Nace CP 1, Course Manual

Page: 384 / 535

RP0177-2000
6 NACE International
Table 1: Maximum 60 Hz Fault Currents—Grounding Cables
(A)
Cable Size Fault Time rms Amperes Cable Size Fault Time rms Amperes
AWG
(B)
Cycles Copper Aluminum AWG Cycles Copper Aluminum
1 15 10,550 6,500 3/0 15 26,500 16,500
30 7,500 4,600 30 18,500 16,500
60 5,300 3,200 60 13,000 8,000
1/0 15 16,500 10,500 4/0 15 30,000 21,000
30 11,500 7,500 30 21,000 15,000
60 8,000 5,300 60 15,000 10,000
2/0 15 21,000 13,000 250 MCM 15 35,000 25,000
30 15,000 9,000 30 25,000 17,500
60 10,000 6,500
(A)
Based on 30°C (86
q
F) ambient and a total temperature of 175°C (347
q
F) established by Insulated Cable Engineers
Association (ICEA)
(2)
for short-circuit characteristic calculations for power cables. Values are approximately 58% of fusing
currents.
(B)
American Wire Gauge (AWG)
where C = average specific heat in BTU/([lb][
q
F])of
annealed soft-drawn copper over the temperature
range T
1
to T
2
, M = mass of copper in pounds, T
1
and T
2
= initial and final temperatures respectively in
degrees Fahrenheit. Figure 1 was developed using C
= 0.104 BTU/([lb][
q
F]), T
1
= 68°F, and T
2
= 1,300°F.
(3)
Typical resistance values are shown in Table 2.
4.9 Isolating Joints
4.9.1 Isolating joints may be installed to divide the
structure into shorter electrical sections or to isolate
a section adjacent to an AC power system from the
remainder of the structure. Isolating joints installed
in areas where a possibility of damage exists
because of induced AC potentials or fault currents
should have lightning arresters, polarization cells,
electrolytic grounding cells, or similar protective
devices installed across the joints. The threshold
voltage characteristics of lightning arresters should
be considered, and installation should include
personnel protection such as dead-front construction.
(The AC and DC isolation provided by isolating joints
is not provided during the conducting mode of some
protective devices.)
4.10 Electrolytic Grounding Cells, Polarization Cells, and
Other Devices
4.10.1 The coordinated selection and installation of
electrolytic grounding cells, polarization cells (2.5-V
DC maximum threshold), or other devices between
the affected structure and suitable grounds should be
considered where arcing and induced AC potentials
could develop. These devices may eliminate or
greatly reduce the induced potentials resulting during
normal operation or surge conditions and also reduce
the possibility of arcing and structure puncture.
4.10.2 Where electrolytic grounding cells,
polarization cells (2.5-V DC maximum threshold), or
other devices are used, they should be properly
sized, spaced, and physically secured in a manner
that safely conducts the maximum amount of
anticipated surge current. Cables connecting these
devices to the structures shall be properly sized as
described in Paragraph 4.8.1. Cables should be kept
as short and straight as possible. An adequately
sized shunting circuit should be provided to permit
electrical isolation of the grounding device during
testing and maintenance.
4.11 Lightning Arresters and Metal Oxide Varistors
(MOVs)
4.11.1 Lightning arresters and MOVs may be
used between structures and across pipeline
electrical isolating devices. However, one restriction
to the use of lightning arresters is that a potential
difference has to develop before the arrester
conducts. With certain types of arresters, this
potential may be high enough to become hazardous
to people coming in contact with the arrester. When
lightning arresters are used, they must be connected
to the structure through adequately sized cables as
described in Paragraph 4.8.1. Lightning arresters
should always be provided with a reliable, low-
resistance ground connection. They should be
located close to the structure being protected and
have a short, straight ground path. An adequately
____________________________
(2)
Insulated Cable Engineers Association (ICEA), P.O. Box 440, South Yarmouth, MA 02664.
(3)
To calculate Q using metric units:
1. Find C (average specific heat) in “(cal/g)(ºC)” or “BTU/([lb][
q
F])” from handbook tables.
2. Substitute M (mass) with “0.002205 x M
(g)
” where M
(g)
= mass of copper in grams.
3. Substitute T
1
= (
q
C
1
+ 17.78)(1.8) and T
2
= (
q
C
2
+ 17.78)(1.8).