Regulation (EU) No. 994/2010 on security of gas supply requires Member States to comply with the infrastructure standard or so called N-1 formula. The N-1 formula is used to estimate whether the gas infrastructure of an area of study has enough technical capacity as to satisfy its total gas demand in the event of disruption of the single largest gas infrastructure during a day of exceptionally high gas demand occurring with a statistical probability of once in 20 years. Compliance with the standard is met for a result equal or greater than 100%. In this paper an assessment of the N-1 formula as a security index is performed. A hydraulic model of two fabricated gas transmission networks of fictitious countries has been developed. Scenarios of disruption of the largest gas infrastructure of each country and at regional level have been simulated in dynamic state under the same conditions required in the N-1 formula. Results from the N-1 formula and the N-1 scenarios are critically evaluated. Results show some limits of the N-1 formula as an index of security of gas supply. The use of hydraulic simulations is suggested as a valuable tool to assess N-1 scenarios and to provide more evidence based indices for the implementation of the nowadays and future versions of the Regulation.