Actually, to be more precise, the scene to which you refer took place before Sammuel L. Jackson cut the power to the compound in order to remedy to computer lock up caused by Nedry. Nedry only shut down the power in order to escape. Apparently, some kind of back up system kicked into place which powered the lights and the computers and just about everything else, except the fences.
In response to the ceiling fans being on while Ellie & Mr Hammond are eating ice cream, there are still a few scenes before the power is even switched off by Mr Arnold. Didn't Nedri's 'White rabbit object' command just turn off the fences, door locks & cameras?
Actually, they are trying to undo the white rabbit program but can't. The only way to stop it is to shut of the power and switch it on again, thereby rebooting the system. It does work, but everything goes to 'stand-by' mode and is waiting for them to be manually turned on from the utility shed on the other side of the park.
I agree, the entire park was not shutdown until Mr Arnold "Shut down the entire system". Before that Dennis Nedry only shut down security systems, Fences and door locks. Not electrictity all together. But understandably this could be mis-interpreted.
The ice cream was out with the rest of the desserts at the buffet ( that the kids goto and eat from when they come back to the visitors center). The ice cream was melting because it was set out for the guests to eat when their tour was done.
ryochan, the ice cream was melting because it was put out with all the rest of the food to be eaten after the tour, when the children arrive back to the visitor center they start eating from the large buffet, the ice cream would have been melting but not because the power was off, the lights and fans etc were only turned off after Samuel L Jackson turned off the power to reboot the system. This scene occurs before that.