Many assume 24 V power supply is safe to handle. But an article in the link below shows that shock is possible even at 12 V, some sensation can be felt and secondary effects of such shock can endanger the person.
Even for DC supply, with a body resistance of 500 ohms in wet condition and 41 mA DC current leading to painful shock to women means 500 x 0.041=20.5 V.
People are reluctant to have earthing for the machinery saying that the voltage is less but we do not know what can happen and are ready to trade safety for a worst experience (shock!!!).
http://www.allaboutcircuits.com/vol_1/chpt_3/4.html