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The amount of Distributed Generation (DG - often from renewable energy sources) connected to the UK power system is increasing. This increase has the potential to affect the reliability of protection system operation (a number of instances of protection maloperation have already been experienced ). Issues associated with protection that have previously been considered in the literature include: blinding of protection, false tripping (sympathetic tripping), increased or decreased grid infeed levels, unwanted islanding and failure of auto-reclosers [2, 3]. The main focus of this paper is the protection cascade tripping effect caused by sympathetic undervoltage tripping of DG interface protection for network faults at the distribution level. This paper aims to comprehensively investigate and quantify the conditions at which sympathetic undervoltage tripping occurs.