The first myth we need to disburse is that you are somehow not responsible for your sub-contractors health and safety while they are working on your job site or working in your name. This is not even a gray area anymore. For example WCB in British Columbia looks at whether the sub-contractor works for anybody else or whether they just work for you. If they just work for you, they are called Dependent Contractors and must be included in your health and safety program.
The best way to deal with the sub-contractor situation is to actually have a sub-contractor management plan and state in this plan what you require your sub-contractors to have before they can work for you. Your plan should cover whether you wish them to have their own WCB, their own health, and safety program, whether you want them to be Cor or Se-cor certified. List any additional certifications that you feel they should have
In addition to the above requirements, you should also be requesting a clearance letter from them on a quarterly basis as well as a wcb rate sheet on a yearly basis. You will want to compare the rate they are paying versus the industry standard rate. Ideally, they should be paying less than the industry rate, and this will show they operate safely on average.
In addition, you should orientate them on what reporting procedures for things like hazards, near misses and incidents. Make them aware that you will require them to attend all job site health and safety meetings or toolbox meetings as your company may call them.
If you have not yet developed a sub-contractor management plan you can google Canada best practices for sub-contractor management and get some helpful tips on how to develop one and what it should include in it.
Please remember above all that unless you actually state in a sub-contractor management plan that you are not responsible for sub-contractor health and safety programs and let the sub-contractors know in writing that you are not responsible then you are going to be found responsible by virtue on nondisclosure. In this case, you must tell them in order not to be responsible. If there is no evidence of this, then you will be responsible.
Hope this helps clear up the difference between workers and sub-contractors as far as responsibility for health and safety.