Perimeter security systems are comprised of between one and three fences, depending on the security level of the facility. Fences are usually 12 feet high; the distance in double-fence configurations ranges from 20 to 25 feet across. Multiple fences also prevent inmates from passing contraband across the fence and prevents anyone from tampering with fence sensors or alarm systems. Fences can incorporate many features including barbed tape, fence-mounted sensors, closed-circuit television, underground seismic systems, and microwave or infrared detection devices near the fence line. The specific combination of features is chosen by correctional security specialists based on the institution's security classification, staff experience and training, and the security threat from outside the facility. 4 references