At the present time, highly actual is to modernize and restructure the Latvian power networks so that they meet the requirements imposed on the integrated energy systems of European Union countries. In the list of requirements the most important are those concerning energy supply and improvement of the network operation reliability. To solve these problems, it is urgent to replace the old equipment with modern one, with modifying, at the same time, the structure of transmission lines. Such measures are expensive, so it is important to prove their economic efficiency. To calculate the electric energy undersupply, in many countries there exists a special criterion - namely, the specific undersupply expenses, which are formed as a result of long-term observation. When calculating the undersupply expenses it is necessary to know the dead load and the time of supply interruption. These values are easy to calculate for distribution networks, however in a transmission network the damaged lines are usually disconnected without taking off the direct load. At the same time, breaking one connection may lead to overloading in other connections. The paper offers a method for calculations of reliability criteria under the described conditions, which consists of two iterations. During the first iteration there is estimated the line condition, the length of a power line, the operation runtime, power flow in the line, etc., and selected the critical connection number. In the second iteration all states with broken critical connections are considered, as well as overloads are determined and the investments needed for their elimination estimated. These investments are to be used as reliability economic equivalent - the reliability expenses. The paper presents the algorithm for calculation of reliability expenses and gives an example of calculations. The calculations show that to estimate economically the unavailability of a transmission line the reliability expenses criterion can be used, which determines the investments needed for estimation of the line's overloading under post-emergency conditions.