistance between primary and secondary windings

Answer

To determine the CT's burden capacity in volt-amperes

Answer

To check the polarity marking on the secondary terminals

Question 2

A CT with a 5A secondary is left open-circuited while the primary carries load current. What hazard does this create?

Accepted Answer

The CT core saturates and the secondary develops dangerously high voltage

Answer

The primary fuse blows due to excessive current

Answer

The CT ratio decreases, causing relay under-reach

Answer

The insulation resistance drops to zero

Question 3

During a polarity test on a current transformer, what does a 'subtractive polarity' result indicate?

Accepted Answer

Current enters the H1 terminal and exits the X1 terminal simultaneously

Answer

The primary and secondary windings are wound in opposite directions

Answer

The CT secondary voltage is in phase with the primary voltage

Answer

The CT has been connected with reversed primary leads

Question 4

What does the 'burden' of a current transformer refer to?

Accepted Answer

The total impedance (in ohms or VA) connected to the secondary winding

Answer

The maximum primary current the CT can carry continuously

Answer

The number of secondary turns relative to primary turns

Answer

The dielectric strength of the CT's insulating medium

Question 5

A potential transformer (PT) is being tested with an insulation resistance test. What minimum insulation resistance is generally acceptable per NETA standards?

Accepted Answer

100 megohms or greater

Answer

1 megohm or greater

Answer

10 kilohms or greater

Answer

500 ohms or greater

Question 6

Which test is used to verify that a CT's accuracy class is maintained at elevated current levels, ensuring it does not saturate prematurely?

Accepted Answer

Excitation (saturation) curve test

Answer

Contact resistance test

Answer

Secondary injection test at rated load

Answer

Dielectric withstand test