I learned something interesting today so I am going to share it with you guys. Contractors depend on being protected for subcontractors workplace accidents by requiring that the sub includes the Contractor as as additionally insured.
If a sub's employee gets injured on the job in most places the Workmens Compensation Laws preclude the employee from suing their employer.
Here is the kicker, for endorsements after 2004 the insurance industry changed the additional insured endorsements to exclude coverage where the contractor's negligence is alleged but no claim is made against the sub.
So in order for you to be protected, just being named additionally insured may not be good enough. You have to get the sub to eliminate the exclusion. If they are unable or unwilling to do that then you have to include an indemnification clause in your contract.
I have a copy of the clause if you are interested.