Internal arcing : issues related to testing and standardization

Internal arc tests are intended to verify the effectiveness of switchgear design in protecting personnel in case of an internal arc. With the IEC 62271-200 & 201 in 2003, new methods and criteria for testing metal / insulation enclosed switchgear under conditions of an internal arc have been formulated. KEMA's experiences with the new standards will be highlighted, including statistics on the failure rates (approx. 20%), differentiated to the various acceptance criteria. A comparison with experiences related to the prior standard (IEC 60298) will be presented. Various problems around certification are highlighted, among which reproducibility of results, initiation and an adequate definition of relevant design features in the stage of verification of drawings. With switchgear having an increasing rated voltage coming on the market, there is an increased tendency to perform internal arc testing with reduced voltage, because of lack of sufficient short-circuit power. With actual examples, it will be demonstrated to what extent such "reduced voltage testing" is acceptable, the main problem being the uncertainty of the arc motion inside the enclosure, and its effect on the current (asymmetry) through its arc voltage. From environmental point of view, internal arc testing in SF6 filled installations is accepted less and less, tempting to replace SF6 (in SF6 insulated MV switchgear) by air in the regular testing of SF6 insulated MV switchgear. A literature overview is presented of several earlier investigations on the differences between arcing in SF6 and air in MV switchgear. New arcing tests are presented with the arcing in an SF6 and air filled model (0.5 m3, 14 kA and arc duration of 0.5 and 1 s). Arc voltage, current and pressure measurements, as well as high-speed infrared and optical imaging are used to demonstrate the differences between arcing in SF6 and air. It is observed that internal pressure rise as a result of arcing in air proceeds significantly faster than in SF6, and reaches higher values than in SF6. After pressure relief, however, the amount of energy supplied to the environment is higher by arcing in SF6 than in air. Concluding, all experimental results suggest that when replacing SF6 (of SF6 insulated switchgear) with air for internal arc testing, the mechanical stress on the switchgear itself is too severe because of higher and faster rising pressure in air. The effects on the environment (indicators as used in testing, pressure and temperature in switchgear compartments & -room) after pressure relief are distinctly different in SF6 and air, but are not part of IEC tests. Results suggest that adequate test procedures need to be designed that should justify replacement of SF6 by air for internal arc testing.