When testing insulation resistance of electric equipment and machinery, ideally when should the insulation resistance be tested for the lowest normal insulation values?

Correct Answer: Option D — immediately after shutting down the machine

Insulation resistance testing per IEEE 45 should be conducted immediately after shutting down a machine while the windings remain at peak operating temperature. This captures the lowest normal insulation resistance value, identifying moisture and thermal degradation before cooling allows insulation to absorb ambient humidity.