The following technical papers describe our instruments and how to use them:

Introduction

Alternating Current (AC) flowing in a pipe can be a portent of trouble, such as corrosion or harm to persons. Interference and fault location, corrosion, and personnel safety is discussed.

AC is best measured with one of our clamp-on AC ammeters. Also accuracy in terms of pipe current and stray pickup, especially in a common corridor shared with an overhead electrical transmission lines discussed.

On the other hand, an indication of AC flowing in a pipe can be had by using the millivolt drop method wherein the pipe itself is used as a shunt -- the potential drop being an indication of the current flowing in the pipe. An advantage is simplicity. A disadvantage is that steel pipe is both resistive and inductive. The inductive component can cause important error. Moreover, it is difficult if not impossible to accurately calibrate.

 Clamp-on ammeters are prefered. They are more accurate, both at 50 to 60 Hz, and at least up to 5000 Hz. By "they" what is meant is those we at the William H. Swain Co. make using concepts shown in U.S. Patent 3,768,011, and in this paper.

Structure of Clamp-on AC Ammeter

Figure 1 is an outline of a clamp-on AC ammeter made at the William H. Swain Co.

The clamp is placed around the pipe and both pairs of lips are secured. This makes the pipe the single turn primary winding carrying current ii to be measured. The clamp's core and secondary winding Ns resemble a fine toroid transformer. Then the ampere turns in the primary (ii)(1) equal the ampere turns in the secondary isNs. Therefore:

          ii
1)   is = --
          Ns

All of secondary current is flows in feedback resistor RF. This connects the input to the output vo of inverting high gain amplifier Amp. Since the error voltage ve is near zero, the voltage across RF is vo. So:

          vo
2)   is = --
          RF

When combined, eq. 1) and eq. 2) become:

          Ns
3)   ii = -- vo
          RF

Figure 1:
Clamp-on AC Ammeter Structure
     

Thus the output vo is a measure of pipe current ii. Ns divided by RF is the calibration factor RT. It is defined as:

          vo
4)   RT = --
          ii

Then:

          Ns
5)   RT = --
          RF

Generally this holds within a few decibels from 5 Hz to 5,000 Hz. Uses of output vo:

• An oscilloscope connected across vo will show the waveform of ii .
• The true rms value will be known when a Fluke meter model 87 is connected to vo .
• The frequency of ii can be measured on the Fluke 87, or estimated with earphones.

Another output is a precision rectifier. It is used for driving a data logger to record the change of ii over time.

Some CP is pulsed at a fairly high frequency. This should have a characteristic sound when earphones are used. If the current is pulsed, this will show on an oscilloscope. Frequency and duty factor can be measured on the F87.

In contrast, foreign water pipe contact or Underground Residential Development (URD) inter- ference is likely strong 60 Hz in the USA, or 50 Hz in some other nations. Headphones or the Fluke 87 in Fig. 1 will measure frequency.

One custoMER™reported sharp increases and decreases in 60 Hz current ii in a large pipe carrying gasoline in a north eastern city. They finally located an intermittent short to a water pipe carrying 60 Hz ground current. Every time a heavy truck went over the pipe, contact was made, then broken. A fairly deep pit was found in the gasoline pipe at the point of contact.

And then there is zero. If the interfering direct current has no comparable AC component, it may be due to a magnesium or zinc anode in a rural area.

In summary, a fault or interference current ii will likely have a characteristic frequency or change over time which will aid in finding and clearing the problem.

Accuracy of a Clamp-on Ammeter

A typical measurement is accurate at 50 - 60 Hz to within ± 1% of reading, ± 3 least significant digits, ± stray pickup. In our lab, the stray is less than 0.1 mA on a 13 inch diameter aperture clamp, or less than 3 mA on a 48" clamp.

Frequency response varies with range sensitivity and clamp size. Figure 2 shows a 12" and Fig. 3 shows a 49".

In the top curve of Figure 2, the "gain" at 25 milliamperes full scale input is flat within ± 2% (0.2 dB) from 17 Hz to 5500 Hz. This was measured on 12" MER Clamp #557 with analog Indicator SN 2194.

 "Gain", or the transfer resistance of the clamp with the indicator. In Figure 1, input current ii multiplied by transfer resistance RT produces output voltage vo. For example, 25 mA input current ii is multiplied by RT = 3.99 ohms to get indicator output voltage vo of 100 millivolts. And this output vo will be in the 98 to 102 millivolt range if the frequency of the input current changes from 17 Hz to 5500 Hz. This is shown in Fig. 2.

Figure 2:
Transfer Resistance RT vs. Frequency

The lower curve in Figure 2 is similar, but the "gain" is reduced so that the full scale input current ii is now 250 milliamperes. The transfer resistance RT here is 0.419 ohms, so 250 mA input will produce 105 millivolts output vo. This output will be the same magnitude within +1, -3 millivolts from 17 Hz to 5500 Hz. In other words, the frequency response is flat within +1% and -3% from 17 Hz to 5500 Hz.

Figure 3 is similar to Figure 2, except that the clamp is a lot bigger. The aperture of standard clamp #46 is 49 inches in diameter. When tested with analog indicator SN 2149, the frequency response was flat within ± 1% from 17 Hz to 5500 Hz.

The top curve in Figure 3 is for an input current ii of 80 milliamperes passing through the aperture of the 4 foot diameter clamp. The transfer resistance RT in equation 4 is 1.24 ohm, so 80 mA input produces 99 millivolts output vo. This is constant within 1 millivolt from 17 Hz to 5500 Hz.

Flat frequency responses enable the user to get a more accurate picture of the current waveform by using an oscilloscope, or by measurement of average and rms voltage with a Fluke meter.

Figure 3:
Transfer Resistance RT vs. Frequency

Stray Pickup

The clamp-on AC ammeter shown in Figure 1 accurately measures the pipe current ii, but it is not perfect. Especially when ii is small, the clamp may have an output current is as a result of stray alternating magnetic fields. Generally this response is small and represents an error less than 1% of measured current, but there can be exceptions.

We placed a 12" and a 49" clamp-on sensor under a 3 phase overhead line carrying an estimated 1000 Amperes at 60 Hz. The clamp was not on a pipe, so any output had to be due to stray pickup. Pickup changes with orientation. We oriented the clamp in what is called the "normal" mode, i.e., as though it were placed around a gas transmission pipe in a common corridor and running under 3 phase transmission lines headed in the same direction as the pipe.

The stray pickup by the 12" clamp was equivalent to less than .03 Amp, and that from the 49" clamp was less than .3 Amp equivalent pipeline current when located about 50 feet under the lower of 3 vertically arrayed lines spaced about 5 feet apart. This fractional ampere stray pickup is not a serious error when measuring 10 Amperes or more flowing in a 12 inch or larger steel pipe.

Stray pickup generally increases with the magnitude of the current in the overhead lines. In a fault condition where the current is 10,000 Amp, we can expect several amperes stray pickup. Still, this is likely a small fraction of the current ii flowing in the pipe, so the measurement accuracy will still be good.

Stray pickup decreases at least as much as the inverse of the distance separating the clamp from the wire carrying the current. Thus a wire twice as far from the clamp induces less than half the pickup.

Stray pickup is also dependent on the way the wires hang over the pipe. The three vertically spaced lines cause more pickup than lines arrayed as in the corners of an equilateral triangle. Widely spaced wires cause more stray pickup, but closely spaced or twisted leads cause very little.

However, if the fault causes a large current to flow in the Earth, or in a far off conductor, then the stray pickup is likely greater.

Clamp position can make a big difference. One of our measurements was unbelievable. Come to find out we were near an underground transforMER™vault. Such things are to be avoided when placing the clamp on the line.

Permanently mounted clamps have been used with success. These are similar to the field portable ones, but with some changes to make them more suited for a long time in soil or in sea water.

Clamp quality has a lot to do with the magnitude of the stray pickup. For example, a .01 inch air gap in one lip of a sensor is likely to increase the stray pickup by 3 to 10 times.

Stray Pickup Summary

The ideal clamp-on AC ammeter responds only to current ii flowing in the walls of the pipe in the aperture of the clamp-on sensor in Figure 1. In practice, we find that current flowing outside the aperture of the clamp generally causes some stray pickup. This is called ie. It is measured as equivalent input amperes ii.

This stray ie is likely less than a fraction of an Ampere in one of our clamps on a pipe in a common corridor with a three phase transmission line having up to 1000 Amps in the lines.

Uses of a Clamp-on AC Ammeter

Interference and fault location, corrosion control, and personnel safety are considered here.

Interference and Fault Location

A corrosion control person may have found that there is direct current interference current flowing in his line, but not know where it is coming from. The wave form or audio tone of the interfering current is a clue to the likely source.

For example, many cathodic protection (CP) rectifiers are unfiltered full wave. These have a strong output at the 2D harmonic of the 50 or 60 Hz main power frequency. Current at 100 or 120 Hz can come from a newly installed anode bed serving a foreign line. You can measure the frequency of the current by connecting a Fluke 87 as shown in Fig. 1.

The panel meter M on the indicator in Fig. 1 shows the pipe line current ii in rms amperes, but M reads proportional to the average value of the ii. It is less prone to jitter and more linear than a true rms meter. And the precision rectifier which drives it is an available output for driving a chart recorder or data logger.

A 24 hour record of current ii likely shows a constant interference current if it is from a foreign CP. A change resulting from rain or dryness indicates CP with automatic current regulation. In contrast, recurring peaks and valleys can point to an electric railroad. A 25 Hz current has been observed on a New England line.

Corrosion Control

Others have written about corrosion resulting from unwanted alternating current.  The direct current equivalent of AC can be 1 to 10%.

More recently there are serious corrosion problems in copper and steel fresh water systems associated with unwanted AC due to illegal grounding architecture. One caller reported that in a large hospital there were over ten places where the white grounded neutral of a 120-240 V system was tied to the green equipment ground. Pipe replacements were required after 2 to 3 years. A pipe current greater than 75 mA rms was deemed a hazard. The DC offset associated with non-linear loads likely exacerbated the problem.

Corrosion can also occur when a defective Underground Residential Development (URD) outer braid spreads 60 Hz power current out into a metal pipe.

AC clamp-on ammeters can be used to measure the magnitude, frequency, and time variation of current in pipe and so aid in locating the fault as well as showing when the fault has been cleared.

Personnel Safety

Gas and oil transmission pipe is frequently laid in a corridor also used by the electric company for power transmission.There can be considerable voltage on, and current induced in pipe in a common corridor, especially if the pipe is well insulated. This can be a hazard to both property and personnel.

Hazard mitigation is better done if the magnitude of the induced current is accurately known. The clamp-on AC ammeter is a tool suited to measure alternating current, and to record the magnitude over a time long enough to include representative fault transients.

One company was concerned for the welfare of compressor operating persons when it appeared that alternating current in the pipeline at times surged to over 300 amperes. The output vo in Figure 1, or the precision full wave rectifier output can be used as input to an alarm system to warn persons of excessive AC. Clamps are available for permanent mount, buried in soil or immersed in sea water.

Clamp-on AC Ammeter Conclusion

This type of clamp-on AC ammeter is a practical tool for measuring alternating current in pipes from 1 inch to 82 inches diameter. All reasonable factors considered, it is accurate, likely to within ± 3% of full scale of range in use. It is wide band -- 17 to 5500 Hz. This fits it for use with a Fluke 87 meter to read true rms, frequency, and duty factor of interfering current in the subject pipe. Together with an oscilloscope, this information helps to find the interfering source.

This type of clamp-on AC ammeter is also useful for measuring true current in a pipe in a common corridor. Safety of persons and control of corrosion are facilitated.

It is also sensitive; 0.1 mA resolution is available in clips to 5" diameter. One milliamp resolution is available to 82 inches diameter.

Millivolt Drop Method

The millivolt drop method is illustrated in Fig. 4. Here the pipe itself is used as a shunt for measuring AC flowing in the pipe. This is similar to the method used for measuring direct current flowing in the pipe, except that in Fig. 4 the millivolt potential is measured using an AC millivoltmeter.

The resistance of the pipe over the span may be looked up in a table or calibrated with direct current (DC) meters. If the resistance R is 0.5 milliohm and the millivolt meter v reads 1.1 millivolt, one may at first think that the pipe current ii is 2.2 Amperes. This would be correct for DC, but there is inductance, skin effect and stray pickup to be considered.

Figure 4:
Structure of the Millivolt Drop Method for AC

Inductance

If the reactance XL of the 100 foot or so measurement span is 0.3 milliohms at 60 Hz, this adds in quadrature to the 0.5 milliohm resistance R so the impedance Z becomes .58 milliohm; a 16% increase. Even at 60 Hz, skin effect may add a bit.

This is at 60 Hz. At 120 Hz - the major ripple frequency for most full wave rectifiers, the reactance is likely doubled to .6 milliohm, so Z goes to .78 milliohm; a 56% increase. Skin effect is also greater.

All this is at a few amperes of pipe current ii. But the incremental permeability of steel changes depending on the current flowing through it. Permeability sets reactance, and also skin effect. So the effect could be a 10% increase, maybe more, if the current ii were to increase to 100 Amperes.

In the lab a 3 inch diameter pipe with 3/16" wall thickness was tested. Even though it was apparently made of cast iron the effect of high frequency on pipe impedance. Cast iron has high resistivity and low permeability relative to the steel in a 24 inch gas pipe. Figure 5 is a Log-Log graph of lab pipe impedance Z against the frequency of the current flowing in the pipe.

Figure 5:
Impedance Z of Lab Pipe vs. Frequency of Current in Pipe

Figure 6 is a Log-Log plot of normalized impedance against frequency. Even in this cast iron, the 60 Hz impedance was 19% greater than the DC resistance. The steel of a 24" gas pipe will show a much greater change with frequency.

Figure 6:
Normalized Impedance Z of Lab Pipe vs. Frequency of Current in Pipe

Stray Pickup

The 60 Hz magnetic field setup by the current flowing in a transmission line over the pipe will induce a voltage in a properly oriented loop of wire beside the pipe. In Figure 4, the AC millivolts induced in the meter wire will depend upon how it is laid with respect to the pipe. A meter wire laid right on the pipe over the whole measurement span can have less than 0.1 mV rms stray pickup if it is far from power lines, motors, etc.

However, induced voltage increases with the enclosed area of the conductor loop which is perpendicular to the field. So the stray pickup on a meter wire which is generally several feet from the pipe over a 100 foot span can be a millivolt or more.

Millivolt Drop Conclusion

The millivolt drop method for measuring alternating current likely can be used as an indication of AC exceeding 1 Ampere. However, calibration will be more of an educated guess than a solid number. It will change a lot with the frequency of the current in the pipe. And this will also change appreciably with the magnitude of the current. Stray pickup will further limit accuracy.

General Conclusion

Clamp-on AC ammeters we make using the concepts of Patent 3768011 and this paper are accurate for measuring 0.1 Ampere to more than 20 Ampere. The typical overall accuracy is ± 3% from 17 Hz to 5000 Hz. This is for clips and clamps from ¾ inch to 82 inch diameter aperture.

Clamp-on AC ammeters are practical tools for measuring and locating current which can cause corrosion or endanger persons. The resolution is 1 milliampere or better, even up to 82 inch diameter pipe. Stray pickup is generally unimportant, even in a common corridor under an AC power transmission line.

The millivolt drop method wherein the pipe itself is used as a current shunt is a lot less sensitive and accurate, even at 60 Hz.