I do not know the answer, but I can offer this. The resistance of a human body varies from person to person and from moment to moment. It is lower when the person is tired or when the person has been sweating. But it is not unreasonable, for the purposes of discussions of electrical safety, to assign a value of 300 ohms. At some point in everyone's life, their body's resistance to electrical current can get that low.
An electrical current of 0.1 amps passing through a human body is sufficient to cause death. Multiply that value by a resistance of 300 ohms (using Ohm's Law, E = IR), and you get a result of 30 volts. Therefore, it is possible that a voltage as low as 30 volts can inflict a fatal shock.
So I wonder why OSHA uses 50 volts, and not something lower.