Electromechanical devices are more prone to circuit failure than semiconductor devices due to their susceptibility to temperature variations, humidity, condensation, frequent usage, and so on. Electromagnetic contactors carry high current and create arc's across contacts as they open. The arc may cause contact surfaces to weld together and the contact will not open when required to do so. Contacts are also subject to mechanical wear and electrical erosion. The reason for this is that most contacts close with a rolling movement combined with a wiping action. This method of closure ensures a good connection between contact surfaces and reduces arcing, but the tips of the contacts eventually erode as a result of this closing motion. The mechanical wear of contacts frequently operated is often more serious than the erosion caused by arcing.
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