As a naturally existing gas of abundant supply

the insulation and interruption capability of air has been extensively investigated. Research has also recently been done to use air as a dilution gas of Fluoroketone for SF

6

free interruption in medium voltage switchgear. Much evidence has shown that air gas-blast circuit breakers have far inferior interruption capability than SF

6

breakers. However

no clear explanation has so far been given regarding why an air switching arc has much larger time constant than an SF

6

arc

and thus exhibiting far inferio

r interruption capability than the latter. Based on reliable experimental measurements

the present work first calibrated the turbulence parameter in a two-dimensional axisymmetrical turbulent arc model which was then used for computational investigation of the air arc behaviour and analysis of the dominant energy transport processes.Through comparative study

it has been found that

for air nozzle arc at low current

the radial and axial cooling mechanisms play almost equal roles

while for SF

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arc the radial cooling mechanism is dominant. The difference in cooling mechanisms for air and SF

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arcs is directly related to the difference in radial temperature profiles which is controlled by the material property

ρCp

through turbulence enhanced thermal conduction. The present work therefore provides clear direction on the explanation of the differing interruption performance of different SF

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replacement gases under transient discharge conditions.