What test voltages are typically used for insulation resistance testing in domestic circuits?

• A. 12 V DC
• B. 1000 V AC
• C. 5 V DC
• D. Commonly 500 V DC or 250 V DC depending on equipment and standards.

Answer: (D)

Insulation resistance testing uses a DC voltage high enough to stress the insulation and reveal leakage paths, but not so high that it damages the equipment. For domestic circuits, the voltage is chosen to match the insulation ratings and the applicable standards, so you’ll commonly see either 500 V DC or 250 V DC depending on the equipment and the standard being followed.

Using 500 V DC provides a meaningful test stress for typical mains insulation without exceeding what most domestic equipment is designed to withstand. Some standards or devices, especially those with lower insulation ratings or specific classifications, specify 250 V DC. Very low voltages like 12 V or 5 V DC don’t stress the insulation enough to reveal defects, and 1000 V DC is generally reserved for higher-voltage equipment beyond standard domestic circuits.