The effects of interrupting elevated currents on the erosion and structure of silver-graphite

The surfaces of silver-5% graphite contacts are greatly disrupted when they are subjected to the interruption of arcs of over 1000 amperes. This paper describes the disruption structures observed and proposes mechanisms for their formation. Large masses of silver and layers rich in graphite develop on the contact surfaces. It is suggested that poor wetting of the graphite by molten silver is a major cause of this phase separation. Even if welds form between silver masses on the surfaces of opposing contacts the total weld strength is low due to the lack of metallic continuity between the silver masses and the silver in the bulk contact. It is shown that silver masses of up to .040 grams can be ejected from the arc region to distances of up to 5 mm. The loss of silver in large masses makes the weight loss in consecutive operations erratic. The probability of arcs restriking increases after the initial interruption operations. The greater probability of restriking is thought to be a result of greater carbon concentration on the contact surfaces and/or the mating through the large silver masses on the surface which can break up into the inter-contact gap during interruption. Test results for pure silver and pure graphite are presented for comparison with the results for the composite materials.