No mine shall be worked at a time when neither the manager of the mine, nor an under-manager thereof nor a person acting in exercise of an appointment made for the purpose of any of the provisions of the two last foregoing sections is present at the mine unless the mine is in the charge of a competent person appointed by the manager and having such qualifications (if any) as may be prescribed:

Provided that nothing in the foregoing provisions of this section shall prevent the working of a mine at a time when no persons are employed thereat below ground.