Notwithstanding the provisions of this regulation, machinery required to be supervised by a person referred to in paragraph (b), (c) or (d) of the definition of ,competent person' may be used in the absence of any such person for a period not exceeding one month in any continuous period of six months, if it is due to circumstances beyond the control of the employer or user of machinery concerned or in the opinion of an inspector, impracticable to comply with the provisions of this regulation: Provided that a person referred to in paragraph (a) of the said definition shall in writing be designated to supervise the machinery in question during such absence.