What is Tan Delta?
Tan Delta (tan δ), also known as the Dissipation Factor (DF), is a measure of the dielectric losses in an insulating material.
Think of a perfect capacitor: The current leads the voltage by exactly 90 degrees. No energy is lost.
A real-world insulation system acts like a perfect capacitor in parallel with a resistor. The resistor represents the imperfection and power loss.
The angle between the current and voltage in this real-world scenario is slightly less than 90 degrees. The difference is the "loss angle" (δ).
Tan δ is the tangent of this loss angle. It's the ratio of the resistive (lossy) current to the capacitive current.
A low tan delta means the material is a good capacitor with very low losses (an excellent insulator). A high tan delta means it's a lossy, poor-quality insulator that heats up and degrades faster.
What Constitutes a "Good" Value? (General Guidelines)
The following table provides a rough guide for tan delta values at room temperature and power frequency (50/60 Hz). Always consult the specific equipment manufacturer's standards for precise limits.
| Insulation Material / System | Good Tan Delta Value (at 20°C, 50/60 Hz) | Context |
|---|---|---|
| New Oil-Impregnated Paper (e.g., in Cables, Transformers) | 0.001 (0.1%) or less | The gold standard for high-voltage AC equipment. |
| EPR / XLPE Cable (Cross-linked Polyethylene) | 0.0005 to 0.002 (0.05% - 0.2%) | Very low loss, excellent for distribution cables. |
| Silicone Rubber | 0.001 to 0.005 (0.1% - 0.5%) | Good for high-temperature applications and coatings. |
| Polyethylene (PE) | 0.0002 to 0.0005 (0.02% - 0.05%) | Extremely low loss, often used in RF applications. |
| Porcelain / Glass (Clean) | 0.002 to 0.01 (0.2% - 1.0%) | For insulators; value is less critical than contamination. |
| Medium Voltage Rotating Machine Stator Windings | < 0.01 (1.0%) at 40°C | IEEE Std 43 recommends corrective action if it exceeds 0.1 (10%) for most form-wound coils. |
| Dry Transformer Insulation | < 0.01 (1.0%) |