Abstract <p>The aim of the paper is to increase electrical strength recovery of the arcing zone for HV AC gas-blast circuit breakers. The synchronous gas injection on a model of mono-flow air arc quenching device is considered. Methods of numerical simulation of gas dynamic processes are used. The intensification of cooling of the nozzle throat zone, an increase in pressure and mass flow in the initial part of the downstream region of the nozzle confirms the effectiveness of synchronous gas injection. The thermophysical and transport properties of an air plasma with an admixture of vapors of an eroded material (copper) in the range of 300–20 000 K are evaluated. The calculation of the composition of the medium confirms a decrease in the concentration of electrons under synchronous gas injection, which provides additional deionization in the nozzles of arc quenching devices. Based on the assessment of plasma electrical conductivity, an increase in the electric field strength is shown. An increase in the rate of electrical strength recovery of the arcing zone by ∼11–24% before current zero is recorded in the considered temperature range of 2000–8000 K.</p>