Arc Commutation Across a Step or a Gap in One of Two Parallel Copper Electrodes

This experimental investigation concerns the commutation of magnetically blown switching arcs up to 1 kA from a contact to an are runner, i.e., across an obstacle formed by a step downwards or by a gap in one of two parallel copper electrodes. The commutation delay of the arc was measured with the aid of an optoelectronical light detector. To obtain also detailed information about the arc commutation mechanism, both an optoelectronical analog arc motion indicator and a streak camera were used. The arc commutation delay depends mainly on the magnetic induction of the blast field, the geometry of the electrode step, and the site where the commutation takes place. The arc commutation behavior across gaps corresponds to that across steps. For both steps and gaps a polarity effect of the commutation mechanism was observed. In most cases an obstacle on the anode causes a shorter commutation delay than one on the cathode. The cathode spot always moves continuously downwards along the vertical front surface of a step or a gap, while the anode spot is able to jump across any obstacle without climbing downwards along its vertical face. All results can be qualitatively derived from an intuitive arc model which might be used to explain unexpected test results and to improve contact arrangements. Finally practical conclusions are drawn.