What test voltage is used for insulation resistance testing in this context?

• A. 250 V
• B. 500 V
• C. 1000 V
• D. 120 V

Answer: (B)

Insulation resistance testing checks how well insulation blocks current by applying a DC voltage across it and measuring the leakage. The voltage must be high enough to reveal defects but not so high that it damages insulation. For typical low‑voltage installations and equipment rated up to about 600 V, 500 V DC is the standard test voltage because it provides a clear, measurable leakage current if insulation is degraded, without overstressing the insulation. A voltage like 120 V is usually too low to expose marginal faults in 230 V systems, while 1000 V can be excessive for ordinary LV equipment and isn’t needed unless testing higher‑voltage gear. So, 500 V is the appropriate choice here.