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Experimental and theoretical investigation of an enclosed free burning arc in SF6

The behaviour of a free burning SF6 arc in a two-volume cylindrical chamber, which simulates the initial arcing in a modern high voltage auto-expansion circuit breaker, has been investigated both experimentally and theoretically. Arc voltage and pressure variations at four strategically chosen locations in the chamber were measured for electrode gap lengths of 30, 40 and 50 mm in the peak current range from 5 to 20 kA of a positive half-cycle alternating current at 50 Hz. A simplified model for free burning arcs in an enclosure was developed by radially integrating the governing equations under the assumptions of a uniform radial temperature profile and a variable radial profile of the axial velocity (equation (1)) in the arc. The effects of Lorentz force, Ohmic heating, radiative energy transfer, and turbulence enhanced momentum exchange are taken into account. The computed arc voltage and pressure variation in the chamber agree with the experimental results within the estimated range of uncertainty.