Effects of currents on human beings: Physical pathology and limits

The impedance of human beings varies between 500 and 4000 Ohms, with the average value being 1000 Ohms. The step and holding currents are of the order of 12 mA for men and 9 mA for women. The currents that cause the ventricular fibrillation follow, in general, a normal distribution whose mean value varies between 20 mA and 100 mA for a shock lasting from 0.3 ms to five seconds. Currents in the order 0.5 A and over can cause burns whose depths depend on the trajectory of the current and the applied voltage. The induced currents due to an electrical field are not dangerous (12 μA). However, their real effects on human organisms are not yet well known. It is, therefore, very important to conceive the protection procedures in relation to the inherent risks associated with each situation. For example, when medical instruments are connected to electrodes situated close to the heart, stringent criteria on electrical isolation must be observed in order to avoid ventricular fibrillation. In low voltage installations, such as domestic applications where 120 V and 220 V are used, North American rules require the ground to be connected to the water mains. In certain European countries, the use of differential circuit breakers ensures a better protection against ground faults.