Generally, the awareness on hazards and safety measures to be followed are better with regular employees as compared to outsourced personnel who are prone to accidents due to poor awareness.
It is always better to have meetings with interested contractors, their site supervisor and site safety supervisor to discuss about the scope of work under the work order, the hazards in the work area, hazards likely to arise during execution of work and safety procedures to be followed along with company's safety policy and requirements. This will help them to understand and to follow. It is better to include penal clauses also for violations by listing so that clarity will be there on both sides.
This practice will help to understand the issues and quote accordingly for the work. It is observed from accident reports in news papers / journals that contractors do not understand or ignore the requirements and once accident takes place, they will run away from the scene / abscond or cut sorry face facing the enquiry.
It is always a good practice to explain safety requirements to prospective agencies before issue of work order.