The paper is a follow-up to the paper written by Stalewski, Goody and Downes for the Second IEE International Conference on Developments in Power System Protection in 1980. The paper discusses the required performance of two types of pole-slipping relay which utilise the principle of impedance measurement and explains the difficulties with this method of measurement, as revealed by model machine and system tests, in distinguishing between pole-slipping and certain types of fault conditions. It describes the logic criteria developed for each relay to overcome these problems. The paper also describes the principles involved for the relay installation at Dinorwig, in determining how far each relay should reach in order to detect remote pole-slips.