The aircraft was AF-4... which has had more major system failures than any other development F-35... this is its 2nd, both involving the electrical system.
That's 2 of the 4 "major incidents" in the development phase... there was another electrical system failure on AA-1, and a software bug that grounded the whole fleet (you kinda need the fuel pump to keep working above 10,000 feet).
F-35 fleet grounded after electrical subsystem failure
The incident marks the third grounding order for the F-35 fleet since last October, and the second in five months involving the AF-4 test aircraft.
In this case, the Honeywell-built integrated power package (IPP) failed during a standard engine test following a maintenance check at 08:30 on 2 August, the JPO said.
The IPP is primarily used as both a starter for the engine and a back-up electrical system, supporting the two main generators. In March, the IPP proved its worth by activating after both generators shut-down with the AF-4 still in flight. The power generated by the IPP allowed the flight control system to keep operating until the pilot landed.
That incident also triggered a fleet-wide grounding in March until Lockheed fixed a flaw with a new generator system design introduced on AF-4. Some aircraft remained grounded for several weeks until the problem was fixed.
I wonder if the IPP was subtly damaged in the March incident.