A mobile generator for generating a direct voltage for a tool supplied with direct current includes an alternating-voltage generator (1), an electronic control unit (3), a rectifier unit (2), which has a direct-voltage output (6) with live conductors (9, 10) for connecting the tool (11), and a protective device (14). The protective device is between the live conductors (9, 10) and a protective potential (18), especially ground or a protective wire. The protective device (14) has an insulating spark gap (15) responding in the case of an error with a defined response voltage (UT) for grounding the live conductors (9, 10).