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 MIL-HDBK-419A
b.
Connect the signal ground terminal of each unit of equipment with a bonding jumper to the signal
ground bus bar in the manner illustrated in Figure 3-2. To provide adequate mechanical strength and a low
impedance path, a broad, flat copper or aluminum strap should be used between the signal ground bus bar and
the signal ground terminal on each unit of equipment. A copper wire, No. 16 AWG or larger, may also be used.
This bonding jumper must be insulated or otherwise kept from contacting either the equipment case, the rack or
the cabinet.
3.2.1.1.5 Isolation.
a.
The signal ground network in lower frequency equipments must be designed and installed to provide
complete electrical isolation between this ground network and the equipment case (see Figure 3-1). For
example, the signal grounds on printed circuit boards must not be connected to the chassis. On the other hand,
if the designer determines that the metal chassis can be used as a signal reference for the lower frequency
circuits without creating interference problems, and it is desirable to do so, the chassis then must be floated
from the equipment case through the use of insulating spacers or standoffs. Care must be exercised in the
mechanical layout of the equipment to insure that screws and fasteners do not compromise this isolation.
Controls, readout and indicating devices, fuses and surge protectors, monitoring jacks, and signal
b.
connectors must be installed in ways that do not compromise this isolation. Both sides of the ac power line
must be isolated from the lower frequency signal ground and from the equipment case. Only transformer-type
power supplies should be used; the commercial ac/dc practice should never be used. The metal portions of
equipments exposed to human contact must be grounded with the green safety wire.
To verify that this isolation is maintained, measure the resistance between the equipment's ground
c.
terminal and the case, between the rack ground bus and the rack frame, and between each side of the ac power
line and the equipment case. Each of these measurements should show a high (> 1 megohm) resistance. Before
making these tests, be sure power lines are not connected and all interfacing cables are disconnected.
3.2.1.1.6 Signal Interfacing.
The preferred method of interfacing lower frequency equipments is to use balanced lines. All signal
a.
inputs and outputs should be balanced with respect to the signal ground; the signal paths between such
equipments should employ balanced, shielded, twisted pair lines in the manner illustrated in Figure 3-3. The
twisting should be as tight as feasible with 18 twists per foot (3-1), the desired goal for small wires.
Where unbalanced signal lines must be used, the signal return must be grounded at one end or the
b.
other, but not both (see Figure 3-4). (Deciding in advance to ground the signal line at only the source (the
driving end) or the load (the receiving end) generally leads to implementation problems in complex installations.
For example, some equipment will be the load for one signal circuit while being, at the same time, the source
for another load as shown in Figure 3-5. In this case, if each line is grounded at the source or at the load, the
single-point ground will be compromised. Most modern electronic installations are so complex and widely
distributed that even if unbalanced interfaces could be implemented which did not violate the lower frequency
signal ground network in the facility, it is improbable that the single-point ground system could be maintained
during future equipment additions and modifications.) As a result, the equipment must be designed to have the
capability of isolating from ground at either the source or load.
3-4
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