Rosberg's day ended prematurely when he stopped on track on Tuesday morning.
"At the start of winter testing, you often encounter small issues because the car you're running is a brand new design," he said.
"We had an electrical problem this morning and, once we had found the root cause, chose to modify the parts to make sure it didn't happen again. Of course, at this time of year all you want to do is spend time in the car and get a feeling for how it's working, but these hiccups are just part of the game. During the laps I completed this morning, I had a good feeling with the car and it felt solid."
Mercedes confirmed that a fault in the wiring loom meant unburnt fuel in the exhaust ignited.
Lewis Hamilton drives the car tomorrow.