Effects of delayed electron emission from the cathodes of circuit breakers

Abstract Electron emission from non-thermionic cathodes of circuit breakers is principally caused by the impact of excited states of neutral molecules on the circuit breaker gas. Solutions of the electron balance equation reveal that if time is required to produce these excited states, a positive ion sheath would form at the cathode from positive ions left in the plasma from the transit of electrons away from the cathode formed by the changed polarity at current zero. A delay of 1 μ s, with a rate of rise of recovery voltage of 1 kV μ s −1 and no electron emission, results in calculated ion sheaths with densities of ∼10 12 cm −3 of thickness of ∼0.1 mm and electric fields near the cathode of ∼10 kV cm −1 . With temperature effects, such fields could cause electrical breakdown and possible circuit breaker failure. The ion sheath also causes a marked reduction of the electric field in the plasma column by a factor of three or more, which would greatly reduce plasma heating after current zero in the plasma column and assist circuit interruption. Calculations of any delay in electron emission are difficult, and no complete calculations of circuit interruptions have been made. If such electrical breakdown is general, circuit breaker performance would be determined by the value of E / N , above which ionization is greater than electron attachment and would explain why SF 6 is a better circuit breaker gas than air and also why circuit interruption improves with an increase of gas pressure; E is the electric field and N the gas number density.