What are EIA Standards? And Where Do They Come From?

A 19-inch rack is a standardized frame or enclosure for mounting electronic equipment modules. Common uses for these racks / enclosures include datacom, telecommunication, audiovisual production equipment, scientific equipment, and networking hardware. The weird thing: almost nothing on a 19″ rack measures 19 inches. The term comes from the width of the front panel of the boxes that are installed in the rack. Other common rack hole-to-hole spacing includes 23″ (566.7 mm) and 24″ racks (592.1 mm).

EIA stands for the Electronic Industries Alliance. It was a group of trade associations for electronic manufacturers in the United States. The current iteration or standard is EIA-310-D. Note: these standards are not enforced by any governing bodies, therefore compliance isn’t enforced.

The document outlines specific guidelines for several components:

  • Rack Units (U) – hole spacing for standard racks on the mounting flange is spaced in groups of three holes. This three-hole group is defined as a Rack Unit (U). 1U is 1.75″ (44.45 mm) of vertical space
  • vertical hole spacing – repeating pattern of holes within one Rack Unit of 1.75″ (44.45 mm). Whole spacing alternates at 1/2″ – 5/8″ – 5/8″ and then repeats. “U” space starts and stops in the middle of the 1/2″ holes
  • horizontal hole spacing – specified at 18 5/16″ (465.1 mm), but this dimension is not universal so most manufacturers will use equipment mounting slots to allow for variations
  • rack opening – specified at a minimum of 17 3/4″ (450 mm)
  • front panel width – 19″ (482.6 mm), but the physical width of the rack itself can vary significantly

A&J’s standard equipment specifications for our three standard racks can be found on our website.

What the EIA-310 Standard Doesn’t Include

Despite having a document defining standardized 19″ racks, there are still several details left out.

  • Are the rack-mounting holes threaded, square or round?
  • How many posts does the rack have? (2, 4 or even 6) Are there any obstructions between these posts?
  • How deep is the racks mounting depth?
  • What is the thread type?
  • How much space is between the front or rear door and the front or rear post?

A Brief History

The term relay rack first appears in the world of telephony, but was also being used in railroad signaling by 1911. However, there is little evidence that these early racks had any standardization.

A 19″ rack format with 1.75″ or 1U was established as a standard by AT&T around 1922 in order to reduce the space required for repeater and termination equipment in a telephone company’s central office. Their engineering department created a family of modular panels “designed to mount on vertical supports spaced 19.5 inches between centers. The height of the different panels will vary, but in all cases to be a whole multiple of 1.75 inches.”

By 1934, it was an established standard with holes tapped for 12-24 screws with alternating spacings of 1.25″ and 0.5″. The standard was again revised in 1992 to comply with the 1988 public law 100-418, setting the standard U as 1.75″ (44.45 mm). The 19-inch format has more or less remained constant since despite the fact that the technology mounted within has changed considerably.

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