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It is generally deemed acceptable in public areas excluding restrooms, etc. The law generally is that employees do not have a reasonable expectation of privacy in their work areas.
do you have to by law post that there cameras? or can you just put them up with out anyone knowing
You are not required to generally but it is good practice to notify the employees so they cannot later claim a violation of privacy. They can only be video not audio. Again, not in private areas like areas where they change clothes or restrooms.