Abstract: The processing circuit of the insulation monitor comprises a microprocessor receiving on a first input a signal representative of the DC component, or the component of preset frequency different from the frequency of the mains system to be monitored, of the voltage applied to the insulation monitor terminals. The microprocessor receives on a second input a signal obtained by making the difference between a measured voltage representative of said voltage component and a preset threshold value and by multiplying this difference by a multiplication coefficient. When the measured voltage is greater than the threshold value, the microprocessor uses the signal applied to the second input so as to improve the precision in the higher voltage range.