Networked devices typically have three possible states: online, failover (impaired, but operational), and offline.
I designed a set of three icons to quickly convey these states to users monitoring such devices.
The offline icon intentionally differs from the other icons in both color and shape. The human ocular cortex is designed to prioritize resources for tracking sharply angled, brightly colored objects in the visual field.
These icons are available as high-resolution TIF files through the archive link below. These icons are offered royalty-free for personal and commercial use without attribution under the terms of the Creative Commons Attribution 3.0 Unported License.