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 MIL-HDBK-419A
3.2.1.1.2 or as otherwise specified. If a wire is used, verify that the size conforms to the 500 cmil per foot
criteria (or as specified). Enter the information requested on Part IC of the Inspection Form.
Verify that the signal ground is correctly identified with a yellow label or color code.
b.
c.
With all cables (signal cables, control lines, power cables, etc.) disconnected, measure the resistance
between the signal ground terminal and the equipment case with an ohmmeter (see Figures 3-33 and 3-34). The
resistance should be greater than one megohm. Also, measure the resistance between each ac input terminal
(ground wire excluded) and the case. A resistance of one megohm or greater should be measured.
Record both readings on the Inspection Form. If the measured resistance is less than one megohm,
d.
proceed as follows:
(1) First check to see that all cables, lines, cords, etc., are disconnected from the equipment or
that the far ends of any such cables are insulated from other equipments and the structure. Disconnect all
cables found still connected.
(2) If no connected cables are found or the low resistance reading still exists after disconnecting
all cables, visually inspect the mounting of the signal ground terminal to see that it is properly insulated from
the case or cabinet (disassemble, if necessary). Alternately, disconnect the signal ground connection inside the
equipment and then measure the resistance between just the terminal and the case. If the terminal is not
insulated from the case or cabinet, it must be redone.
(3)
If the preceding two steps fail to identify the reason for the lack of isolation, the equipment
schematics and mechanical layout should be analyzed and inspected to locate the compromise in the signal
ground isolation. Be particularly alert for sneak paths through components (transformers, switches, relays,
etc.), readout devices (meters, lights), physical contact between the case or cabinet and the signal ground, and
wiring errors.
Measure the resistance between the green safety wire and the case; the resistance reading should be
e.
0.1 ohm or less. If a higher resistance reading is obtained, inspect the equipment to see if the green wire in the
power cord has been connected to the case or cabinet. If the connection is there physically, was the paint
removed from the area of attachment? Are screws or nuts fastened securely? If any of these deficiencies
exist, they must be corrected before installing or energizing the equipment.
Inspect all cabling and connectors to see that balanced signal lines are used for lower frequency
f.
interfacing lines and that cable shields are grounded only at one end. The shields of individual cable pairs must
be isolated from each other except at the common ground points. Check overall shields for grounding in
accordance with Section 3.2.1.1.6. Record any specifically noted deficiencies under Part IE of the Inspection
Form.
If the equipment is already installed, verify that the signal ground terminal is connected to the
g.
nearest feeder ground plate of the lower frequency signal ground network for the facility. Check the size of
the cable to see that it conforms to the 500 cmil per foot criteria of Section 3.2.1.1.3 or as otherwise specified.
3-44
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