There are a lot of factors at play in this question. In-house at Gear Hungry, none of us have ever broken a ½-inch drive breaker bar. However, it’s more possible to break a ⅜-inch drive, and one tester has sheared one-off at the knuckle. Many manufacturers list a bar’s torque rating online.
As a very general rule, err on the side of caution and use plenty of bar for the job at hand.