Touch Voltage

During an earth fault, current flowing through the earthing system creates voltage gradients on exposed metalwork and the ground surface. Touch voltage is the potential difference a person would experience if they simultaneously contacted an energised conductive part (such as a faulted equipment enclosure) and the ground or another earthed part. The conventional touch voltage limit of 50V AC rms is based on body impedance research: at this voltage, the current through the human body remains below the threshold for ventricular fibrillation under most conditions. In wet locations, bathrooms, swimming pools, and medical facilities, the limit is reduced to 25V or even 12V because wet skin has lower impedance. Protective equipotential bonding connects all accessible metalwork to the same earthing point, minimising voltage differences between touchable surfaces. In TN systems, touch voltage during a fault is Ut = If × RPE, where If is the fault current and RPE is the protective conductor resistance. The protective device must disconnect the supply before touch voltage becomes dangerous — hence the time-dependent touch voltage curves in IEC 60479-1.