Why does the human body not feel an electric shock at low voltages such as 12 volts?

The leakage current or faulty current that flows through the human body, because of ineffective earthing arrangement, causes electric shock. This faulty current is derived by the equation E/R, where E is the system voltage and R the skin resistance of the human body, measured in ohms. 

Under worst conditions, that is, `wet', this resistance could reach as low as 1000 ohms. The severity of electric shock is directly proportional to the system voltage and inversely to the body resistance. The system voltage for domestic supply is taken as 230 volts at 50Hz frequency. Under worst conditions in 230 volts system, the leakage current through human body is arrived at as 230/1000 i.e. 0.23 ampere which when passes through the chest of the victim, could cause stoppage of breathing. If the system voltage is low, to the tune of 12 volts, the leakage current will be 0.012 ampere, consistent with 1000 ohms body resistance and no detrimental effects of electric shock will be felt. In dry conditions, the body resistance is taken as 50,000 ohms on average and the faulty current through the body will be 0.0046 and 0.00024 ampere, commensurate with 230 volt and 12 volt systems respectively, which is safe. On the whole, 12 volts is reasonably safe to handle under faulty conditions.

Source : thehindu.com