In the Jurassic park novel whats the deal with the second blackout?

In the Jurassic Park novels the raptors first appear as enemies for the protagonists when the auxiliary generator runs out of fuel. But Arnold claims that the fences were not active since they restarted the system some time before, since the auxiliary generator does not have enough power for it.

And unlike in the movie, Nedry did switch off all fences and other security measurements including the raptors’ fence the evening before.

And in fact they were out all along anyways.

So, why are the raptors only appearing once the auxiliary generator is out of fuel?