Page 244 - Pipeline Rules of Thumb Handbook

Corrosion/Coatings 231

Reference 1 gives the equations relating input current (i i ), to a water pipe carrying 60Hz ground current was finally
winding turns (N s ), and output voltage (v o ), along with further located. Every time a heavy truck went over the pipe, contact
information about clamp-on ammeters. to a water line was made, then broken. A fairly deep pit was
An undesired current will likely have an identifying char- found in the gasoline pipe at the point of contact.
acteristic (i.e., a “signature”). An oscilloscope connected
across v o in Figure 1 will show the waveform of i i . The fre-
quency of i i can be measured on a portable meter, such as a Accuracy of a clamp-on ammeter
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Fluke 87 , or estimated with earphones. Another output is a
precision rectifier. It is used for driving a data logger to record A typical measurement is accurate at 50 to 60Hz to within
the change of i i over time. ±1% of reading, ±3 least significant digits, and ± stray pickup.
Some cathodic protection (CP) is pulsed at a fairly high fre- Frequency response varies with range sensitivity and clamp
quency. This should have a characteristic sound when ear- size. Figure 2 shows results on a 12-in. (30-cm) clamp, and
phones are used. If the current is pulsed, this will show on an Figure 3 shows results on a 49-in. (125-cm) clamp.
oscilloscope. Frequency and duty factor can be measured on In the top curve of Figure 2, the sensitivity or transfer resis-
the meter. tance (R T ) at 25mA full-scale input is flat within ±2% (0.2dB)
A foreign water pipe contact or underground residential from 17 to 5,500Hz. This was measured on a 12-in. clamp
distribution (URD) interference will likely have a strong with an analog indicator.
signal—60Hz in the U.S. or 50Hz in some other nations. The lower curve in Figure 2 is similar, but the sensitivity
Either headphones or the meter will measure frequency. is reduced so that the full-scale i i is now 250mA. The
One owner reported sharp increases and decreases in 60Hz frequency response is flat within +1% and -3% from 17 to
i i in a large-diameter gasoline pipeline. An intermittent short 5,500Hz.
Figure 3 is similar to Figure 2 except that the clamp
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Trade name. was much larger. When tested with an analog indictor, the

Figure 1. Clamp-on AC ammeter structure.