A combinatorial model for bus LAN structures with backup buses is derived. The model assumes immediate detection of faulty standby elements and same failure rates for both active and standby elements. The model is used to analyze the improvement attained by the addition of backup buses and the effect of catastrophic failures induced by faulty stattions. A more general Markov model is used for the double-bus structure. The model accounts for lower failure rates in and limited test of standby elements. An overview of the model is given and the methodology followed for its specification is explained. Results how that for typical dormancy factors preventive test may be no necessary and then design can be simplified.