Data Center Rack Standards

Introduction
 A data center can occupy one room of a building, one or more floors, or an entire building. Most of the equipment is often in the form of servers mounted in 19” rack cabinets, which are usually placed in single rows forming corridors between them. This allows people to access to the front and rear of each cabinet.

The 19” (482.6- mm) racks are widely used for computer server equipment, which allow for dense hardware configurations without occupying excessive floor space or requiring shelving. The Western Electric 23-inch standard, with holes on 1” (25.4 mm) centers, prevails in telecommunications.

Standards
 The formal standards for a 19” (482.6 mm) rack are

  • Electronic Industries Alliance: EIA-310-D
  • Consumer Electronics Association: CEA-310-E design requirements for Cabinets, Panels, Racks, and Subracks.
  • International Electrotechnical Commission
  1. IEC 60297 Mechanical structures for electronic equipment – Dimensions of mechanical structures of the 19” (482,6 mm) series
  2. IEC 60297-1 Replaced by IEC 60297-3-100
  3. IEC 60297-2 Replaced by IEC 60297-3-100
  4. IEC 60297-3-100 Part 3-100: Basic dimensions of front panels, subracks, chassis, racks, and cabinets
  5. IEC 60297-3-101 Part 3-101: Subracks and associated plug-in units
  6. IEC 60297-3-102 Part 3-102: Injector/extractor handle
  7. IEC 60297-3-102 Part 3-103: Keying and alignment pin
  8. IEC 60297-3-104 Part 3-104: Connector dependent interface dimensions of subracks and plug-in units
  9. IEC 60297-3-105 Part 3-105: Dimensions and design aspects for 1U chassis
  10. IEC 60297-4 Replaced by IEC 60297-3-102
  11. IEC 60297-5 Multiple documents, -100, 101, 102, … 107, replaced by IEC 60297-3-101
  • Deutsches Institut für Normung DIN 41494
  1. DIN 41494 Equipment practices for electronic equipment; mechanical structures of the 482,6 mm (19”) series
  2. DIN 41494-7 Dimensions of cabinets and suites of racks
  3. DIN 41494-8 Components on front panels; mounting conditions, dimensions
  4. DIN IEC 60297-3-100

These specifications standardize several important features of 19 racks, such as the Rack Unit (RU or simply U), vertical hole spacing, horizontal hole spacing, rack opening, and front panel width.

Rack Unit “U” or “RU”
The hole spacing for standard 19 racks on the mounting flange is spaced in groups of three holes. This three-hole group is defined as a Rack Unit (RU) or is sometimes just called a “U”.

Vertical Hole Spacing
Vertical hole spacing is defined as a repeating pattern of holes within one Rack Unit of 1.75″. The hole spacing alternated at: 1/2″ – 5/8″ – 5/8″ and repeats. The start and stop of the “U” space is in the middle of the 1/2″ spaced holes.

Horizontal Spacing
The horizontal spacing of the vertical rows of holes is specified by EIA-310 at 18 5/16” (18.312) (465.1 mm).

Rack Opening
The opening in the rack is specified as a minimum of 17.72″ (450mm). Post or relay racks tend to have a larger than normal opening as do many other racks with threaded holes. Square hole racks tend to be very close to the minimum opening.

Front Panel Width
The only dimension on a 19 rack that actually measures 19 is the width of the front panel of equipment.

Rack Measurements
Interestingly, no measurement on a 19 rack actually equals 19”. The term 19” comes from the width of the front panel of the boxes that are installed in the rack. To determine rack size as 19 or otherwise, the hole to hole spacing is taken into account.

Rack hole-to-hole spacing
19″ Rack = 18-5/16″ (18.312″) (465.1 mm)
23″ Rack = 22-5/16″ (22.312″) (566.7 mm)
24″ Rack = 23-5/16″ (23.312″) (592.1 mm)

Conclusion
Rack standards were a result of problems in the installation of rack enclosures due to multi-vendor compatibility. In 1984, the International Electrotechnical Commission (IEC) approved the IEC 297-3 standard as a means to standardize the mechanical dimensions of 19” 
(482.6- mm) enclosures. Other standards such as the EIA 310 (Electronic Industries Alliance) provided further standardization to rack mounting telecommunications and IT equipment. 12

12 ptsdcs.com