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Platinum Metals Rev., 1957, 1, (2), 57

Arc Erosion of Electrical Contacts

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Erosion under the action of the arc is one of the determining factors in the operating life of electrical contacts which are required to interrupt currents of the order of several amperes. The loss or transfer of material from the contact surfaces which takes place during arcing may occur by a number of processes, but that most frequently encountered involves a loss of material from the negative contact under the action of the so-called “normal arc”.

Relatively little experimental data are available on the magnitude of arc transfer, but a study now published by Dr. W. B. Ittner and H. B. Ulsh of the International Business Machines Corporation, New York, (Proc. Inst. Elec. Eng., Part B, 1957, 104, Jan., 63-68), provides a valuable contribution to this subject, more particularly as the materials investigated include a number normally used in industrial practice.

The paper describes measurements of the “normal arc” transfer in resistive and inductive 50 volt circuits interrupting currents of 1.5 to 5 amp. Within the experimental errors the material transfer from the cathode was found to be directly proportional to the total charge passed in the arc, confirming a relationship first proposed by R. Holm. A summary of the results is shown in the table, which gives the average measured values of the cathode loss in microgrammes per coulomb, and—by dividing by the density of the material—in cubic centimetres per coulomb.


MaterialCathode loss
g × 10−6/Ccm3 × 10−6/C
Palladium 19.4 1.59
Platinum 8.7 0.41
Silver 3.6 0.34
Tungsten 1.1 0.06
50% Copper-palladium 20.5 1.94
10% Iridium-platinum 17.6 0.82
15% Iridium-platinum 16.5 0.77
20% Iridium-platinum 14.5 0.67
35% Iridium-platinum 11.6 0.54
5% Ruthenium-palladium 14.1 1.15
11% Ruthenium-platinum 13.2 0.65

Agreement between the experimental data and those computed from a modified Llewellyn Jones formula was found to be good, from which it follows that the desirable properties for minimum arc erosion are a high boiling temperature, a high thermal conductivity and a high density.

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