The vendor contract, a statement of who directs the electrician's work there, and a description of the testing as work on the company's own equipment. Control by the petitioner must be visible in the paper, not asserted.
Prove control with the contract and the reporting line
The offsite rule denies L-1B where the employee would be stationed principally at an unaffiliated worksite and either controlled and supervised principally by that employer or supplied essentially as labour. Factory acceptance testing of a line the company has purchased is usually work for the company's benefit under the company's direction, but the file has to show it. Attach the purchase and testing agreement with the vendor, identify the company engineer or manager who will supervise the electrician during the visit, state the number of weeks and confirm that the vendor pays nothing for his time and gives him no instructions beyond site safety.
Contrast that with the balance of the assignment at the company's own plant. Keep the vendor period short relative to the whole assignment; a petition where most of the time is spent at third-party sites invites the conclusion that the placement is really a staffing arrangement.