Arcing at Electrical Contacts on Closure. Part I. Dependence upon Surface Conditions and Circuit Parameters

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Abstract
In a low voltage circuit the occurrence of an arc between approaching electrodes is dependent upon the nature of the surfaces and upon the circuit inductance. For carbon surfaces, or noble metal surfaces which have been ``activated'' by operation in various organic vapors resulting in a carbonaceous layer, the limiting circuit inductance is somewhat above 10−3h, which is much higher than the limiting inductance for clean noble metal surfaces. This activation by organic vapors occurs for noble metals only and for certain vapors; for example, benzene derivatives. In the case of silver and benzene vapor, it has been shown that the activation is due to adsorption of benzene onto a greasy surface layer and its decomposition there by the heat of subsequent closures. A metal surface, which has been activated by organic vapor, remains active indefinitely if there is no arcing at the surfaces; but with continued operation and accompanying arcing, the activating material is burned away, and the surface returns to the inactive condition if no activating vapor is supplied. Arc voltages, which are independent of current and of ambient gas, as far as tested, have been measured for a number of metals and for carbon; the arc voltage for carbon is quite erratic in the range between 20 and 30 volts, but for each of a number of metals the arc voltage is steady. Arcing at noble metal surfaces, similar to that induced by carbonaceous material from organic vapors, can be produced also by insulating particles or insulating films. The active condition gradually disappears with continued arcing, unless there is a steady supply of insulating material to the surface. The minimum arc current has been measured to be 0.6 amp for active silver and for carbon, and 0.03 amp for inactive silver. These are the currents at which an established arc is extinguished.

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