The time constant of high voltage circuit breaker arcs before current zero

It has been shown that the relative rate of decrease of conductance of the arc, in the following called RRDC, can be split in terms depending on the changing of the quantities: arc length, arc cross section, peak arc temperature and temperature profile, and the order of magnitude of the different terms has been estimated. For an experimental air blast breaker current and voltage near current zero have been measured for the case of a short line fault and the RRDC function has been calculated in the time interval 20-1 µs before current zero. It has been shown that the often used theory of Mayr is not applicable over the whole of the investigated time interval and also that the theory of Cassie does not apply in the time interval preceding 10 µs before current zero. In the last 10 µs before current zero Cassie´s theory describes the investigated air blast breaker quite well. The transient behavior of the arc conductance (e.g., Cassie´s time constant) in the air blast breaker has been found to be of the same order of magnitude (approximately 0.5 µs) as has been reported for SF6gas blast breakers. This has been explained by the fact that the RRDC function is mainly dependent on the variation in the arc cross section, which seems to be only slightly dependent on the quenching medium. Because this reasoning is only valid before current zero, greater differences of the transient arc behavior between different types of breakers may occur at and after current zero.