Page 243 - Pipeline Rules of Thumb Handbook

230 Pipeline Rules of Thumb Handbook

• Security fencing around aboveground appurtenances do The right-of-way must be vacated if thunderstorms come
not mitigate induced voltages but they do limit access to within 10 miles of the pipeline right-of-way or the power
the tructure. line(s) that are influencing the pipeline. Never work on an
influenced pipeline during a thunderstorm because of the
Whenever there is a pipeline closely parallel to a power potential for a direct lightning strike. Power system fault cur-
line for any significant distance (for example, closer than rents will flow in the earth if lightning causes flashover of a
500ft or more than 2,000ft), further investigation is war- power line insulator. A person on the right-of-way is in danger,
ranted. If 5 volts AC or more is measured at any point on even if they are not touching the pipeline.
the parallel section, this voltage should be related to line Corrosion and maintenance personnel should be very cau-
loading to estimate the pipeline-induced AC voltage at peak tious about stringing test lead wire on the right-of-way where
line-loading conditions. The induced voltage should be miti- the lead wire parallels the influencing HVAC line. Extremely
gated if the calculated values approach or exceed 15 volts AC. hazardous voltages can be induced on a significant length of
Fault conditions and step-and-touch voltages also must be test lead wire laid on top of the ground.
considered. The practice of using insulating rubber gloves should also
be discouraged unless the work is being performed by a
trained power company employee. “Rubber Goods” must be
Personnel safety specially cared for and tested to ensure their reliability.
The only safe alternative in the pipeline environment is to
On any construction or maintenance project, safety is an test the level of induced AC on the pipeline before contact-
attitude. This attitude is developed by proper training. A ing anything that may be a conductor. If voltages are safe,
valuable source for such information is Section 4 of NACE normal measurement or repair and maintenance techniques
Standard RPO177. NACE also offers an audiovisual presen- may be employed. If unacceptably high voltages are encoun-
tation that deals with the effects of electric shock in such sit- tered, one must work on a ground mat. A ground mat can be
uations. RPO177 should be included in the construction as simple as a piece of 6-foot road mesh laid on top of the
specifications whenever a pipeline is built or exposed for ground and bonded to the pipeline or appurtenance with an
maintenance on an energized HVAC right-of-way. One of automotive jumper cable. Since the individual, the ground
the inspectors should be designated in charge of electrical mat, and the pipeline appurtenance are all at equal potential
safety. This individual must be familiar with and properly it matters little as to how high the actual measured voltage is
equipped to test for safe levels of induced voltage in the on the pipeline.
pipeline. A safety meeting should be held prior to construc-
tion with all construction employees to discuss electrical
safety requirements. Reprinted with permission—Pipeline & Gas Journal

MEASURING UNWANTED ALTERNATING CURRENT IN PIPE

William H. Swain, William H. Swain Co.

Alternating current (AC) flowing on a pipeline can be voltage drop indicating current flowing in the pipe. An advan-
a portent of trouble, such as corrosion or harm to tage of this method is simplicity. A disadvantage is that steel
persons. Clamp-on ammeters represent an accurate pipe is both resistive and inductive. The inductive component
1
way to measure AC. AC voltage drop measurements on can cause critical errors. Moreover, accurate calibration is
a pipeline can be seriously erroneous. This article difficult, if not impossible.
describes the use of clamp-on AC ammeters for inter-
ference and fault location, corrosion control, and per-
sonnel safety purposes.
Alternating current (AC) is best measured with a clamp-on
AC ammeter. Accuracy is important in terms of pipe current Structure of a clamp-on AC ammeter
and stray pickup, especially in a common corridor shared with
an overhead electrical transmission line. Figure 1 is an outline of a clamp-on AC ammeter. The aper-
AC flowing in a pipe can also be detected by using the mV ture of the clamp may range from 3/4 to 60in. (2 to 152cm)
drop method wherein the pipe itself is used as a shunt—the in diameter.