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 TM 5-685/NAVFAC MO-912
cable trays, metal enclosures, etc.) should be electri-
safety and well being of plant personnel or the pub-
cally continuous and bonded to the protective
lic at large. A ground grid system should also pro-
grounding scheme. Continuous grounding conduc-
vide a significantly low resistance path to ground
__
tors such as a metallic raceway or conduit or desig-
and have the capability to minimize rise in ground
nated ground wires should always be run from the
potential during ground faults.
ground grid system (i.e., location of generators) to
(3) The conductive sheath or armor of cables
downstream distribution switchboards to ensure
and exposed conductive material (usually sheet
adequate grounding throughout the electrical distri-
metal) enclosing electrical equipment or conductors
bution system. Permanent grounding jumper cables
(such as panelboards, raceways, busducts, switch-
must effectively provide a ground current path to
boards, utilization equipment, and fixtures) must be
and around flexible metallic conduit and removable
grounded to prevent electrical shock. All parts of the
meters. Shielded cables must be grounded per
grounding system must be continuous.
manufacturers' requirements. Shielded coaxial
(4) Personnel should verify that grounding for
cable requires special grounding depending on use
the system is adequate by performing ground resis-
and function. A voltmeter must be used for detecting
tance tests.
potential differences across the break in a bonding
(5) The ground grid of the plant should be the
strap or conductor before handling.
primary system. In some cases a metallic under-
(9) A typical grounding system for a building
ground water piping system may be used in lieu of a
containing heavy electrical equipment and related
plant ground grid, provided adequate galvanic and
apparatus is shown in figure 2-3. The illustration
stray current corrosion protection for the piping is
shows the following:
installed, used and tested periodically. This practice
(a) Grounding electrodes (driven into the
is not acceptable in hazardous areas and is not
earth) to maintain ground potential on all con-
recommended if the piping system becomes sacrifi-
nected conductors. This is used to dissipate (into the
cial.
earth) currents conducted to the electrodes.
(6) The plant ground grid should have a system
(b) Ground bus (forming a protective ground-
resistance of 10 ohms or less. Ground grid system
ing network) which is solidly connected to the
resistance may be decreased by driving multiple
grounding electrodes.
--
ground electrode rods. A few rods, deeply driven and
(c) Grounding conductors (installed as neces-
widely spaced, are more effective than a large num-
sary) to connect equipment frames, conduits, cable
ber of short, closely spaced rods. Solid hard copper
trays, enclosures, etc., to the ground bus.
rods should be used, not copperplated steel. When
(10) Radio frequency interference (RFI) is in-
low resistance soils are deep, the surface extension
terference of communications transmission and re-
rods may be used to reach the low resistance stra-
ception caused by spurious emissions. These can be
tum. Bonding of ground conductors to rods should
generated by communications equipment, switching
be by permanent exothermic weld (preferred) or
of DC power circuits or operations of AC generation,
compression sleeve, and not by bolted clamp (corro-
transmission, and power consumers. The fre-
sion results in high resistance connection). Resis-
quencies and sources of RFI can be determined by
tance at each rod in a multiple system should not
tests. Proper enclosures, shielding and grounding of
exceed 15 ohms.
AC equipment and devices should eliminate RFI.
(7) Reliable ground fault protection requires
RFI can be carried by conductive material or be
proper design and installation of the grounding sys-
broadcast. Lamp ballasts, off-spec radio equipment
tem. In addition, routine maintenance of circuit pro-
and certain controls may be the prime suspects. The
tective equipment, system grounding, and equip-
radio engineer or technician can trace and recom-
ment grounding is required (refer to ground
mend actions to eliminate or suppress the emis-
resistance testing, chap 7).
sions. Pickup of RFI can also be suppressed by in-
(8) Equipment grounding refers to the method
creasing the separation distance between power and
in which conductive enclosures, conduits, supports,
communication conductor runs.
and equipment frames are positively and perma-
nently interconnected and connected to the ground-
2-9. Load shedding.
ing system. Grounding is necessary to protect per-
Load shedding is sometimes required during emer-
sonnel from electric shock hazards, to provide
gency situations or while operating from an auxil-
adequate ground fault current-carrying capability
iary power source in order to ensure enough power
and to contribute to satisfactory performance of the
gets to the critical circuits (such as the circuits re-
electrical system. Electrical supporting structures
quired for classified communications or aircraft
within the substation (i.e., metal conduit, metal
2-8
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