So Far as is Reasonably Practicable
The employer should identify all the circumstances in which his employees could be at risk whilst at work, and all the circumstances in which anyone else could be at risk as a result of anything done in his business, and determine the level of resource that is needed to manage each risk effectively taking account of the level of risk. Only by directing and applying a level and effectiveness of resource that is commensurate with the risk can the employer perform his general duties so far as is reasonably practicable.
The term "so far as is reasonably practicable" means that the degree of risk in a particular situation can be balanced against the time, trouble, cost and physical difficulty of taking measures to avoid the risk. If these resources are so disproportionate to the risk that it would be unreasonable to expect any employer to have to incur them to prevent it, the employer is not obliged to do so unless there is a specific requirement that he does.
The greater the risk, the more likely it is that it is reasonable to go to very substantial expense, trouble and invention to reduce it. But if the consequences and extent of a risk are small, insistence on great expense would not be considered reasonable. It is important to remember that the judgement is an objective one and the size or financial position of the employer are immaterial.
Self-employed persons have a general duty in the Health and Safety at Work Act to conduct their undertakings in a way that ensures, so far as is reasonably practicable, that they, and any others affected by what they do, are not exposed to health and safety risks.