Elecrode vapour effects in high current gas blast switchgear arcs

In this paper it is suggested that the magnetic pinch effect at the electrode/arc column junction is responsible for local current density enhancement and the consequent copious emission of electrode vapour into the column of an SF 6 arc. This hypothesis is supported by measurements of electron density and arc temperature in the column vapour core and by the agreement of pinch theory with empirical relationships. Since the vapour core decay current is less than the inception current (∼ 20 kA) then for very high peak currents an enhanced vapour density can exist at current zero. The high electrical conductivity associated with this vapour can thus have a deleterious effect on voltage recovery in a practical circuit breaker.