NETA Insulating Liquids and Gases Testing

Question 1

What does Dissolved Gas Analysis (DGA) of transformer oil primarily detect?

Accepted Answer

Incipient faults inside the transformer by identifying gases produced by thermal or electrical degradation

Answer

DGA identifies and quantifies gases (H2, CH4, C2H2, CO, CO2, etc.) dissolved in transformer oil. Different fault types produce characteristic gas patterns: arcing produces acetylene (C2H2), overheating produces ethylene (C2H4), and partial discharge produces hydrogen (H2). DGA is the most sensitive method for detecting developing faults before catastrophic failure.

Question 2

Which dissolved gas in transformer oil is most indicative of high-energy electrical arcing?

Accepted Answer

Acetylene (C2H2)

Answer

Acetylene is only produced at very high temperatures (above ~700°C), such as those generated during electrical arcing or a flashover inside the transformer. Even small concentrations of acetylene are considered a serious fault indicator and typically trigger immediate follow-up investigation or removal from service.

Question 3

What does the dielectric breakdown voltage test of insulating oil measure?

Accepted Answer

The voltage at which the oil fails to insulate and allows a spark to jump between test electrodes

Answer

The dielectric breakdown voltage (BDV) test, performed per ASTM D877 or D1816, measures how much AC voltage the oil can withstand before it conducts. Contamination with moisture or particles significantly lowers BDV. NETA ATS specifies minimum acceptable BDV levels (typically 26–28 kV minimum for ASTM D877 for medium-voltage equipment), below which oil should be filtered or replaced.

Question 4

SF6 (sulfur hexafluoride) is used in gas-insulated switchgear (GIS). What is the primary concern when the SF6 gas density drops below the minimum rated pressure?

Accepted Answer

The insulating and arc-interrupting capability of the equipment is compromised

Answer

SF6 provides both electrical insulation and arc interruption in GIS equipment. Its effectiveness scales with density (pressure adjusted for temperature). If density falls below the manufacturer's minimum—due to leaks—the dielectric strength drops, increasing the risk of internal flashover, and the arc-interrupting capability is reduced, potentially causing a circuit breaker to fail to clear a fault.

Question 5

The interfacial tension (IFT) test on transformer oil is used to assess which oil property?

Accepted Answer

The degree of oxidation and polar contaminant buildup in the oil

Answer

IFT measures the surface tension between the oil and distilled water. Fresh, unoxidized transformer oil has a high IFT (typically >40 dynes/cm). As the oil oxidizes and accumulates polar contaminants (soaps, acids, and sludge), IFT decreases. A low IFT value (below 25 dynes/cm per NETA guidelines) indicates significant oil deterioration and potential sludge formation on windings.

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

(NETA) InterNational Electrical Testing Association Certified Technicians Practice Test

NETA Insulating Liquids and Gases Testing