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

The ratio test confirms that the CT's actual transformation ratio matches its nameplate rating (e.g., 600:5). An incorrect ratio means the metering or protective relays connected to the CT receive inaccurate current signals, leading to metering errors or relay misoperation.

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

Unlike a power transformer, a CT is a constant-current device. When the secondary is open while primary current flows, all the magnetizing force drives the core into deep saturation, inducing voltages on the secondary that can reach thousands of volts—enough to be lethal and to destroy CT insulation.

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

Subtractive polarity (standard for most power CTs) means that when current enters the primary at the H1 terminal, it exits the secondary at the X1 terminal at the same instant. Knowing polarity is critical for differential protection schemes where CT secondaries must be correctly phased to prevent false tripping.

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

CT burden is the load placed on the secondary, expressed in ohms or VA at a rated secondary current. If the actual burden exceeds the CT's rated burden, the CT can saturate, causing ratio errors and inaccurate relay inputs. NETA procedures include measuring burden to confirm it is within the CT's rating.

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

NETA Acceptance Testing Specifications (ATS) require a minimum of 100 MΩ for instrument transformer insulation. Lower values indicate deteriorated insulation that could cause ratio errors, safety hazards, or dielectric failure under normal operating voltage.

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

NETA Instrument Transformer Testing (CTs and PTs)