Without prejudice to section 6, an employer shall not be obliged to comply with section 11, 12, 13, 16, 17 or 18A where due to exceptional circumstances or an emergency (including an accident or the imminent risk of an accident), the consequences of which could not have been avoided despite the exercise of all due care, or otherwise to the occurrence of unusual and unforeseeable circumstances beyond the employer's control, it would not be practicable for the employer to comply with the section concerned.