What diagnostic test assesses insulation quality by measuring capacitance and dissipation factor?

• A. Vibration test for rotor balancing.
• B. Fuel quality test.
• C. Dielectric test such as tan delta (dissipation factor) testing.
• D. Oil viscosity test.

Answer: (C)

Assessing insulation health is done with a dielectric test, which treats the insulation like a capacitor and looks at two key properties: capacitance and dissipation factor. Capacitance gives a sense of the insulation’s geometry and dielectric constant; it should stay within a tight tolerance because a change can indicate moisture, voids, or aging. The dissipation factor, or tan delta, measures how much current is lost as heat within the insulation material. A low dissipation factor means the insulation is behaving ideally with minimal losses, while a higher value signals deteriorating insulation, moisture ingress, contamination, or aging. By applying an AC voltage and measuring both the capacitance and the dissipation factor, you get a clear picture of insulation quality.

The other tests listed focus on mechanical or fluid properties rather than electrical insulation. A vibration test checks rotor balance and mechanical integrity, a fuel quality test looks at fuel properties, and an oil viscosity test gauges the lubricant's performance. None of these assess the electrical insulation’s condition, which is why the dielectric test with tan delta is the appropriate choice.