In the ACU-ODU you find 2 power supplies: one is for the core processor of the ACU-ODM, it is labelled with PS ODM. The other one supplies the external contacts of the ACU e.g. the control signals for the POL drivers relais. This one is labelled with 'PS EXT'. By this, the core processor of the sat-nms ACU-ODM will not break down if something externally happens (e.g. overload/ short circuit)