(a)
Limits. It is recommended that the combined resistance of the
ground wires and connections of any grounded circuit, equipment or lightning
arrester should not exceed the values given below, if grounded connections made
according to §
39.66 (relating to nature of
ground connection) will sufficiently limit the resistance. It will frequently
be impracticable with artificial grounds to obtain resistances in dry or other
high resistance soils as low as the values given below for ordinary soils. In
such cases two grounds shall be used as defined in §
39.68 (relating to methods of
ground connection), and no requirement shall be made as to resistance.
Reference should also be made to §
39.66. The current stated opposite
the different resistances in the table is either the current capacity of a
circuit from which leakage may occur to the ground circuit, or the continuous
current capacity to which the grounded equipment or arrester is limited by
design or by automatic cutouts. If the secondary is exposed only through
transformer windings, this current capacity shall be that of the primary fuse
of the transformer. If the secondary is exposed by the conductors of
conflicting or crossing high voltage circuits, the current capacities shall be
those of the automatic cutouts in such circuits. The product of the
corresponding numbers in the first and second columns shall never be greater
than 150, that is, the potential difference due to the stated current is never
greater than 150 volts, if connections are made to water pipes. If more than
one ground is made on the same circuit, equipment or arrester, in the same
vicinity, all such grounds shall be considered collectively in respect to
meeting the requirements of this section:
|
Amperes
|
Water pipe grounds (in
ohms)
|
Artificial grounds ordinary soils (in
ohms)
|
|
less than 10
|
15
|
1025
|
|
10 to 25
|
6
|
25
|
|
25 and above
|
less than 6
|
25
|
(b)
Checking. The resistance
of station grounds shall be checked when made. With artificial grounds this
check may be made by measuring the voltage between the grounded point of the
circuit, or the grounded frame of the equipment or the grounded point of the
lightning arrester and an auxiliary metal reference rod or pipe driven into the
ground, while a measured current is flowing through the ground connection and
any exposed metal piping or other artificial ground in the vicinity, but not
within 20 feet. If the station ground is to water piping, the check may be made
with current flowing through the water piping and some independent piping
system or artificial ground in the vicinity, but not within 20 feet. The
auxiliary road or pipe shall be at least ten feet from any artificial ground or
piping systems through which the measured current is made to flow. All ground
connections shall be inspected periodically. Ground connections on distribution
circuits shall, when installed, be tested for resistance unless multiple
grounding to water piping systems is used.
