Apparatus for measuring the electrical energy supplied to a rail traction unit by a high voltage line, the apparatus being intended to be installed on the roof high voltage line connecting the pantograph to power supply means of the traction unit, the apparatus comprising means for measuring the intensity of the power current and means for measuring the power voltage, - The means for measuring the intensity of the power current comprising a current sensor, a first analog-to-digital converter and a first electrical/optical converter, the first electrical/optical converter being capable of transforming the digital electric signal outputted by the first analog-to-digital converter into an optical signal transmitted by a first optical fiber toward a third optical / electrical converter provided with outputs to be connected to processing electronics; and, - The means for measuring the power voltage comprising a voltage divider, a discriminator to sense DC or AC voltages, a second analog-to-digital converter and a second electrical/optical converter, the second electrical/optical converter being capable of transforming the digital electric signal outputted by the second analog-to-digital converter into an optical signal transmitted by a second optical fiber at the input of a third optical/electrical converter, the apparatus comprising a battery connected to a third electrical/ optical converter, capable of generating a laser beam conveyed by third optical fibers (21a, 21b) to first and second optical/electrical converters (22a, 22b) respectively connected to the first electrical/optical converter and the second electrical/optical converter for their respective electric power supply, the third optical/electrical converter being power supplied by the battery directly, and the apparatus comprises an insulator body comprising a top space for the confinement of the high voltage environment, regrouping the means for measuring the intensity of the power current, the means for measuring the power voltage, and the first and second optical/electrical converters (22a, 22b), and bottom recess for the confinement of the low voltage environment, regrouping the third electrical/optical converter and the third optical/electrical converter, only the first, second and third optical fibers providing a link between the top space and the bottom recess. - The means for measuring the intensity of the power current comprising a current sensor, a first analog-to-digital converter and a first electrical/optical converter, the first electrical/optical converter being capable of transforming the digital electric signal outputted by the first analog-to-digital converter into an optical signal transmitted by a first optical fiber toward a third optical / electrical converter provided with outputs to be connected to processing electronics; and, - The means for measuring the power voltage comprising a voltage divider, a discriminator to sense DC or AC voltages, a second analog-to-digital converter and a second electrical/optical converter, the second electrical/optical converter being capable of transforming the digital electric signal outputted by the second analog-to-digital converter into an optical signal transmitted by a second optical fiber at the input of a third optical/electrical converter,