As a consequence of substantial changes to the railway track layout and the implementation of a new approach to the renewal of negative bonding as part of a Major United Kingdom (UK) Railway Signaling Infrastructure Project, the need for the reassessment of the existing electrical circuit impedances arose. This was necessary to ensure that circuit breakers and relays, protecting the portion of the third rail DC traction power supply network in question, had safe protection settings employed to continue to ensure safety under electrical fault conditions. This paper outlines the challenges that needed to be overcome in gathering, processing and reconciling input data to arrive at a complete, coherent and consistent set of data necessary for the calculation of the maximum fault impedances seen from each of the circuit breakers. The paper goes on to present the development, implementation and application of the methodology for the calculation of the protection settings. The process, illustrated with a flow-chart, was developed based on the relevant railway standards and guidance documents reflecting best practice, taking into account previous experience and the lessons learnt from other recent Power Upgrade projects. The methodology is based on modeling the feeding arrangement with an equivalent electrical circuit and was implemented into a spreadsheet based calculation tool. The tool facilitates the choice of feeding scenarios, the input and validation of data and enables sensitivity studies of the effect of various circuit components. A particular challenge of the application of this methodology and tool was the identification of the worst fault scenarios, especially in large switches and crossings areas with multiple ‘Tee’ feed connections at various points along the track. Such problems were solved by modeling and carrying out simulations for various scenarios to determine the worst case. The methodology and computational tool were validated for every application by comparing the results with those obtained by an independent computer program. The analysis of the protection setting calculation results demonstrated surprising conclusions contrary to original expectations.