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Installing the IRUs in a Rack
007-4857-002 25
Determining Where to Attach Rails in a Rack
The base IRU, and any additional IRUs, each require five units (5U) of vertical space within the
rack (one unit is equivalent to 1.75 inches [44.5 mm]). To determine where you should install the
unit’s shelf rails in the rack, you must count mounting holes. Each U contains three mounting
holes; therefore, in the 5U of space that the module occupies, there are 15 mounting holes. The
bottom hole of the 5U space is hole 1. The top mounting hole in the 5U space is hole 15. See
Figure 1-11.
Note: An IRU in the rack is identified by the lowest U number that it occupies. For example, in
Figure 1-11 the IRU’s bottom is in U1 (the first unit within the rack).
Figure 1-11 Mounting Hole Pattern of Rack Rails
To determine how many mounting holes you must count, use the following formula: 3x(the
lowest U number that the module will occupy) - 2. For example, when you want to install the
module in locations U6 through U11, count 16 mounting holes (3 x 6 - 2) starting from the bottom
of the rack. The 16th hole is the first mounting hole of U6.
3
2
1
4
5
6
7
8
3
2
1
4
5
6
7
8
1
6th hole
5
U
COMPACT
inside
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