The high voltages carried by transmission lines and high earthfault currents that flow in the event of an earth fault can lead to an high earth potential rise (EPR). In some cases the EPR can reach a potential of over 20kV on a transmission structure located near where people live and move. The high magnitude of the prospective touch voltage hazards is a well-known fact yet design tolerable touch voltage targets in many standards are maintained at relatively low magnitudes and many transmission tower touch voltages can exceed the tolerable touch voltage targets specified in standards documents. As transmission lines have a very high reliability and are subject to few earthfault events the likelihood that a person will be exposed to a hazard voltage is usually very low. This situation is often assumed implicitly to yield a tolerable risk profile and the earthing design task focussed on installing a tower footing of less than a certain resistance value to meet the insulation coordination requirements of the network. The ability to use quantified risk analysis (QRA) to explicitly quantify the touch voltage risk profile for a given touch or transfer hazard is providing asset owners with the means to more responsibly manage this risk scenario and justify the most effective and economically hazard mitigation measures.
This paper firstly describes the history of the application of QRA to managing transmission line risks, as well as a practical QRA approach that may be used, and the key parameters and interactions driving the risk magnitude for workers and the public. An example of the application of a staged design process for new lines is then introduced to provide a valuable input to the concept design stage of the planning process. As many structures already exist in locations near where people work and live and are susceptible to changing land use placing people in more exposed situations a process is required for assessing the changed situation. The basic design process is extended to enable the risk at an existing site to be quantified and where required determine the most cost-effective means to mitigate the risk to within a tolerable range. The paper describes how current injection testing can be used to measure the actual hazard levels thereby reducing uncertainty, simplifying the software analysis, and strengthening the justification of any risk mitigation measures.