In this article the authors analyze the yield of firefighting external water supply (FEWS) subject to possible damage of pipelines at different sites. Standard methods of assessing water loss dead-end and ring networks FEWS does not take into account the numerous factors which affect the flow of water and also the scenario of possible collision damage different sections of a piping network FEWS isn’t considered. For the calculation the authors used a numerical method of mathematical modeling of water loss ring network FEWS. On the basis of Darcy’s law - Weisbach and mathematical description of the pressure-flow characteristics used in the pumping station pumps there are composed a system of equations for pressure balances of the ring network FEWS with a set number of fire hydrants (FH) and damage to different pipeline sections. To solve systems of equations there are used methods of discrete mathematics and the mathematical program package Mathcad 14. The obtained results are compared with the reference data and the calculation results obtained in a similar way in the simula¬tion of ring network FEWS without considering the damage. For the first time the authors introduce the notion of survivability of the ring network FEWS. To evaluate the effectiveness of yield ring network FEWS in terms of the damage some sections it is pro-posed to use the coefficient of survivability, which is calculated as the ratio of the number of involved fire hydrants that provide water loss when the network FEWS damaged, the total number of fire hydrants in-volved network FEWS. With respect to the models of analyzed ring networks FEWS disclosed the structure of the location of water sources (depending on the number of fire hydrants and distances between them), it’s provided a mathematical description of each of the considered models. The performance of the proposed models and method of quantitative assessment of the network yield FEWS is demonstrated. Recommendations are given for further development and practical use of the developed method for the needs of specialists in the field of fire safety. In particular, it is proposed to use the methodology to design networks of passports FEWS subject to the height differences of the terrain, the amount of FH involved, approximate their locations and characteristics of pumping stations. In the preparation of these passports of networks FEWS it’s proposed in a separate Annex to put the calculated yield table real network FEWS based on the expected factors of internal and external negative effects (including damage to sections of pipelines) on the network. In addition, it’s proposed to reflect the methodology of calculation of loss networks FEWS in the special supplement to the a set of rules 8.13130.2009.