For specialized applications, fiber-optic, or 10BASE-FL, Ethernet segments are popular. Fiber-optic cable is more expensive, but it is invaluable for situations where electronic emissions and environmental hazards are a concern. Fiber-optic cable is often used in interbuilding applications to insulate networking equipment from electrical damage caused by lightning. Because it does not conduct electricity, fiber-optic cable can also be useful in areas where large amounts of electromagnetic interference are present, such as on a factory floor. The Ethernet standard allows for fiber-optic cable segments up to 2 kilometers long, making fiber optic Ethernet perfect for connecting nodes and buildings that are otherwise not reachable with copper media.

(passage copied from http://www.lantronix.com/learning/tutorials/etntba.html)