Abstract: A protection circuit for a cable transmission system comprising a multipaired cable core surrounded by a conductive sheath wherein at least some of the cable pairs have one or more line powered repeater amplifiers connected in series therewith at discrete locations. Gas tubes connect each of the amplifiers input and output terminals to a common bus at each location. In addition a further gas tube having a breakdown voltage several times higher than the others at that location, is connected between the common bus and the conductive sheath, so as to provide controlled breakdown therebetween.