What is insulation resistance testing and how is it performed?

Insulation resistance testing measures the resistance of electrical insulation to identify deterioration before failure. A megohmmeter (megger) applies high DC voltage (typically 500V, 1000V, or higher) and measures leakage current. Good insulation shows high resistance (megohms to gigaohms). Testing methods include: spot reading (single measurement), time-resistance (plot R vs time, good insulation increases), polarization index (ratio of 10-minute to 1-minute reading, should be >2), and step voltage (increase voltage in steps). Temperature correction is applied as insulation resistance decreases with temperature.