To ensure that none of Jurassic Park’s dinosaurs were able to escape from their enclosures and wreak havoc, InGen erected fifty miles of twelve-foot-high electric fencing on Isla Nublar. Twenty-two miles of this was constructed around the perimeter of the island, while the rest was used to repel any direct attempts by the park’s dinosaurs to break out of their paddocks. All of the electrified fences were charged with ten thousand volts; according to John Arnold, all of the animals had quickly learned not to go near them. Certain portions of the fencing, such as the barrier which surrounded the Velociraptor holding pen, were reinforced with barbed wire, while others formed part of a double barrier with thirty-foot deep concrete moats.
Despite the extreme security measures put into practice by the park’s management team, which were described by Dr. Alan Grant as being “paranoid”, several dinosaurs were able to escape from their enclosures, and even travel to the Costa Rican mainland. These break outs were caused by the security system failure caused by Dennis Nedry, although several smaller dinosaurs had escaped before the Isla Nublar Incident.
Source: Jurassic Park, by Michael Crichton, pages 14, 55, 118, 131, 153. (Fifth Reprint)