Abstract: A measuring device to measure the insulation resistance (Riso) of a fuel cell system with respect to an electrical ground point comprises a resistor arrangement—formed by a series connection of reference resistors—between two load-current lines of the fuel cell system as well as one reference voltage source, which is arranged between the ground point and a node between the reference resistors and which is switchable between at least two different reference voltages (Urefo, Urefu). The measuring device further comprises one voltage-sensing device that acquires the voltage (Uo, Uu) between at least one of the load-current lines and the ground point.