A novel fault approximation tool (FAT) is presented for use in distribution networks with a high penetration of inverter-interfaced distributed generation. The FAT does not rely on balanced operation nor an absolute reference point for voltage calculations, both of which are required for Newton-Raphson load flow calculations. Simulations show that the FAT provides an accurate representation of the transient and steady-state fault response for unbalanced faults when juxtaposed with a detailed small-signal model. The `sub-transient' response of the inverter-interfaced distributed generation is omitted to remove data considered extraneous from the perspective of distribution line protection analysis. The FAT is not appropriate for analysing the efficacy of distributed generation local protection and is inappropriate for islanded applications. The FAT is designed explicitly for the purpose of utility line protection design and analysis.