skip153 - this is the kind of question that gets asked after technical & engineering people have told plant management that "We can't run like this", and get the response of "Well, show me the code where it says we can't". If this is the case, you have my sympathy...I've been there. You're basically being told to drive a car with bad brakes. You won't likely find a specific clause that forbids the operation of a motor vehicle with bad brakes, but it will fall under one of the catch-all "maintained in a safe operating condition" paragraphs. It'll be the same for turbines with faulty overspeed trips.
Nobody cares until there's an accident. Then the investigator will want to see the records regarding the overspeed protection. At that point, you either get to show that you have no records, and were negligent in performing the proper tests, or that you DO have records that showed a faulty safety device, and yet you continued to operate the turbine anyway. Either scenario is very awkward to deal with.