Do I need an UL approval if I build a machine and use it in my own company?
Check your contract with your insurance company. They might require all equipment used to have appropriate safety ratings in order to maintain your coverage.

Remember the "Underwriters" in Underwriters Laboratories means "insurance companies". UL is an organization set up by insurance companies to set standards for what kinds of equipment are safe enough to insure (i.e. to reduce the risk that insurance companies have to pay out on policies).