Flexible rockfall barriers are protection systems against risks of falling rocks. Their complex behaviour is still not well understood and the development of relevant models allowing for quick calculations is a need toward the optimisation of such structures. The key issue is the modelling of the so-called ”curtain effect”, where the cable net slides along the supporting cables. The present paper proposes hence a model of sliding cable submitted to concentrated forces. It addresses the stability conditions of the centred finite difference scheme, especially in the framework of the Dynamic Relaxation Method where estimates are found for the best possible fictitious masses at nodes. Expressions are derived in the general case, but also for monotonically curved cables and looped cable elements or rings. Elementary problems are shown for validation, before a case study based on experiments conducted during the French National project C2ROP demonstrates the accuracy and the reliability of the proposed methodology. The present study is however limited to quasi-static loadings and therefore needs further developments to be extended to realistic dynamic cases which are discussed in the conclusion.