Hey Rajalakshmi.
The answer for the question shall be it completely depends on the HR-Policies and Management style of the company . Suppose the employee has made problems or caused an inconvenience in the operations of the plant, then the Operation department shall communicate the same to the HR dept. I'm pretty sure HR will not be aware of the Operations part. So an investigation has to be done through the HR dept. The guilty has to be given a chance to Speak. On conclusion form the External Agengy (HR), the employee shall be given warning letter, If found Guilty.
If the Employee has some Behavioral problems or Disciplinary action it can be directly done through the HR dept, Provided HR-Dept gives an information to the Operations dept too. So it actually Depends on the HR dept Directly or Indirectly. This is because HR handles Appraisal.
Regards,
Yashwanth R