Equipment designed to be placed in a rack is typically described as rack-mount, rack-mount instrument, a rack mounted system, a rack mount chassis, subrack, rack mountable, or occasionally simply shelf. The height of the electronic modules is also standardized as multiples of 1.752 inches (44.50 mm) or one rack unit or U (less commonly RU). The industry standard rack cabinet is 42U tall.
The term relay rack appeared first in the world of telephony. By 1911, the term was also being used in railroad signaling. There is little evidence that the dimensions of these early racks were standardized. The 19-inch rack format with rack-units of 1.75 inches and holes tapped for 12-24 screws with alternating spacings of 1.25 inches and 0.5 inch was an established standard by 1934. The EIA standard was revised again in 1992 to comply with the 1988 public law 100-418, setting the standard U as 44.5 mm (15.9 mm + 15.9 mm + 12.7 mm), making each "U" officially 1.752 inches.
The 19-inch rack format has remained constant while the technology that is mounted within it has changed considerably and the set of fields to which racks are applied has greatly expanded. The 19-inch (482.6 mm) standard rack arrangement is widely used throughout the telecommunication,computing, audio, video, entertainment and other industries, though the Western Electric 23-inch standard, with holes on 1-inch (25.4 mm) centers, is still used in legacy ILEC/CLEC facilities.
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A 19-inch rack used for switches at the DE-CIX in Frankfurt, Germany
Nineteen-inch racks in two-post or four-post form hold most equipment in modern data centers, ISP facilities, and professionally designed corporate server rooms. They allow for dense hardware configurations without occupying excessive floorspace or requiring shelving.
Nineteen-inch racks are also often used to house professional audio and video equipment, including amplifiers, effects units, interfaces, headphone amplifiers, and even small scale audio mixers. A third common use for rack-mounted equipment is industrial power, control, and automation hardware.
Typically, a piece of equipment being installed has a front panel height 1⁄32 inch (0.031 inches or 0.787 millimetres) less than the allotted number of Us. Thus, a 1U rackmount computer is not 1.75 inches (44.4 mm) tall but is 1.719 inches (43.7 mm) tall. 2U would be 3.469 inches (88.1 mm) instead of 3.5 inches (88.9 mm). This gap allows a bit of room above and below an installed piece of equipment so it may be removed without binding on the adjacent equipment.
In 1965, a durable fiber reinforced plastic 19-inch rackmount case was patented by ECS Composites and became widely used in military and commercial applications for electronic deployment and operation. State-of-the-art rackmount cases are now also constructed of thermo stamped composite, carbon fiber, and DuPont’s Kevlarfor demanding military and commercial uses.