What is the difference between isolation and disconnection for any switch disconnector ?

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Isolation : it is switching the ciruit to open it with a suitable space between moving and fixed contact to prevent ionization of air in this space ( complete safety )

Disconnection : it is switching the ciruit to open it but with a space between fixed and moving contact not suitable to prevent ionization of air in this space , in this case arcs can be done and it is not completely safe