Small changes in element parameters which can be caused by manufacturing tolerances, wear, aging, and maintenance can cause significant changes in the overvoltage performance of a power system in which they are incorporated. To study the effect of such dispersion on the performance, a computer-controlled transient-network analyser capable of rapid, accurate estimation of overvoltage-probability distributions has been developed. This system has been used to study the sensitivity of the probability distribution of overvoltage peaks to changes in circuit-breaker characteristics. The effects of prestrike, mechanical dispersion of contact-closure instants between phases, and preferred pole-closing sequences were investigated in typical networks using both plain and resistor-equipped breakers. Significant changes in overvoltage-probability distributions which could cause up to tenfold increases in line-flashover probabilities were found.