The insulation resistance measurement provides information about the insulation resistance of electrical windings and commutators. Direct current is used to measure the resistance of the insulation against the earth potential. As in the tangent-delta measurement, the conductivity of the insulation is investigated or the resistance capacity of the insulation against a leakage current.

The measurement must last exactly 60 seconds, as per the VDE standard, to achieve comparable results. The winding, insulation and housing all form a capacitor here that is charged up. The level of charging voltage after the measurement duration indicates the level of the insulation resistance. According to applicable regulations, the industrial sector requires that a limit of 1000 Ω per 1V measurement or insulation voltage should not be undershot at 75°C winding temperature.

If the absolute level of the insulation value is too low, this can be an indicator for contamination. Dust, moisture and other contaminants can divert the measurement current to earth and the above-mentioned capacitor, consisting of the housing, winding and insulation as the dielectric, cannot charge up. In practice, insulation values in the high GigaOhm range (1,000,000,000 Ω) are not a rarity.