Amp Clips for Corrosion Control
Contents:
Introduction
Swain Meters® are non-contact ammeters for
corrosion protection, fault location, and quality
assurance.
DC Amp Clips™ and AC Amp Clips™ are ¼ inch to 5
foot diameter aperture, field-portable, clamp-on
ammeters with high sensitivity and accuracy.
They measure current from 10 mA to 200 amp.,
while the system is on line and working normally.
Clips are widely used. Examples include auto
plants, wooded areas, the desert, shipboard, and
hundreds of feet under the sea.
DC Amp Clips are used in cathodic protection of
gas, oil, & water lines and offshore oil platforms.
Automotive and power people measure the charge,
load, and parasitic drain current of batteries
without disconnecting the cables.
DC Amp Clips measure direct current, both
magnitude and direction of flow, with the system
intact. They are used to find Differential control
leakage with the control still operational.
Telephone cable and pipe line ground faults are
located by measuring the line current and its
direction of flow, with the line in service.
AC Amp Clips are used to find leakage in grounds
& power cable, and stray current in dairy farms.
They measure alternating current in pipe lines which
can cause corrosion & magnetic fields. They also
measure induced or foreign short current which can
be a hazard to persons and equipment.
Digital models are error resistant and tend to be
smaller and lighter. The analog models show the
trend of current changes better, and they are more
resistant to severe environments.
The recorder or rectifier output provides DC for
a data logger or oscilloscope. It is also a backup
if the panel meter is damaged.
Fig. 1: Digital DC
Amp Clip indicator with 1 mA resolution and 200 amp.
maximum reading, with ¾" Sea Clip® and 13" Sea
Clamp™ sensors.
Contents
What can be done, and how to do it are given in
the first section of this paper.
How are they built, what accuracy can be
expected, & which model is best for a particular use
is included in the second section.
This paper is an update of DC Amp Clips for
Corrosion Control given at Morgantown, W.V. in
May of 1986 at the Appalachian Underground Corrosion
Short Course.
DC Amp Clip
Introduction
Batteries were returned to the manufacturer
because "they would not hold a charge". Auto-marine
¾" clips were used to show that the fault was
parasitic drain in the .1 to 1 amp. range.
It was time consuming to find defective anodes
and open feed cable along the cathodically protected
lines of a major gas transmission company. Time was
saved using the CP-¾" model, especially in the 20 mA
to 1 amp range.
In some spots transcontinental telephone cable
was losing cathodic protection current to the earth,
causing local corrosion. DC Amp Clips with 4" clips
were used to find leaks in the 20 to 100 mA range.
Interference to telephone cable, gas pipe and
even the reinforcing steel in concrete pillars was
traced to electric rail lines. Clips and clamps to
30" dia. were used to measure interference current
from under 1 amp. to over 10 amp.
It was difficult or not feasible to locate
foreign shorts to gas transmission line in an urban
area. There was too much cross over of the high
frequency current used in conventional pipe tracers.
However, the gas company did find the shorts using a
DC current injection process they originated
together with an 18" DC Amp Clamp. This is shown in
Fig. 8.
Fig. 2: Automarine
Indicator with 5 mA resolution and 400 amp. maximum
reading. The cable from the car to the battery
carries about 60 mA. It is charging current because
the ¾" Sea Clip's cabled arm is up, and the
Indicator reads to the right.
Fig. 3: Analog DC
Amp Clip Indicator with 1 mA resolution and 100 amp.
maximum reading, with 10" Sea Clamp Sensor, in a
carrying case (Box).
Clamp & Clip Defined
The Clip shown in Fig. 2 can be used with one
hand. The Clamp shown in Fig 3 comprises two half
circles ("C"s) bolted together at the flat sectors
called lips. Clamps are further defined on p. 5.
Earth Field Effect (He)
These DC Amp Clips measure current by measuring
the magnetic field intensity set up by the current
flowing in the aperture of the clip or clamp
(sensor). Therefore a magnet acting on the sensor
will shift its zero (more so in large sensors than
in small ones) if the sensor is not perfect.
It isn't. One departure from the ideal is that
the sensor splits into two halves when the jaws are
opened so that it can be closed around a conductor
without the need to unhook anything.
We specify the change in zero offset in terms of
the magnetic field of the Earth. This is the Earth
Field Effect -- He for short.
To measure He the sensor is rotated full circle
in a North-South vertical plane and the peak-to-peak
change in zero offset noted. For a 4" Clip this
should be less than 60 mA equivalent input current.
We call this 0 ± 30 mA peak.
In the "how to" sections which follow it is
assumed that the reader is familiar with the
operating instructions and has had some practice
using his DC Amp Clip.
Cars, Trucks, and Marine
The Auto-Marine, digital, and 200 amp analog DC
Amp Clips are used on vehicles and offshore
platforms to measure at least 4 classes of direct
current while the vehicle is working normally:
Parasitic drain, usually 0.05 to 3 amp.; battery
charging, usually ½ to 40 amp.; heavy load, and
cranking, 20 to 400 amp.; and marine cathodic
protection, 0.1 to 20 amp.
To measure these there is no need to unhook any
cables or wires.
Parasitic Drain
At some auto dealer's shops perfectly good
batteries were pulled out of cars on warranty
because they "would not hold a charge." It turned
out that the fault was in the cars: a small current
was being drained from the battery when the ignition
was off. This was a glove or trunk compartment lamp
which stayed on (1 amp.), or excessive drain in the
electronic package (1/3 amp.), etc.
Parasitic drain trouble shooting requires
measuring the total battery current. To do this,
turn the motor off, turn the DC Amp Clip on, set the
zero knob so that the meter reads near zero on the
most sensitive range, and then put the clip around
all the cables connected to either the (+), or
alternatively the (-) battery terminal.
Polarity
Positive current direction is marked on the clip.
The meter reads (+) when current flows from left
to right through the aperture of the clip when the
cabled arm is up., as shown in Fig. 4A. The meter
reads (-) when current flows from left to right with
the cabled arm down, as shown in Fig. 4B.
Fig. 4A: The meter
reads positive when current flows from left to right
through the aperture of the clip when the clip's
cabled arm is up.
Parasitic Drain, continued
Observe polarity, so that you know whether the
meter is reading charge or discharge current. Charge
current enters the (+) terminal, or leaves the (-)
terminal. This helps to avoid error when reading
current less than 100 mA. The battery is not being
charged when the motor is off and no charger is
connected.
The first meter reading may be off scale. There
is no harm in overloading the meter for a few
seconds. Turn to a higher range until the meter
reads well up on the scale. The reading may be +0.9
amp. leaving the (+) terminal. It looks like the
battery has a drain of about 1 amp., but this should
be checked with the Floating Zero Procedure.
Fig. 4B: The meter
reads negative when current flows from left to right
through the aperture of the clip when the clip's
cabled arm is down.
Floating Zero, Short Form
To check the +0.9 amp. reading turn the clip over
and put it back on the battery cables in exactly the
same place and way as before (with care, to have the
same clip placement, but turned over. This is
important). Ideally the meter will read -0.9 amp.
However, if the meter was off zero a bit to start,
or there was a magnetized bolt or sector of sheet
metal near by, the zero will have been shifted
somewhat, probably for both readings. The meter may
read -1.1 amp.
The most probable true current is the average of
the two readings, +1.0 amp., discharge.
This is the short form Floating Zero (FZ)
Procedure. It was done by putting the clip on the
battery line, once with the clip's cabled arm up,
and another time with the clip's cabled arm down.
This reversed the current in the aperture of the
clip so the meter read positive one time and
negative another time. When the average of the two
readings was used (reversing the sign of the
negative reading) there was no need to carefully set
the zero, because zero offset was likely largely
canceled out in the process.
Battery Charging
One way to measure battery charge is shown in
Fig. 2, which represents a clip on the battery
cable. The indicator is reading about 1/3 amp.
charge on the 1 amp. range. If the motor stops with
the ignition on the meter will swing to the left of
zero, indicating a negative or load current. With
this setup it is convenient to cycle the vehicle
through a variety of loads, engine speeds, etc. to
see how the alternator and regulator respond to
service conditions.
Heavy Load
Full service load, or cranking current up to 400
amp. can be measured as shown in Fig. 2. The current
should not exceed 500 amp. because this might damage
the DC Amp Clip. Models that are rated for less than
200 amp. should not be used for cranking current
measurement.
Marine Cathodic Protection
Shipboard rectifier & anode current in an
impressed current cathodic protection system can be
measured with a ¾" Sea Clip and one of the
indicators shown in Figs. 1, 2, or 5. Measuring
sacrificial anode current can be harder, but there
are ways.
Undersea Current Measurement
Sea Clips are used underwater to hundreds of feet
depth when they are equipped with water-proof
connectors, cables, and a suitable surface
Indicator.
Offshore oil & gas platforms frequently have
sacrificial anodes held to the structure by 2" to 4"
standoffs. The anode current is measured by
measuring the current in the standoffs as shown in
Fig. 5B, preferably using at least the two step
Floating Zero Procedure. This has been done with
divers, and more recently, with ROVs. It is best to
call us about your particular need.
Fig. 5A: CP-¾" Sea
Clip with 5 mA resolution and 40 amp. maximum
reading. This ¾" dia. aperture Sea Clip has Earth
Field Effect rating He = 0 ± 5 mA peak.
Some offshore platforms use a large sacrificial
anode at some distance from the platform, with
connection through a heavy cable on the sea bottom.
The current is measured by placing the clip around
the cable.
One reason for measuring the current is to make
sure that the anodes on standoffs (and the sea bed
anode with cable) are connected electrically as well
as physically to the platform. Apparently the
connection can look good to a camera on an ROV, but
not be good electrically. Another is to find what
current is required to protect the platform, and
thus to determine what anode weight is required for
this, and future platforms.
Water Current
Apparently it is not always feasible to locate an
anode cable on a ship, or there may be none - as in
an anode on a prop shaft. In this case we suggest
the less accurate but useful method of holding a sea
clamp around the anode's surface so that the anode
current flowing in the sea water passes through the
aperture of the clamp. This works. DC Amp Clamps
measure the current flowing through the aperture, be
it in water, plasma, steel, etc.
Shaft Current
Pump and propeller shafts may carry a direct
current (or alternating current -- we make meters
for AC also). This can be measured by placing the
clamp (loose fitting) around the shaft, whether
rotating or stationary. There may be noise, so we
suggest you phone to discuss the conditions. Of
course, if the shaft is rotating very special care
is required to avoid harming a person.
Fig. 5B: Analog
indicator with 1 mA resolution and 100 amp. maximum
reading. The 5" Sea Clip is suited for use measuring
current in pipes, either on land or under the sea.
Corrosion Control
Anodes, Bonds, and Rectifiers
Parts of an impressed current cathodic protection
system for a pipeline are outlined in Fig. 6. The
rectifier is connected between the pipe and the
anodes so that current flows in the earth between
them. The current in each part is measured by
placing the clip around the cable, observing
polarity. The system stays on line and operates
normally during the test. There is no need to
disconnect any lines. If the current is under 100 mA
at least the short form Floating Zero Procedure
should be used.
The DC Amp Clip model CP-¾" Sea Clip shown in
Fig. 5A can be used to measure the current output of
the rectifier IR, and compare this with the drains
to anodes distributed along a string. Anodes 1, 2,
3, & 4. each represent one of a group.
The rectifier output in this illustration is IR =
3 amperes.
Anode 1 is typical of a good unit in this
illustration. Its drain is I1 = 1 amp. String
current Ia is measured and found to be 2 amp., which
seems reasonable.
Conduit
Rectifier or anode current can be measured
through conduit - plastic, copper, or steel. This
was shown in the lab., and later verified at a power
plant on an anode line in steel conduit over 50 ft.
long. There was just one wire in the conduit, and
the conduit itself did not carry a current, so we
were able to read the anode line current to within
3% of what it showed outside the conduit.
Fig. 6: Rectifier
and anodes in a distributed impressed current
cathodic protection system. DC Amp Clip model CP-¾"
Sea Clip shown in Fig. 5A may be used to measure the
anode cable current.
The clip reads the algebraic sum of all currents
flowing through the aperture. If there are two lines
enclosed by the clip it will indicate the one plus
the other. If these happen to be the (+) and (-)
leads of a rectifier carrying 1 amp. the clip will
show zero if all the current going out on one line
comes back on the other line.
Leakage to Ground
However, if the rectifier sends +1 amp. out on
the (+) line, but there is leakage to earth at the
rectifier so that only -0.9 amp. returns on the (-)
line, then the clip around both lines will show +0.1
amp., i.e., the leakage.
Foreign Interference
There is foreign line interference, represented
by VF, at anode 2. This causes normal anode 2 to
draw an unusually large current I2 = 2 ½ amp. Since
Ia is only 2 amp., one would expect that I2 would be
2 amp at the very most. This is a clue that foreign
interference is present.
Stringer current Ib is ½ amp., but in the wrong
direction. This suggests that anode 3 is weak, and
that anode 2 is subject to interference.
The current Id at anode 4 is practically zero,
meaning that the cable is broken between anodes 3
and 4. Back a bit more than half way the clip is put
on the cable at Ic, which is found to be a few
milliamperes, headed the wrong way. This probably
means that the break (X) is in earth between Ic and
Id.
The pipeline may be broken into electrical
sections by insulating flanges with bonds across.
The DC Amp Clip is used to measure the bond currents
with a view to finding coating insulation flaws,
shorts to foreign pipes, interference from foreign
lines, or newly installed foreign anode beds.
If the insulating flanges are widely separated,
and if there is a severe problem as happens in some
urban areas, then the corrosion engineer will want
to get a clamp that fits around the whole pipe. This
way he can measure the actual pipe current at
several locations, and thus locate the interference.
This is discussed in the section- Locating an
Electrical Short.
Clamp & Clip Defined
A Sensor is either a Clip or a Clamp.
The Clip shown in Fig. 2 can be used with one
hand. The Clamp shown in Fig 3 comprises two half
circles ("C"s) bolted together at the flat sectors
called lips. The stud bolts are captive to the low C
which has only the bridle cable attached. There are
oversize holes in the lips of the top C which has
attached both the bridle and the main cable. The
stud bolts are put through the holes and secured
with two brass hex thumb nuts.
The "nose" is the pair of lips far from the
bridle. The "tail" is the pair of lips near the
bridle. The main cable has a female connector to the
Indicator.
Clamp Polarity
Positive current direction is marked on the
clamp.
The direction of current flow through a clamp
(Fig.7) is analogous to the clip. In a clip positive
current flows from left to right through the
aperture when the cabled arm is up. Similarly, in a
clamp positive current flows from left to right when
the half circle ("C") having both the main cable to
the indicator and the bridle cable to the other "C"
is on top. This is shown in Fig. 7.
Another way to look at it is that positive
current flows from the bridle side of the clamp,
through the aperture, to the side more remote from
the bridle.
Fig. 7: Clamp
polarity. The arrow shows the direction of positive
current flow through a clamp.
Measuring Direct Current in Pipe
The process for measuring DC in 4" pipe or
smaller is, in some ways, similar to that for
parasitic drain given on p. 3. Differences appear
because the clip is larger and the pipe may be
steel.
If the current to be measured in copper pipe is
more than 10 times the He rating of the clip the
straight foreword method should work, but it is
prudent to use the short form FZ procedure on p. 3.
The He spec. for a 5" clip is 0 ± 40 mA.
If the current is small it is likely to be more
difficult to get accurate results with a simple
process on iron or steel pipe because it usually has
high magnetic permeability which distorts the
Earth's magnetic field and so shifts the zero of the
clip. An interrupter discussed on page 11, or the
full FZ procedure given on page 12 may be needed.
On steel pipe the accuracy expected when the full
FZ is used by a skilled and experienced person is 0
± 2/3 He. Then if 1 amp. is measured with a 5" clip
on steel pipe the result should be within 2/3 of 40
mA, i.e., 0.97 to 1.03 amp. using the process shown
on page 12.
Large Clamps
Large clamps differ from clips in that they have:
- Less gain. A 30" dia clamp may have ¼ the
sensitivity of a 2 ½" clip.
- Greater He. The He rating of a 30" clamp is
0 ± 410 mA peak, but a 2 ½" clip is 0 ± 10 mA.
The large He usually causes a substantial zero
offset change as the clamp is moved near a big steel
pipe. Fortunately, the current that is important in
a 30" pipe is much greater than that of concern in a
2" pipe, so the real problem of the larger He is
mitigated. However, it is usually a good idea to
plan on using the FZ procedure. "Nose" and other
terms are defined on p. 5, "Clip and Clamp Defined".
Two people are needed to do the FZ procedure
properly when a 30" clamp is used. Shims will be
needed to hold the clamp in one set position. Two or
3 pieces of plastic foam, ¾" or 2" board will
usually do.
Special Adapters; No Thumb Nuts
When feasible we make things a bit special at the
request of customers. One example is an 8" clamp
with lips adapted to be secured with large copper
clips instead of thumb nuts. One of our Canadian
customers said it was a trouble to put thumb nuts on
the captive studs in snow, so we flattened and
grooved the lips to match the clips. Please
telephone if you need something special.
Locating an Electrical Short
An electrical short of a pipeline may make
cathodic protection difficult or impossible. If a
water pipe shorts a 12" gas line in an urban area it
can be located as shown in Fig. 8.
A battery (VA) is temporarily connected to the
12" line in manhole A. The negative terminal of a
truck battery in series with a lamp to limit the
current to 5 or 10 amp. can be connected to the
pipeline. A water main usually works as a ground.
The clamp is placed over the line at position 1
to the left of VA in manhole A. The current i1 is
small, and does not change much when VA is connected
and disconnected i.e., interrupted. This means that
the short is not likely to be to the left of manhole
A.
The clamp is moved to position 2 to the right of
VA. Current i2 is much larger, probably several
amp., flowing toward the battery when VA is
connected, and greatly reduced when VA is
disconnected. Since i2 is large and follows the
interrupt pattern of VA the operator expects to find
the short to the right of manhole A.
The battery and DC Amp Clamp are moved to the
right of manhole A, to manhole B. The process is
repeated. This time the large current following the
interrupt pattern of VB is i3, and it is flowing
toward the battery. A check of i4 shows that it is
small and does not much follow the interrupt pattern
of VB.
Fig. 8: DC Amp
Clamp used to locate an electrical short to a
pipeline by driving the pipe with a switched current
at first one manhole and then another until the
short is bracketed. A SEA Clamp similar to that in
Fig. 1 or Fig. 3 is suited for this work.
The large current following the interrupt pattern
of VB is i3, and it is flowing toward the battery. A
check of i4 shows that it is small and does not much
follow the interrupt pattern of VB.
This says that the short is most likely to the
left of manhole B. And it is between holes A & B.
This may be a good enough clue to the location of
the short. If not, the suggested procedure is to
repeat the process in a manhole half way between A &
B.
Telephone Cable
Lead sheath cable, and to some extent, the steel
armor on fiber optic cable are apt to corrode where
current leaves the cable. Fig. 9 shows a way to find
out where current is leaving the cable by measuring
the normal direct current on the cable's sheath.
Phone cable is shown going underground from bell
hole to bell hole, probably in an urban area. The
current is 70 mA at a start in bell hole #1, and the
same in #2. Since there were no additions, it is
likely that there is no leakage to earth between
holes 1 & 2.
However, at bell hole #3 the current is only 30
mA, so the operator expects that 40 mA left the
cable and went in to the earth between holes 2 & 3.
This may be a good enough clue to find the current
leak, but if not, the user will want to drill a new
hole between bell holes 2 & 3, and repeat the
process.
Fig. 9: Finding
telephone cable leakage between bell holes. A 4"
version of the DC Amp Clip shown in Fig. 5B is
suited for this work.
Clip Definitions
The attitude of a clip is specified in terms of
the "nose" and the axis of the aperture.
"Lips" are the flat areas where the aperture of
the clip opens up to admit the guy wire when the
arms are squeezed.
The "nose" is the pair of lips of the clip
farthest from the arms, or handles. The clip points
north when the lips are nearer the north pole than
the arms.
The "tail" is the pair of lips nearer the arms.
The lips should always be clean and free to close
when the arms are released.
When a vertical guy wire is in the aperture of
the clip, it is best held so that the axis of the
aperture lines up with the wire.
Flange Leakage (Simplest Way)
Cathodic protection current leakage across a
defective insulated flange in a pipe line can be
measured using one or more of the methods shown in
Fig. 10.
If the current is quite high, say ½ to 5 amp. the
simplest way is to put the clamp on the pipe close
enough to the flange so that there is no path for
current to enter or leave other than through the
flange, and measure the current with the DC Amp
Clamp. The battery and voltmeter are not required.
However, three or four checks are advisable,
especially if the current is small, say less than
ten times the He rating of the clamp used.
To verify the reading of flange current, repeat
the measurement of pipe current on the other side of
the flange. The two measurements should be in
reasonable agreement if there is no other way for
current to enter or leave the pipe line between the
measurement points.
Use the full Floating Zero Procedure, or at least
the short form. Results should be consistent.
Use a current interrupter as shown in Fig. 11 if
the current source can be switched.
Use the full switched battery setup shown in Fig.
10. In each case results should be consistent, or
the discrepancy explained.
Fig. 10: DC Amp
Clamp used on a pipeline to measure insulating
flange leakage current if. The flange voltage Vf is
also measured as current iB from a battery in series
with a lamp is switched on and off. The flange
resistance Rf is equal to the flange voltage Vf
divided by the flange current if.
Flange Current c/o Flange Resistance
Fig. 10 shows the arrangement for measuring small
flange leakage current. First flange resistance is
determined, and then flange voltage is used to get
the current.
The DC Amp Clamp is placed on the pipeline close
to the flange so that there will be no current
entering or leaving the pipe between the clamp and
the far side of the flange.
A dc voltmeter (Vf) is connected across the
flange. It may be connected spanning the clamp and
flange, or directly across the flange if this is
more convenient, because the pipe has low
resistance.
A switched current source (iB) consisting a
battery, lamp, and interrupter (switch) are
connected in series across the flange and spanning
both the voltmeter and clamp. The current source may
put out something like 1 amp. This 1 amp. will
likely go both through the flange and back away from
the flange and around the system the long way. The
purpose of the DC Amp Clamp is to measure the change
in the actual flange current.
In operation the change in flange current is
noted together with the corresponding change in
flange voltage. Their ratio is the flange
resistance.
For example, with the switch open the DC Amp
Clamp may read +0.15 amp. while the voltmeter Vf
reads +0.03 volt. When the switch is closed the
clamp may read +0.35 amp., while the voltmeter reads
+0.23 volt. The current change with the switch
closed was +0.2 amp, and the corresponding voltage
change was +0.2 volt. So the flange resistance is 1
ohm. At least under these conditions. Some leaky
type resistors are non-linear. They change
resistance when the current changes, temperature,
etc. And it may be that it is not the flange, but a
conductive liquid in the pipe.
Finally we get the flange current. Since the
flange voltage was +0.03 volt when the switch was
open this is presumed to be the normal voltage
across the flange. Since the resistance is 1 ohm,
the normal flange current is +0.03 amp.
This process also helps solve other problems.
URD Concentric Neutral Cable Resistance
Corrosion can eat away part of the neutral on URD
power cable, especially when the concentric neutral
is bare metal in the earth. But since this corrosion
is usually concentrated in a small sector of land,
it is useful to be able to measure the neutral
resistance.
The problem of locating a corroded sector is
complicated by the fact that AC load current can
take alternate paths, so that the AC voltage drop is
not reliable in locating corrosion.
A solution is the setup in Fig. 10. Replace the
leaky flange with a sector of URD cable between 2
manholes. And the clamp with a clip. Usually 2 ½" is
big enough. The process of finding the resistance
between the 2 manholes is the same as for the
flange. The key is that the DC Amp Clip measures the
current in the section of URD under test.
One Man Does the Work of Two
One of our Canadian customers said that, with the
right equipment, he can work alone and do the work
of two men when the second man would be needed for
remote switching or data taking. For example, in the
case of the flange current measurement above,
suppose that an interrupter is not available. Then a
co-worker can switch the breaker as required if
voice or radio communication is available.
To avoid having to hire another man, our Canadian
friend bought a well known digital data logger
(names on request) which includes a clock and
connected it to the recorder (Rcdr.) jack voltage
output of the DC Amp Clip so as to record both pipe
current and the time of day for a specific current
magnitude. And in Fig. 10, he would also record the
flange voltage on another channel. Then he would go
to the breaker and key it off and on, recording in
his notebook the time of each action.
When the needed tests are done he would go to his
office, down load the data logger into his computer,
and work to a solution.
Zero Cancelled by Current Interrupter
A current interrupter can be used to remove the
zero offset problem associated with measuring small
currents with a large clamp; PROVIDED that it is
known that the only current of interest comes from a
rectifier or some interruptible source. Current from
other sources will be missed when using an
interrupter. The full floating zero procedure must
be used to be sure of getting all currents in the
pipeline.
If all the flange current is known to come from a
rectifier, arrange to have the rectifier turned on
and off (10 sec. on, 20 sec. off) by a current
interrupter. Then the flange current is the
difference between the on and off readings. Zero
setting, and zero offset do not matter. The
direction of flow of the current is known, because
the direction of change of the meter reading during
the 10 sec. on time shows the polarity.
For example, if the meter reads -45 mA during the
20 sec. off time, but -15 mA during the 10 sec on
time, the pipe and flange current during the on time
is +30 mA because the meter changed to a more
positive reading when the rectifier was switched on.
Fig. 11: Current
interrupter used to eliminate the zero offset
problem when measuring flange current. The digital
DC Amp Clip shown in Fig. 1 is suited for this work.
This process is also good for measuring a small
current with a small clip.
Guy Wires
Current flowing from a guy wire or its anchor
into the earth generally causes corrosion. Real
attention was given to this some time ago when a
not-too-strong windstorm took down a whole line of
poles. Examination showed that the guy wire anchor
rods pulled out of the earth easily because there
were only vestiges of anchor left. The anchors had
all been corroded away.
It was found that the cause was current going
down the guy wire to the anchor and into the earth.
This often corrodes the anchor, but at times it
corrodes the guy rod itself. Since the guy rod has a
smaller mass than the anchor, a current which would
represent only slow corrosion to the anchor is
serious to the rod
Mr. Orville W. Zastrow*, Mr. S. A. Potocny**, and
others have written at length about this. From a
casual reading it, 10 mA for 30 years
is significant for the typical anchor. If it is
primarily the rod that is primarily corroding, 10 mA
could be serious in a few years.
The process for measuring guy wire current
greater than 100 mA is similar to that for that
given on p. 3 for measuring parasitic drain current
at a battery. At least the short form Floating Zero
Procedure should be used.
* Zastrow, Orville W., REA Bulletin
161-23, pp. 13-15.
** Potocny, S. A., Telephone guy rod corrosion by
stray currents, Materials Performance, Dec.
1979, pp. 19-25.
Full Floating Zero Procedure
To measure guy wire current around 20 mA requires
one of our best DC Amp Clips and the full Floating
Zero Procedure (full FZ).
Most guy wires are under ¾" dia., so a ¾" Sea
Clip will do nicely. Some heavy guys are up to .85"
dia. If this is the case at your plant, call and we
will arrange to supply oversize clips to you.
The best ¾" Sea Clip is specified to have Earth
Field Effect (He) less (better) than 0 ± 1 ½ mA
peak. This would seem to be stable enough to measure
10 mA, and it is, on copper wire away from
magnetized bolts or metal.
However, most guy wires are steel which can be
magnetized by the Earth's magnetic field. And if not
magnetized, they tend to distort and concentrate the
Earth's magnetic field. This causes a zero shift
which can lead to important error if ignored.
Most of the error due to zero offset is likely to
be removed if the full FZ is used properly. By this
the average measured current will probably be
within ± 2/3 the present He of the clip, provided
the user has studied the operating instructions and
this procedure, and uses the same care and skill as
when he successfully practiced measuring a known
small current flowing in steel rod or cable,
preferably in the comfort of his shop.
It is not easy, and it is not absolutely sure to
work every time. It can be done. One of the Delco
engineers told me that he could get 0 ± 2 mA
accuracy reading parasitic battery drain- a similar
challenge. This has been done similarly.
Setup for FZ
Look for a location on the guy wire (or rod)
which is relatively free of magnets. Do this by
sliding the clip longitudinally up and down on the
wire while looking at the meter. Choose a spot where
the meter reading does not change much. Set the
range switch for 1 mA resolution. Avoid a location
near bolts or special fittings.
For simplicity, the guy
wire is vertical.
Prepare a data sheet convenient for writing down
readings in pairs. Leave space for average
calculations.
FZ Process
The objective is to get at least 3 pairs of guy
wire current readings, all at the same position on
the wire, all on the same current range, and all
with the same zero setting. The Zero setting does
not have to cause the meter to read particularly
close to zero. It is more convenient to set the Zero
control so that it reads less than ± 50 mA when the
clip is not on the wire.
The pairs of guy wire current readings are taken
at at least three different attitudes of the clip on
the wire. four are preferred. Each pair includes a
"normal" or positive polarity reading as in Fig 4A
on p. 3, and also a "reverse" or negative reading,
with the clip turned over, as in Fig 4B. The
attitudes are preferably symmetrical about the wire;
the four points of the compass if the wire is
vertical.
By the same position on the wire it is meant that the
wire is centered inside the aperture of the clip to
the same extent for all readings. And it holds its
position vertically. It is not necessary that the
clip be exactly centered on the wire, but this will
not hurt. It is necessary that the clip be off
center the same amount for each reading, whether the
clip is normal or reversed. And the clip should keep
the same longitudinal position along the length of
the wire for each reading. This is quite important.
Some users make wood or plastic jigs to center
the wire in the clip. Others use a free hand. For
example, ..IF SAFE- WATCH OUT FOR ELECTRIC SHOCK..
the left hand can be put on the wire so as to feel
the clip - just touching - no force. The gentle
touch of the side of the clip serves to locate the
clip with respect to the wire as it is positioned
for first the normal and then the reverse reading.
Hold the clip by just the cabled arm. This avoids
the problem of cracking the lips open by a mild
squeeze on the two arms. When proceeding to the next
attitude, keep the hand at the same height on the
wire, just rotate around the wire.
Attitude is specified in terms of the "nose" and
the axis of the aperture. The nose is the pair of
lips of the clip farthest from the arms, or handles.
The clip points north when the lips are nearer the
north pole than the arms. When a vertical guy wire
is in the aperture the clip is best held so that the
axis of the aperture lines up with the wire.
FZ Data Example
In the example below the first reading is the
normal or (+) meter indication in mA, with the
cabled arm on the right. Then current from the top
of the pole to the anchor reads positive, and tends
to be corrosive. The second is the reverse or (-)
reading. The average is the sum of the normal and
reverse readings, but with the sign of the reverse
reading inverted.
The zero control is not touched during the whole
series of readings. This is important. Start over if
the Zero control is moved by accident.
| Heading of pair |
Readings
mA |
Average
mA |
| North |
+40 |
+33 |
| -25 |
| East |
+30 |
+25 |
| -20 |
| South* |
+55 |
+25 |
| +5 |
| West |
+25 |
+20 |
| -15 |
| Sum of averages |
+103 |
| Average of 4 pairs |
+26 |
* Note that the meter read (+5) at the reverse
reading. When the sign of the +5 is inverted and
added to the +55 normal reading the sum of the two
is +50, so the average is +25.
The accuracy expected when the full FZ is used by
a skilled person on a steel conductor is 0 ± 2/3 He
rating for the clip. This is He = 0 ± 1.5 mA for our
best ¾" Sea Clip. So we expect that the true current
is between 25 and 27 mA. However, especially at
small current levels, it is better to add one or two
mA to the likely uncertainty. For example, the
accuracy rating of an analog Indicator alone is 0 ±
3 % of full scale. If this is 100 mA, we need to add
± 3 mA to the uncertainty.
From this it is concluded that the true guy wire
current in this example is likely to be +26 mA, ± 2
or 3 mA; i.e., +23 to 29 mA. It is positive, so it
tends to be corrosive. However, the damage to the
anchor may not be serious until the current has
persisted for several years. But if the current is
primarily leaving the rod the damage could be
serious much sooner*.
* Please note: I am an instrument builder, not a
corrosion engineer. These are my impressions from
casual readings. The user should consult the
literature.
Checks
Each pair of readings should be checked to see if
it is fairly consistent with the others. In the
example above the maximum deviation of the average
of a pair is 7 mA from the average of all pairs.
This is about 5 times the He = 0 ± .5 mA rating of
the clip. It is reasonable on steel wire. In
contrast, if a pair of readings had been +70 and -80
mA the data should be rejected. Use 3 instead of 4
headings, or go to a different spot on the guy wire.
If time permits, the whole series of readings
should be repeated in the same spot on the wire. In
general they will differ somewhat, but the
average of all pairs to be the same within 2 or 3 mA
is expected.
If feasible, a complete set of readings should be
taken at another spot on the wire at least a foot
away from the first. Again, individual readings may
differ by 10 mA, but the average of all pairs should
be the same within 2 or 3 mA.
The user should also be on the lookout for errors
due to RFI, AC flowing in the guy wire, etc. If not
sure, call us at 941-957-3110, or Fax: 941-378-0712.
AC Rejection
DC Amp Clips reject AC, and vice versa. The best
analog and digital DC Amp Clips will change their
reading less than 0 ± 5 mA because of up to 10 amp.
AC flowing in the line. And the change due to 30
amp. AC may be less than 15 mA.
RFI
Radio Frequency Interference (RFI) can shift the
zero and may make readings erratic. If RFI is
suspected, cover the bottom and sides of the DC Amp
Clip with aluminum foil, and then put it back inside
the leather case. This usually stabilizes the meter.
Alternating Current
Alternating current needs to be measured because
a large AC flowing in a pipe or cable can be a
threat to property and personal safety. For example:
It has been estimated that 1% to 3% of AC in a
pipe is direct current. If so, 2 amp. AC pipe to
soil current can represent 20 mA DC. A DC Amp Clip
is usually able to read the 20 mA, give or take 5 to
10 mA, and say which way it is going.
An oil company's engineer told me how AC put a
good sized pit in one of their gasoline lines.
Somehow a water line had come close to their line,
and it was carrying substantial alternating current.
Trucks going over the lines pushed them together,
but they separated when the truck passed. The AC
shifted in and out of the gas line until the arc had
worn a fair pit. It is fortunate that they found the
fault before the pit went deeper.
Another engineer told me of occasional surges of
induced current to over 100 amp. in a gas pipeline
running in the same corridor as a major power
transmission line. Apparently there was concern that
this would cause corrosion, and might endanger
persons at the pumping station.
A power company engineer told me about URD cable
that had corroded until AC shifted from the line to
the local water system. Residents got a small shock
when the turned the water spigot.
Another power engineer told of concern about
considerable current in the leg of a transmission
line tower.
A small AC can also be a problem. For example:
A contractor in an urban area is concerned that
AC flowing in copper plumbing can cause corrosion,
and/or create magnetic fields that some may consider
harmful to persons.
A dairy farMER™told me that his estimate of lost
production of milk due to stray power current in
dairy cows was a million dollars a day. Ten mA is
said to be the threshold.
How to Measure AC
There are at least two ways to measure AC with
the system operating normally. There is no need to
unhook any wires or break open any pipe lines.
Simplest
The simplest (least equipment to carry) is shown
in Fig. 11, where the clamp is shunted with a few
ohms resistance and then a Fluke meter is used to
read the voltage across the shunt. The resistor R
can be adjusted so that the Fluke model 87 meter,
while really reading AC millivolts rms, shows a
number the same as the current in amperes.
Suppose the line current ii is 12 amp., and the
sensor (a clip or clamp) has N = 1000 turns. Then
under ideal conditions the sensor current is = 12
mA. If the shunt R = 1 ohm, the voltage across R
will be 10 mV, so the user can interpret this as ii
= 12 amp.
The problem is that sensitivity is limited. If
two digit readings are needed and the resolution of
the Fluke (one of the best for field portable use)
is 0.1 mV, the minimum current reading is 1 amp. The dairy cattle and people begin to get
uneasy at 0.01 amp. More sensitivity is needed for
some uses.
Fig. 12: Measuring
AC on a pipe with a 1 ohm shunt resistor across the
sensor's coil. Calibration resistor "R" is set so
that a Fluke 87 true rms meter reads 12 mV when 12
amp flows in the pipe.
Fig. 13: Digital AC
Amp Clip having 0.1 mA resolution and 20 amp maximum
reading on a 5" clip. The output voltage can be
either AC, or rectified AC, proportional to the
current.
High Sensitivity
We make AC Amp Clips and Clamps from ¾" to 5 feet
diameter. If properly calibrated they use the same
sensors as the DC Amp Clips. The maximum sensitivity
of the ¾" analog model is 1.0 mA rms on a sine wave,
full scale. The digital model has a resolution of
0.1 mA, and like the analog, a maximum current
reading of 20 amp. Larger clips and clamps have more
turns (N), more stray pickup, and less sensitivity.
In addition for viewing the waveform with an
oscilloscope both the analog digital models can have
an AC voltage output proportional to the current
input (ii) usable from 10 Hz to over 2000 Hz. The
digital meters can have an output which is a
rectified form of the current input. This also
permits use of an oscilloscope to spot pulse
distortion, and in addition it provides a DC signal
suited for data logging. The user can record current
surges which occur when he is not there.
Stray Pickup & DC Rejection
On the more sensitive range a direct current
flowing in the aperture of a ¾" clip may reduce the
sensitivity about 3% when 5 amp. DC is present.
Of course, the DC must be really steady. The AC
component of a fluctuating direct current is the
peak-to-peak of the change divided by 2.8. For
example, 1 amp. DC which fluctuates only 1% p-p has
a 3.6 mA rms AC component. The meter will show this
3.6 mA, but depending on frequency, it may add
algebraically, or rms wise, to the AC intended for
measurement.
Stray pickup in a ¾" clip is usually magnetic. If
an interfering current is flowing in a cable
touching a ¾" clip the rejection may be 100 amp. per
amp. For example, If the clip is measuring 20 mA but
a wire carrying 1 amp. touches the outside of the
clip there may be induced an interfering current of
10 mA. This can add phasewise (algebraically) if the
currents are at the same frequency. But if they are
at a different frequency they tend to add rms. wise.
This 100 amp. per amp. magnetic rejection may
increase to 1000 if the wire touching the side of
the clip is oriented for least pickup. If the wire
carrying the interfering current is moved away from
the ¾" clip by 1 ¼" the rejection may again increase
by a factor of 10 to 10,000.
A nearby transforMER™can cause magnetic pickup.
For example, a 2 ½" clip had 0.1 mA magnetic pickup
when held above the bench. But this might increase
to 0.3 mA if there were a fluorescent light ballast
transforMER™within 3 feet.
Larger sensors have greater pickup, magnetically,
and electrostatically.
Electrostatic pickup is basically capacitive
coupling of the clip to an AC voltage source. It
increases with the voltage of the interfering wire,
not its current. It also increases when the user
grounds the Indicator by connecting it to an
oscilloscope or recorder, especially if these run on
the AC mains power. A 2 ½" Clip located about a foot
from a small radio plugged into the AC mains but
turned off caused the meter to read 0.6 mA when the
Indicator was connected to an oscilloscope.
[Text from page 16 on is
missing!]
DC & AC Amp Clip Characteristics
Gain & Zero Offset
Maintenance
Selecting a Swain Meter™
AC Amp Clip
Conclusion
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