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 MIL-HDBK-419A
1.5.1.1.1.4.1 Bolted-Grid (Stringer) or Rigid Grid System Raised Floors. Shown in Figures 1-58, 1-59, and 1-60
are bolted-grid floor systems. The systems are similar to the drop-in grid except the grids, when properly
installed, are securely bolted or clamped in place. The drop-in panels must be metal or wood with metal plate
on both sides with a selected floor covering. They should be no larger than 24" x 24". Although the panels may
not make a good low resistance contact with the stringers, the high distributed capacity makes the floor appear
to be an electrically continuous sheet at rf frequencies. The equipment cabinets shall be connected to the floor
stringers by bonding straps which must be kept as short as possible. This will provide a low impedance path to
earth at the lower frequencies. Materials used for stringers and pedestal heads are steel and aluminum. Raised
flooring to be used for equipotential planes should be purchased to conform to the requirements of
MIL-F-29046 (TD).
In general, the grounding aspects of raised flooring have been excellent. Problem areas that designers should be
aware of are:
a.
Installation practices.
Inadequate bonded joints between pedestal heads and stringers have
sometimes resulted from poor installation practices primarily due to:
pedestal heads heavily oxidized and dirty when bolted,
(1)
use of poor bolting hardware (speed nuts, sheet metal screws), or
(2)
bolting hardware not installed or not properly tightened.
(3)
Clipnuts shall not be used in place of standard nuts since they generally will deform and therefore produce loose
joints.
The installer may receive aluminum pedestal heads from the manufacturing plant that are heavily oxidized.
The joint surface requires minor abrasion and perhaps a light coating of a joint protective compound. The joint
compound should be particularly considered for non-carpeted floors where moisture, cleaning compounds and
wax would settle, degrading the joint (sometimes severely) in a several year period.
The use of improper bolting hardware has caused unreliable joints. All bolted bonds must meet the
requirements of Table 1-23. Sheet metal screws have on occasion been employed to support the stringer to the
pedestal. Clipnuts employed in lieu of standard nuts will deform and in turn produce loose joints, and shall not
therefore be used. This "clip" nut called the grip lock nut is shown in Figure 1-61. MIL-F-29046 has been
modified to specifically prohibit the use of such type of hardware.
It is recommended that the installation crew be briefed and the floor tested, before the floor panels are
installed.
b.
Floor system checkout. Composite bonds between cabinet chassis and the raised floor shall not
exceed a specified resistance value, usually 1 milliohm. Typically, a pedestal head to stringer resistance will
read about 40 micro-ohms and should not exceed 100 micro-ohms.
For additional information see
MIL-F-29046.
c.
Resistance measurement equipment.
The instrument recommended to obtain resistance
measurements of 100 micro-ohms for these measurements is a modified Shallcross Model 670A Milliohmmeter
or equal.
1-125
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