Quantcast Section III- Testing

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
a. Stainless steel ground rods. Do not use stain-
corrosion should be reported, as this may indicate
the need for cathodic protection in the area.
less steel ground rods. Their performance can be
-
unpredictable because of their tendency toward lo-
10-5. Galvanic corrosion of grounds.
calized corrosion.
The use of dissimilar metals embedded in the earth
b. Underground pipe lines. The bonding of inte-
in and around generating stations and substations
rior metallic pipelines to an electrical system's
results in the formation of a huge galvanic cell.
ground provisions of copper (which is required by
Steel or galvanized structures, including conduits,
code) if done incorrectly, can result in galvanic cor-
cable sheaths, pipes, and structural footings, where
rosion of the underground pipeline. Installation of a
used either purposely or inadvertently in the
dielectrically-insulated fitting on the pipe above
ground system, are subject to galvanic corrosion.
ground, and before the copper ground connection,
Attention should be given to the necessity of provid-
will eliminate the earth's electrolytic coupling be-
ing corrosion mitigation measures under such cir-
tween the underground cable and the ground wire.
cumstances.
Section III-TESTING
10-6. Ground resistance tests.
connection, on the structure, equipment enclosure,
or neutral conductor, to the earthing connection.
In addition to visual inspections of grounding sys-
The earthing connection may be the top of a single
tems and connections, resistance measurements
ground rod, a water pipe, a counterpoise, or a
will be made periodically to determine whether
ground grid.
there is any trend toward an increase in the ground
b. Measure the resistance of the earthing connec-
resistance of an installation. Maximum permissible
tion whether it is a ground rod, a water pipe, a
resistance for grounds and grounding systems will
counterpoise, or a ground grid to the earth itself.
be in accordance with departmental standards,
c. Wherever the total resistance of the total
ANSI C2, or the National Electrical Safety Code,
ground circuit is in excess of the values established,
whichever is lower.
measure resistance of individual portions of the cir-
a. ANSI C2 requirements. No specific ground re-
--
cuit to determine the point or points where resis-
sistance is given, except that a single-grounded,
tance is excessive and corrective action can be
individually-made electrode, with a ground resis-
taken.
tance exceeding 25 ohms, requires two parallel and
d. Measure resistance between gates and
interconnected electrodes. Supply stations (depen-
gateposts to ensure that flexible ground connections
dent upon size) require an extensive grounding sys-
are adequate. Resistance higher than one-half ohm
tem, consisting of either multiple buried conductors
indicates a deficiency.
or electrodes or both, to limit touch, step, mesh, and
e. Measure resistance between operating rods
transferred potentials in accordance with industry
and handles of group-operated switches and the
practices. All grounding systems must be designed
supporting structure to determine that the flexible
to minimize hazard to personnel and have resis-
connections are adequate. Resistance higher than
tances low enough to permit prompt operation of
one-half ohm indicates a deficiency.
circuit protective devices.
f. Measure resistance of all bonds between
b. Departmental standards. Departmental stan-
metallic-cable sheathing and its ground path. Resis-
dards will require values ranging from 1 ohm up to
tance higher than one-half ohm indicates a defi-
a maximum of 25 ohms depending on the size of the
ciency.
system.
g. Testing of grounds may create hazardous con-
c. Measurement records. Continuous records will
ditions if care is not exercised. Fault or surge cur-
be kept for all grounding installations, which re-
rents can build up dangerous voltages between the
quire a ground resistance of 10 ohms or less, to
point of equipment ground connection and the point
verify that design resistances are still being pro-
of the earthing connection. Rubber gloves, blankets,
vided.
and such are recommended for the protection of
personnel. Ground resistance measurements should
1O-7. Ground value measurements.
never be attempted during lightning storms.
The following ground resistance measurements
10-8. Methods of measuring ground resis-
should be made in order to ensure safe operating
tances.
practices.
a. Measure the ground path resistance of all
All methods of measuring ground resistance are
similar in that a suitable source of current is neces-
branches of the grounding system from the point of
10-2





 


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