Arcing of Electrical Contacts in Telephone Switching Systems: Part IV - Mechanism of the Initiation of the Short Arc

It has been shown 1 that an arc may be initiated between contacts at close separations at voltages well below the spark breakdown potential of the surrounding atmosphere. These arcs were found 2 to be initiated at approximately a constant field of the order of millions of volts/cm. The magnitude of the field is dependent primarily on the contact surface conditions. Essentially the same results were obtained in vacuum at small separations. 3 In this paper, arc initiation measurements were extended to a lower range of separation between 200 and 2000A for palladium ar-i gold contacts in air. Approximately constant field lines were still tained for arc initiation with fields as high as 30 X 10 volts/cm. These are the highest observed fields for arc initiation. The corresponding emission currents are appreciable and are shown to be responsible for setting off the chain of events leading to establishment of the arc. NOTATION a d jk I m r s t u f(y) z F I Effective radius of electron field emission beam from cathode from a point on cathode surface Separation between the contacts Electron current density Thermal conductivity Height of surface irregularity on cathode surface Mass of metal atom Radius of curvature of a point on the cathode surface Electrostatic stress Time Dimensionless parameter in heat conduction equations u = a/2(at)m Nordheim elliptic function Distance from the surface of the anode in the metal True field strength Electron current