No, in such circumstances there will usually be a claim. If the employer has breached health and safety regulations, and therefore their own statutory and/or common law duty to the employee, then that is a good basis for a personal injury claim (assuming that the employee sustained personal injury as a result). The extent to which the employee is responsible for their injuries due to their own negligence will be reflected in a reduction for contributory negligence. To give an example, it's generally the case that contributory negligence in a road traffic accident if you're not wearing a seat belt is around 25%. So if someone drives into your car they are responsible for the accident either way, but if you're not wearing a seatbelt your damages can be reduced to reflect your own contributory negligence (i.e. you would not have been hurt as badly if you had been wearing a seatbelt). In some cases contributory negligence can be 100%, although that is rare. It all depends on the facts of the case.
The criminal law question is a little different. If a person is convicted for breaching criminal law they will have been convicted beyond reasonable doubt (which is the standard of proof in criminal cases), so if the same question arises in civil proceedings it will be taken as having been decided the same way, because the standard of proof in civil cases is on the balance of probabilities (i.e. 51% instead of 98%). Again, to give an example, if there is a personal injury claim involving a road traffic accident, a party will be found to have been negligent if they've previously been found guilty of driving without due care and attention (or more serious charges). So in an employment context, if an employer has been convicted of health and safety breaches in the criminal courts, they will have been found to have breached their duties to an employee for the same actions in the civil courts. So it won't determine the entire claim (the negligence/criminal act will still have to have caused injury, and the extent of the injury may be in dispute), but criminal proceedings may determine the question of whether a breach has occurred.