Kubota J106-STD Operator's Manual page 2

WARNING
3
To prevent electrical shock the following instruction must be followed.
Before the generator can be connected to a building's electrical
system, a licensed electrician must install an isolation (transfer)
switch in the building's main fuse box. The switch is the connection
point for generator power and allows selection of generator or main
line power to the building.
This will prevent the generator from charging the main power line
(backfeeding) when the main power supply has failed or has been
turned off for line repair. Backfeeding can electrocute or injure line
maintenance personnel. Also, generator and building electrical
system damage can occur when normal operating power returns if
unit is used without an isolation switch.