What is the significance of the words "so far as is reasonably practicable" with regard to health and safety legislation?

Section 2(1) of the Health and Safety at Work etc Act 1974 contains the criminal law duty on an employer to ensure, "so far as is reasonably practicable", the health, safety and welfare at work of its employees. These words offer a duty holder a statutory defence in the event of an alleged breach. A statutory duty subject to a defence of reasonable practicability is satisfied if the employer can show that the cost of any further preventive steps to make the situation safer would be grossly disproportionate to the further benefit from their introduction. It will be a question of fact in each case as to whether or not the employer has complied with that qualified standard.