Analysis of transfer touch voltages in low-voltage electrical installations

Protection against electric shock in our homes and work places is one of the most important priorities for electrical services engineers who are now designing electrical installations to conform to the requirements of the 17th edition IEE Wiring Regulations (BS7671: 2008). Now Chapter 41 of BS7671: 2008 requires that additional protection by means of a 30mA 40ms residual current protection device be provided for final circuits supplying general purpose socket outlets that are intended for use by ‘Ordinary persons’. However, there are few publications and little information available on the theory describing how touch voltage of a dangerous magnitude could be transferred from a faulty circuit onto the exposed-conductive-parts of Class 1 equipment of another healthy circuit. This paper summarises the theory of transfer touch voltage calculations and expands on it to show how to carry out a sensitivity analysis in relation to the design parameters that are being used by designers and installers. Based on the results of a real case study, it appears that there is sufficient evidence to show that it may not be sufficiently safe to use the nominal external earth fault loop impedance quoted by the electricity utility companies for adopting a low touch voltage design. Practical application: The touch voltage sensitivity equation derived in this paper is a powerful tool for designers and installers of electrical installations to investigate and identify the sensitivity of resulting touch voltages of any electrical circuits. Nomenclature Ut = touch voltage Uoc = open circuit voltage of the mains supply R2 = circuit protective-conductor resistance R1 = phase conductor resistance Ze = earth fault loop impedance external to the faulty circuit Usource = the potential drop ofZe MET = the main earthing terminal (or main earth bar) of the electrical installation Uo = the nominal declared voltage of the mains supply