ASTM D924 Standard Test Method for Dissipation Factor (or Power Factor) and Relative Permittivity (Dielectric Constant) of Electrical Insulating Liquids

ASTM D924 Introduction

ASTM D924 (Standard Test Method for Dissipation Factor and Relative Permittivity of Electrical Insulating Liquids) provides uniform procedures for testing the dielectric properties of electrical insulating liquids. The dissipation factor and the power factor are both measures of how much electrical energy is dissipated as heat in an alternating current circuit. Hence, low values generally signify low dielectric losses and good insulation properties. Dielectric constant (or relative permittivity) is the property of an insulating liquid by which it stores electrical energy. These properties directly affect the reliability of transformers, oil circuit breakers, and electrical cables. Therefore, ASTM D924 can be used for quality control, maintenance testing, research, and material selection for commercial-frequency electrical insulation systems.

Get Certified ASTM D924 Testing for Reliable Electrical Insulation Performance

Oxidation, contamination, and moisture cause electrical insulating liquids to deteriorate over time. As a result, service conditions can also increase dielectric losses. With accurate testing in the laboratory, manufacturers and utilities can recognize insulation issues before equipment failure. Also, regular dielectric analysis enhances the reliability and safety of transformer operation.

ASTM D924 Testing Procedure and Requirements

The ASTM D924 test measures the dielectric losses and the relative permittivity at commercial frequencies 45 Hz to 65 Hz. First, technicians prepare clean insulating liquid samples and put them into calibrated dielectric test cells. Then the testing system creates an alternating electric field and measures the dissipation factor, power factor, and capacitance characteristics. Lastly, operators determine the dielectric properties based on the electrical responses. The analyst measures the oil sample’s Power Factor at 100°C. A minimum of 100mL of sample is needed for analysis; however, we prefer that you send 500mL in case we have to duplicate or redo the sample. Pure oil samples are preferred as the presence of free water would damage the instrument and give poor results.

Scope of ASTM D924 Test Standard

• Tests of in-service and new insulating liquids
• Works on transformers and oil circuit breakers
• Measures the losses in the material under AC field conditions
• Measures the relative permittivity properties

Equipment and Sample Preparation

Use of ASTM D924 

• Detects deterioration of insulating liquid
• Identifies contamination and moisture issues
• Carries out transformer maintenance programs
• Helps to perform the quality control of dielectrics

Why does the dissipation factor matter in electrical insulating liquids?

The dissipation factor is a measure of the losses in insulating liquids that occur as heat. So the lower the value, the better the electrical insulation. Higher dissipation factor values indicate contamination, oxidation, or moisture. Engineers, therefore, take advantage of this property to check the state of insulation and the reliability of the equipment.

How does relative permittivity affect electrical equipment performance?

Relative permittivity affects the distribution of the electric field and capacitance characteristics within an electrical system. The lower the dielectric constant is, the lower the capacitance between parts and ground. But the intermediate values help achieve good AC voltage distribution between liquid and solid insulation materials. The higher the relative permittivity, the smaller the equipment size in capacitors.

Common Challenges and Troubleshooting

One of the most common issues technologists have is getting contaminated samples, dirty test cells, and moisture absorption. Thus, these problems could lead to higher dielectric losses and inaccurate measurements. Another factor that can impact test consistency is improper temperature control and unstable frequency conditions. In addition, capacitance readings may be affected if the spacing of the electrodes is not accurate. Laboratories minimize these errors by using calibrated dielectric systems, keeping equipment clean, and carefully controlling environmental factors.

Testing Technique, Process, and Data Collection

Testing starts with careful sampling and preparation in a contamination-free environment. Then, the dielectric test cell is filled and subjected to a controlled commercial frequency alternating voltage. During the process, the measuring circuit takes note of the capacitance behavior and dielectric losses. Finally, from the electrical data collected, operators determine dissipation factor, power factor, and relative permittivity. Accurate measurements help with condition monitoring and the evaluation of insulation quality.

Analysis Results and Interpretation

ASTM D924 provides values for the dissipation factor and the power factor in both decimal and percentage forms. Generally, a low dissipation factor indicates low AC dielectric losses and good insulation stability. Higher values, however, can indicate contamination, oxidation, or deterioration of the liquids. The results are compared to acceptable operational limits to ensure insulating liquid suitability.

Factors to Consider For ASTM D924 Testing

• Speed: Fast dielectric testing is a valuable utility tool for detecting insulation issues before equipment failure disrupts power delivery.
• Expertise: Clean testing conditions are maintained by experienced technicians, and dielectric measurements are interpreted correctly to deliver reliable evaluations.
• Cost: A consistent program of insulating liquid testing helps prevent costly damage to electrical equipment and identifies problems before they get out of hand.

Link to ASTM D924

By Malhar Khole

Malhar Khole is a Material Testing Associate at MaTestLab Inc. and holds a postgraduate degree in Material Science and Technology from IIT...