A switching apparatus includes an energy store and two measuring devices connected to the control apparatus, the energy store being connected in series between the supply connection and the power supply via the first measuring device, the control apparatus can ascertain if the supply voltage attached to the primary side of the power supply falls short of a first voltage threshold value and, via the second measuring device, if the supply voltage of the power supply attached to the secondary side of the power supply falls short of a second voltage threshold value, the control apparatus evaluates the time between falling short of the first voltage threshold value and falling short of the second voltage threshold value.