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Corrosion/Coatings 235

General conclusion 2. R. A. Gummow, et al., “AC Corrosion—A Challenge to
Pipeline Integrity,” MP 38, 2 (1999).
Clamp-on AC ammeters, as described in this article, are 3. Private communication from B. Sheehan at San Diego
accurate for measuring from 0.1A to more than 20A. The County Water Authority, August 28, 2000.
typical overall accuracy is ±3% from 17 to 5,000Hz. This is
for clips and clamps from 3/4- to 60-in. diameter aperture. WILLIAM H. SWAIN is President of William H. Swain
Clamp-on AC ammeters are practical tools for measuring Co., 239 Field End St., Sarasota, FL 34240. He founded the
and locating current that can cause corrosion or endanger company in 1964 and has invented and marketed DC and AC
humans. The resolution is 1mA or better, even up to 60-in.- amp clamps, meters, and other products for corrosion control
diameter pipe. Stray pickup is generally unimportant, even in on lines up to 5-ft (1.5-m) diameter. He has an M.S. degree
a common corridor under an AC power transmission line. in electrical engineering and is a professional engineer in the
The mV drop method, wherein the pipe itself is used as a state of Florida. He has been a NACE member for more than
current shunt, is much less sensitive and accurate, even at 16 years. MP
60Hz.

References
Reproduced with permission from NACE International, Houston, TX. All
1. W. H. Swain, “Measuring Unwanted Alternating Current in rights reserved. Published in the January 2001 issue of Materials Per-
Pipe,” W. H. Swain Co. Web site: www.SwainMeter.com. formance. © NACE International 2001

Minimizing shock hazards on pipelines near HVAC lines

As right-of-way for pipelines becomes more difficult to suitable length of flexible welding cable firmly to the metal-
obtain, especially in congested areas, rights-of-ways are being lic mat. The other end of the cable should be equipped with
shared by pipelines and high voltage transmission lines. a suitable copper or brass clamp that can easily be connected
Sharing a common right-of-way can pose problems during to the tower ground. Personnel should always place the mat
operation and especially during pipeline construction unless on the ground and stand on it while connecting the cable
special precautions are taken. clamp to the tower ground. When work is complete, person-
Considerable voltage can be induced in pipelines above nel should remain standing on the mat while disconnecting
ground on skids unless the section of pipeline is grounded. the cable clamp from the tower ground wire. This will make
Without grounding, a lethal shock hazard to construction per- it safe to carry the mat to the next tower ground connection
sonnel can occur. To minimize the voltage on the pipe, drive site.
ground rods at each end of the section of above ground pipe. When tie-ins are made, additional personnel protection is
The ground rods should be driven at least 4 feet in the ground required. Use a metallic mat or driven ground rod connected
and connected to the above ground section of pipe with to the pipe at each end of the tie-in, and connect an insulated
insulated copper wire and a “C” clamp. If the above ground copper cable (bond) across the tie-in point. The pipe and
1
section is longer than /2 mile, additional ground rods should ground mat will be essentially at the same potential if a high-
be driven and connected to the pipe. voltage surge occurs during the tie-in operation. The bond
Work should be discontinued if thunderstorms are occur- will prevent buildup of voltage between the unconnected
ring in the vicinity (approximately 10 miles) of the pipeline ends at the tie-in location.
construction. If lightning should strike the power line, even After the pipeline has been buried and placed in service,
at a distant location, while personnel are in contact with the personnel should always connect a bonding cable across any
pipe, they could suffer a severe shock or even electrocution. location where the pipe is to be separated, i.e., a valve to be
To eliminate this danger, personnel should always stand removed or a section of pipe is to be removed and replaced.
on a 3-ft. ¥ 3-ft. (minimum) steel mat made of 1≤ square The bonding cable should remain in place until all connec-
(maximum) hardware cloth that is well grounded electrically tions have been completed.
to the nearest electrical tower ground via a copper cable. Equipment with steel tracks will not require any special
The mat should be fabricated so that personnel can easily grounding procedures for travelling along the power line
move it from tower to tower. This can be done by bolting a right-of-way, but machinery with rubber tires should have a