What test voltages are typically used for insulation resistance testing on low-voltage circuits?

• A. 12 V DC or 24 V DC.
• B. 500 V DC or 1000 V DC.
• C. 208 V AC.
• D. 3-phase 400 V AC.

Answer: (B)

Insulation resistance testing relies on applying a DC voltage across the insulation to stress it and measure leakage current, giving a resistance in megohms. For low-voltage circuits, the typical test voltages are 500 V DC or 1000 V DC because these levels are appropriate for the insulation ratings encountered in LV equipment: 500 V DC is commonly used for systems up to about 600 V, while 1000 V DC is used for systems up to about 1000 V. Using these higher DC levels helps reveal defects that lower voltages wouldn’t stress, without exceeding the insulation’s limits. Voltages like 12 V DC, 24 V DC, or any AC voltages (208 V AC, 400 V AC) are operating voltages, not test voltages, so they wouldn’t reliably indicate insulation integrity.