1) There is no obligation for an employer to provide a locked or secure break or sleeping room for their employees.2) That said, if an assault happens, then your employer may well be liable. You state that you have to be on call, and therefore you need a  place to wait, rest, etc. If it would be considered negligent, or unreasonably careless, for your employer to not provide you with a secure place and you were injured, that could give rise to liability. Whether it is negligent would depend on the circumstances--patient population, overall  level of security and monitoring in the building, now easy it is for a patient or outsider to get where he or she should not be, etc.