The invention relates to a method and a device for measuring a first voltage and a second voltage by means of a differential voltmeter. The differential voltmeter comprises a first inlet and a second inlet and a known voltage potential is applied to the second inlet. The voltage measurement comprises the following; a first voltage is applied to a first inlet of the differential voltmeter, a first differential voltage is measured, the first voltage from the measured first differential voltmeter and the known voltage potential is determined, the second voltage is applied to the second inlet of the differential voltmeter, a second differential voltage is measured, and the second voltage from the measured second differential voltage and the previously determined first voltage is determined.