Subpoenas for witnesses may be signed and issued by the Secretary or by the Commission or a Commissioner, and may be served by any person of full age designated by the Commission, or by a Commissioner or the Secretary, or through the sheriff's office of any city or county of the State. The Commission may cause subpoenas and subpoenas duces tecum to be issued upon its own motion, or upon the application of any party to a hearing or investigation, but subpoenas duces tecum may not be issued upon application, unless the application is in writing and describes the books, papers, or other documents to be produced.