The coupling device with electrical isolation comprises at least one input signal, an output of an output signal representative of said input signal, and signal transfer means with electrical isolation receiving the input signal and supplying said output signal. The transfer means comprise at least one signal transformer having at least one primary winding to receive a primary signal representative of said input signal, switching means to switch the input signal and supply the latter to said primary winding, and electrically isolated control means of the switching means comprising a control input receiving control signals during switching periods. The electrical switchgear unit comprises one such coupling device connected to measuring resistors and to a processing unit processing electrical protection functions.