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 MIL-HDBK-419A
Using a clamp-on ammeter measure the stray current levels (see Figure 2-10) in the safety ground
d.
network at selected points throughout the facility. Choose a sufficient number of points to give an indication
of the relative stray current level in the facility. In facilities containing electronic equipments, record and
take action to correct all levels greater than 0.1 ampere. Enter results of measurements in Section C, Part IV,
and see Section 2.2.3, which follows, for suggested steps for reducing these stray current levels.
2.2.2.2.5 Signal Grounding Practices.
a.
Prepare a general description of the methods, techniques, and practices of signal grounding in the
facility (see Sections 1.5 and 5.3.2 of Vol I). Determine if an identifiable, dedicated signal grounding network is
present. Note the equipments served by the network. Include the description of this network in Section A, Part
V of the Form. In Section B, record the location and size of wires or buses used as signal conductors. Also
determine the existence of an equipotential plane.
b.
Using a double balanced bridge or a four-terminal milliohmmeter, measure the resistance (see
Section 2.2.2.3.1) between selected points of the signal ground network. Where lower frequency ground
networks exist, measure the resistance between those points on the network where equipment interconnections
are made. On Figure 2-11, typical examples of this measurement are between equipments D and E, between E
and G, and between G and K. This resistance should not exceed 20 milliohms. Further, measure the resistance
between the ground terminals of equipments that are also interconnected with signal cables and control lines.
On systems employing equipment chassis or cabinet as signal ground (e.g., most higher frequency and rf
systems), measure the cabinet-to-cabinet (or chassis-to-chassis) cables (see Connections B-F and F-H on
Figure 2-11). Also measure the cabinet-to-structure resistance (see Connections B-C and K-J). These two
resistances should be less than five milliohms. Record the test point locations and measured resistances in
Section C, Part V, of the Survey Form.
2.2.2.2.6 Ground System Noise Survey. Stray currents on the fault protection subsystem, signal reference
subsystem, and cable shields are frequent causes of common-mode noise interference in facilities. The
assessment of these currents is a vital part of the survey of electronic facilities. Measure both stray power
currents and other non-power related currents as follows:
a.
Using a clamp-on ammeter, check the ac load currents on the conductors of three phase supply
lines. Note particularly any differences in line currents greater than 10 percent. Also check and record ac
currents on neutrals.
b.
Using the clamp-on ammeter, probe signal ground wires, signal reference subsystems, cable shields,
or other conductors likely to be carrying stray power currents. Note particularly the current levels in the
grounds of lower frequency equipments and in the shields of cables carrying video, data, or other types of
signals with operating frequencies in the power frequency range. Typical locations in the illustration of Figure
2-11 where stray current measurements should be made are on the connection B-C between the rack of higher
frequency equipment and the structure, on connection A-F between the equipment bay and the power panel, on
the cabinet ground connection between lower frequency equipments E and G, and on connection J-K.
c.
Record the results of the current measurements in Part VI of the Survey Form.
2-17
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