since you are in a union, your first (required) steps would be to grieve this through your union. They are required to allow you to file a grievance and represent you.

I do not know what your rules are but if you were in charge of safety and you failed to ensure the machine in question was locked out, it may be an unforgivable error. Your diabetes isn't going to get you such special treatment that you would be allowed to endanger the safety of yourself or others. While the ADA has requirements an employer provide accommodations to allow a disabled worker to perform their job, it never goes to the point of excusing actions that endanger the safety of anybody.