Wis. Stat. § 167.13: Operation.

No person, firm or corporation shall use, operate or permit to be used or operated any such machine purchased prior to June 12, 1909, unless during all the time such machine shall be used and operated it shall be in charge of a competent person whose sole duty shall be to oversee and attend to the operation and use of the same; nor use, operate or permit to be used or operated any such machine whatever while the safety devices or guards are detached.