No. Productive work at the vendor's site before approval requires its own authorization, and it also subtracts from the year abroad if that year is not yet complete. File first, then travel.
Approval before the first productive day
It is tempting to send the electrician to the acceptance test under a business-visitor entry because the visit is short. Hands-on testing and modification of equipment is productive work, and a border officer who hears that description may refuse entry, which then appears in every later application. The safer sequence is to complete the specialized-knowledge evidence, file the petition, and schedule the vendor test after approval and admission in L-1B.
Where the vendor's calendar cannot move, the company can send a different employee who already holds status, or ask the vendor to conduct the test with the electrician joining remotely. If the year abroad is still being accumulated, every U.S. working day delays the eligible filing date.
Initial L-1B approval for an operating U.S. plant may be up to three years, so an eighteen-month assignment with the testing weeks inside it is easily covered once the paperwork is in order.