In insulation testing, tan delta is a measure of what?

• A. It measures the dissipation factor in a dielectric test (tan delta).
• B. It measures the engine fuel consumption rate.
• C. It measures cooling fan efficiency.
• D. It measures the mechanical wear.

Answer: (A)

Tan delta is the dielectric loss factor, also called the dissipation factor, in an AC insulation test. When voltage is applied to insulation, some current is the ideal displacement current through the capacitance, while some leaks through imperfections in the insulation. The phase difference between voltage and current reflects energy being lost as heat, and tan delta is the tangent of that phase angle. In practical terms, it’s the ratio of the resistive (loss) component to the reactive (capacity) component of the current. A low tan delta means minimal dielectric losses and good insulation, while a higher tan delta indicates greater losses due to moisture, aging, contamination, or micro‑cracks. The other options describe phenomena unrelated to insulation testing, such as engine fuel use, cooling fan efficiency, or mechanical wear.