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In some cases it is necessary to change from the normal control action andattempt to prevent a process variable from exceeding an allowable upper or lowerlimit. This can be achieved by the use of special type switches called limitswitches.
it's a switch that has a part that could be moved to change it's contacts state
open to close or the other way around so you can act upon it
i don't know if i said it in an understandable way
