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What are positive opening contacts?

Per IEC 60050 - International Electrotechnical Vocabulary: IEV ref 441-16-11
"The positive opening operation (of a mechanical switching device) is defined as:
An opening operation which, in accordance with specified requirements, ensures that all the main contacts are in the open position when the actuator is in the position corresponding to the open position of the device."
For limit switches, implementation of positive opening means the normally closed contacts are mechanically forced apart by direct pressure from the actuator.  Should the contacts stick together as a result of arcing or other failure mechanism, the "direct" pressure from the actuator must force them apart.  While the contacts may be either snap action or slow make-break, they must not depend on spring tension alone to open the contacts.
Eaton's LS - Titan Series of limit switches feature positive opening contacts.