This is by way of further note to what Mr. KK!HR has correctly stated directly in response to the query raised.
Purpose of a Preliminary Enquiry
The object of the preliminary enquiry is only to find out whether a prima facie case is made against the employee. Therefore, a preliminary enquiry is just advisable or desirable but not a statutory obligation. Since it is only a fact-finding investigation conducted by the employer before framing a charge of misconduct, it is not necessary to afford an opportunity to the employee during the preliminary enquiry; it can be done without the employee's knowledge.
Preliminary Enquiry vs. Domestic Enquiry
A preliminary enquiry is considered purely informal and not subject to any specific rules. In contrast, a domestic enquiry is formal and subject to the rules of natural justice. The findings of the enquiry officer, who is distinct from the disciplinary authority, form the basis for the punishment to be awarded to the delinquent or otherwise.
Therefore, even if the delinquent employee involved in any way in the preliminary enquiry admits to the misconduct, in my considered opinion, they cannot be immediately punished without issuing a formal charge sheet. However, if the employee admits the charges unconditionally, the process of a domestic enquiry can be bypassed, and punishment can be awarded accordingly.
Analysis of Proposed Suspension
Based on the analysis of the process and position of a preliminary enquiry in disciplinary proceedings, it is not justifiable to impose the punishment of a 10-day suspension as proposed by the Plant Head. Since this would constitute punitive suspension, the entire period of suspension would be considered a break in service.
Therefore, the poster may revert back to the Plant Head with a polite note incorporating all the above legal points.