Insulation measurements are performed at either ____,____, or ______.

• A. 250, 500 or 1000 volts
• B. 125, 250 or 500 volts
• C. 100, 250 or 500 volts
• D. 300, 600 or 1200 volts

Answer: (A)

When checking insulation, you stress it with voltages higher than normal operating levels to reveal weak spots or any leakage paths. The voltages most commonly used for these tests are 250, 500, and 1000 volts because they cover a wide range of equipment ratings. Using these three options lets technicians assess insulation integrity for equipment rated from lower to higher voltages, providing a meaningful margin to catch problems before a fault occurs in service. If insulation holds at these test voltages, it’s a good indication that it will withstand normal operation and occasional surges; a poor reading shows a leakage path or breakdown tendency that needs repair.

Other voltage options aren’t as standard across the common range. Some voltages like 125 V aren’t as universally used for insulation testing across all equipment classes, and 300, 600, or 1200 V aren’t the typical triad relied on in many standards.