Outline the procedure and purpose of an insulation resistance test.

• A. Apply a low AC voltage and measure heating
• B. Apply a high DC voltage between conductors and/or to earth and measure leakage resistance; high resistance indicates good insulation, low indicates leakage or damage.
• C. Apply a high DC voltage and measure current
• D. Apply an AC signal and measure capacitance

Answer: (B)

An insulation resistance test is used to check how well the insulation between live parts and earth (or between conductors) can prevent current leakage. It relies on applying a high DC voltage and measuring the leakage resistance with a megohmmeter. If the insulation is in good condition, the leakage resistance will be high; if there are leakage paths or damage, the resistance drops.

To perform it, first ensure the equipment is completely de-energized and isolated from the supply. Disconnect any components or circuits that could affect the reading. Connect the test leads so one end is on the conductor (or windings) under test and the other end is on earth or on the other conductor as required. Apply the specified high DC test voltage for a short period, allowing leakage current to stabilize, and read the resistance value in megohms. After the test, discharge the insulation safely and restore the equipment. Compare the reading with the equipment’s specifications or relevant standards; a high resistance indicates good insulation, while a low resistance points to leakage paths, moisture, contamination, or damage. Temperature, humidity, and cleanliness can influence the reading, so consider those factors when interpreting the result.