Abstract: A large LTD device capable of delivering about 50 MA pulsed current to load consists of hundreds of thousands of capacitors, switches and trigger units. To achieve high reliability of such an enormous system is quite difficult with the series system reliability model, the required reliability of each switch is almost impossible to realize. The main fault modes of LTD device are switch prefire and trigger unit fault. The switch prefire will result in the failure of the LTD cavity where the prefire occurs. The fault of trigger unit would affect more LTD cavities because one trigger unit triggers several LTD cavities. The key to achieve high reliability level and meet the required performance, it is key that some fault LTD cavities are allowed and the number of fault LTD cavities is limited in different place. In this paper a reliability model of the LTD system is built from a conceptual design of the LTD device. The number of the fault LTD cavities would be strictly limited in the system, module and layer in the reliability model. For a given system reliability, the reliability values of switch and trigger unit are calculated by Monte-Carlo method. The method of fault isolation is also analyzed.