Insulating Varnish Testing Demystified: Tan Delta, BDV, and Thermal Class Explained

Quality assurance in insulating varnishes isn’t complete without rigorous testing. These tests determine whether a varnish will perform reliably under electrical and thermal stress, contributing significantly to electrical insulation reliability.

Key Varnish Testing Methods

1. Tan Delta (Dissipation Factor)
This test evaluates the dielectric loss. A lower tan delta value indicates better insulation performance and fewer leakage currents. It’s crucial for assessing thermal aging and varnish breakdown over time. In line with tan delta test varnish procedures, this method is a key indicator of long-term dielectric efficiency.

2. Breakdown Voltage (BDV)
BDV test electrical varnish checks how much voltage a varnish-coated wire or surface can withstand before dielectric failure. High BDV indicates strong resistance to electrical stress, making it vital for transformer and high-voltage applications. This test directly reflects the varnish dielectric strength, ensuring consistent performance under load.

3. Thermal Class Testing (per IEC 60085)
Thermal aging tests simulate prolonged heat exposure. A varnish must retain at least 50% of its dielectric strength after 20,000 hours at its rated class temperature (e.g., 155°C for Class F). This thermal class insulation test, aligned with IEC insulation test methods, validates the varnish’s capability to withstand elevated temperatures over time.

4. Additional Tests

• Viscosity & Solids Content: Determines flow behavior during application, essential for maintaining varnish quality control.
  
  
  

• Adhesion: Tests bond strength between varnish and conductor.
  
  
  

• Moisture Resistance: Evaluates how well the varnish performs under humid or wet conditions, reinforcing compliance with insulating varnish testing standards.
  
  
  

Understanding these metrics helps OEMs ensure reliability, safety, and compliance with international standards like IEC, ASTM, and UL.