An employer has a legal duty of care to its employee's. As long as it can be proven they provided suitable mitigation to prevent such an injury occuring they will already be suitable covered. Think of it this way, if I work in an environment with suitable noise my employer must provide me with suitable ear defense. If I choose to forgoe that defense that is my decision as an individual. If use the gear and still develop hearing problem I cannot sue my employers for not providing adequate ear defense if they did so. What they cannot do is require me to sign away my right to adequate ear defense and if they were to do so said contract would not be considered legally binding.