1. A cooling element comprising a cooling body,the cooling body comprising a first surface and a second surface, the first surface and the second surface being on the opposing sides of the body and the distance between the first and second surfaces defining the height of the cooling body,
multiple of channels for cooling liquid arranged inside the cooling body, the channels extending in a longitudinal direction of the cooling body, wherein
the channels are arranged in two layers in the direction of the height of the cooling body inside the cooling body and the cooling element is adapted to hold electronic components on the first surface and on the second surface for cooling electronic components on both surfaces,
wherein channels of the separate layers of the cooling body are formed as upper and lower channels, whereby at least one of the upper channels is arranged to be in liquid communication to a lower channel at least in one position inside the cooling body,
wherein the cooling body comprises
at least one upper channel which is open to a lower channel within the whole length of the channels, and
an opening extending through the cooling body in the longitudinal direction of the cooling body, the opening being arranged between the at least one upper channel which is open to a lower channel and the mentioned lower channel, wherein
the cooling element comprises at least one insert piece adapted to be inserted to the opening of the cooling body, and the at least one insert piece comprises at least one hole, whereby the insert piece is adapted to separate the at least one upper channel from the lower channel and to provide liquid communication between the at least one upper channel and the lower channel through the at least one hole of the insert piece.