Nace CP 1, Course Manual

Page: 388 / 535

RP0177-2000
10 NACE International
Table 2: Average Impedance for Various Conductor Sizes
(A)
Conductor
(B)
Average 60-Hz Impedance
(Ohms/1,000 ft)
Average 60-Hz Impedance
(Ohms/km)
#6 AWG 0.923 3.03
#2 AWG 0.366 1.20
#1/0 AWG 0.2295 0.753
#4/0 AWG 0.1097 0.360
250 MCM 0.0968 0.318
500 MCM 0.0492 0.161
1,000 MCM 0.0259 0.0850
2,000 MCM 0.0151 0.0495
4,000 MCM 0.00972 0.0319
(A)
Fusing current is 10% higher than current for 684°C (1,232°F) temperature rise.
(B)
For cable sizes in metric units, see Appendix A.
sized shunting circuit should be provided to permit
isolation of the grounding device during testing or
maintenance.
4.11.2 Certain types of sealed, explosion-proof,
enclosed, or self-healing lightning arresters may be
used in locations where a combustible atmosphere is
anticipated, but only if it can be determined that the
maximum possible power fault current does not
exceed the design rating of the arrester. Open spark
gaps shall not be used in these locations.
4.12 Stray Direct Current Areas
4.12.1 In areas where stray direct currents are
present, galvanic anodes (including those in
electrolytic grounding cells), grounding grids, or
grounds directly connected to the structure may pick
up stray direct current. This current could possibly
discharge directly to earth from the structure at other
locations, resulting in corrosion of the structure at
those points. Also, direct current pickup by the
structure could lead to direct current discharge to
earth through the galvanic anodes or grounding
devices, resulting in increased consumption of the
anode material or corrosion of grounding rods and an
increase in their effective resistance to earth. The
use of DC decoupling devices should be considered
in these cases.
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Section 5: Personnel Protection
5.1 Introduction
5.1.1 This section recommends practices that
contribute to the safety of people who, during
construction, system operation, corrosion survey, or
cathodic protection maintenance of metallic
structures, may be exposed to the hazards of AC
potentials on those structures. The possibility of
hazards to personnel during construction and system
operation because of contact with metallic structures
exposed to AC electrical and/or lightning effects must
be recognized and provisions made to alleviate such
hazards. The severity of the personnel hazard is
usually proportional to the magnitude of the potential
difference between the structure and the earth or
between separate structures. The severity also
depends on the duration of the exposure. Before
construction work is started, coordination with the
appropriate utilities in the area must be made so that
proper work procedures are established and the
construction does not damage or interfere with other
utilities’ equipment or operations.
(4)
5.1.2 Each utility should be aware of the others’
facilities and cooperate in the mitigation of the
electrical effects of one installation on the other. The
mitigation required for a specific situation must be
based on safety considerations with good
engineering judgment.
5.1.3 Increasing the separation distance between
facilities is generally effective in reducing the
electrical effects of one installation on another.
5.2 Recognition of Shock Hazards to Personnel
5.2.1 AC potentials on structures must be reduced to
and maintained at safe levels to prevent shock
hazards to personnel. The degree of shock hazard
and the threshold levels of current that can be
____________________________
(4)
In some cases, the electric utility can shut down the electrical transmission facility or block the reclosing features. The utility may designate a
coordinator while the project is in progress. These possibilities should be explored with the electric utility.