The motion of short circuit arcs in low-voltage current limiting miniature circuit breakers

Jeffery, Peter Andrew (1999) The motion of short circuit arcs in low-voltage current limiting miniature circuit breakers. University of Southampton, School of Engineering Sciences, Doctoral Thesis.

Abstract

Investigations have been made into the effect of the Miniature Circuit Breaker configuration on the behaviour of short circuit arcs. A Flexible Test Apparatus was developed to recreate the operation of a MCB, igniting the arc between opening contacts. The short circuit fault was simulated using a capacitor discharge system, with a prospective peak current of 3.5 kA. A high-speed digital Arc Imaging System was used to record the arc behaviour. Analysis techniques have been developed that plot the individual trajectories of the anode and cathode root from the digital data. The time that each arc root remains in the contact region can be deduced and is defined as the arc root contact time. It is shown that the traditional arc voltage to measure arc contact times is not accurate.
In the Arc Contact Time Investigation the contact material, contact geometry, arc runner configuration were varied. At 3m/s contact velocity silver graphite contacts and step geometry led to longer cathode root contact times. Cathode root motion was prevented by a corner on the arc runner. The polarity of connection affected the arc movement away from the contact region. Increasing the contact velocity reduced cathode root contact times, but did not guarantee arc movement away from the contact region.
The Taguchi Method was applied to the Arc Behaviour Investigation. The contact material, contact geometry, arc chamber geometry, arc chamber material and arc chamber venting were varied. Tests were carried out at lOm/s contact velocity. The cathode root contact time, anode root contact times and let through energy were calculated. The arc voltage and current, arc root trajectory plots and selected arc images are presented. The cathode and anode root contact times were independent. Generally, the cathode root moved away from the contact region before the anode root. Lower let-though energies were recorded for open arc chamber vents. The arc failed to move from the contact region, when silver graphite contacts were used in combination with acrylic arc chamber, copper arc runners and choked arc chamber vents.
The mobility of the arc is dominated by the cathode root, and can only run at high velocity if the electrode surface conditions promote oxide layer type emission. The arc is drawn between the contacts with metal vapour type emission dominating and is initially limited to low velocity. During the arc contact time the arc root interaction effects damage the oxide layer on the surrounding conductor surface. As the arc column length increases, deflection of the arc column by the self-blast magnetic field may cause a discontinuous jump, after which the arc may run at high velocity. Increasing the contact velocity and steel arc runner backing strips encourage a discontinuous jump away from the contact region. Deposition on the arc runner of silver and carbon discourages both discontinuous jumps and high velocity motion. Changes in geometry, restricting the arc chamber venting and the anode root commutation can reduce the cathode root velocity. When this occurs, the arc root interaction effects damage the oxide layer, and cause the arc to revert to metal vapour type emission. Additionally, the surface of the acrylic arc chamber is carbonised during prolonged arc contact times. The partial conducting path through the carbonised material further encourages the arc to remain in the contact region.

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