Cable and Cable Fault Locating - Part 2
This is the second of a four part series on cable and fault locating technologies that are in
common use today. This month’s installment is on the 'A-Frame' method of cable fault location.
Future issues will cover Time Domain Reflectometry (TDR) and high-voltage 'thumpers'. (Part I
was on basic cable locating).
Buried cables do occasionally fail, for a variety of reasons and in many different ways.
Lightening strikes, overloads or surges, installation problems, shovel and rodent damage are
some of the common causes of damage that can lead to cable failure. Any discontinuity in the
cable jacket which allows moisture in over time corrodes the conductors. Cables fail either open
circuit, short circuit or somewhere in between, to earth and/or to another conductor in the cable.
The type of fault should be determined as different faults require different approaches. A
persistent fault to earth (a ground fault) is usually most accurately and most easily found with an
A-frame. Open and short circuits are best found with a TDR, and a 'flashing' fault that only
happens at high voltages usually requires a high voltage surge generator or 'thumper'. Most
transmitters in a fault finding system will have some way to indicate if there is a path to ground
such as an ammeter or ohmmeter and some have both. If there is a path to earth, the a-frame is
still one of the most popular and most recommended methods when the conductor is not
enclosed in duct.
We can build on the methods of basic locating to detect persistent earth faults. We showed that
cable locating is achieved by creating an alternating current (AC) on a cable and tracing the
resulting AC electromagnetic (EM) field with a tuned receiver. A-frame systems differ in that in
addition to the locate signal, we add a pulsed DC current (for ground fault locating) to the cable
under test (CUT). The use of
DC allows the detection of
current direction and will lead the
user towards a fault. Where there
is contact with the earth, this
current will flow out of the CUT
at the fault and back to the
ground stake of the transmitter.
The current will be concentrated
near the fault(s) and the ground
stake but from those points will
travel very wide and deep in
search of the path of least
resistance. Current through a resistance makes a voltage. The flow of the pulsed DC through the
impedance of the earth will create a slight DC voltage and that is how we find faults. The
addition of an A-frame to a locator basically turns the locator into a very sensitive voltmeter and
following the pulsing DC through the earth with the A-frame gives a direction to and magnitude
of the fault(s).
Because we are following the path of current through the earth, cables in duct cause us problems.
Even if we have fault find signal flow, it doesn't necessarily lead us to the fault. If the cable
problem happens to be at the point of duct damage, we may have sufficient current flow through
the earth from that location. Quite possible though is that there is moisture inside the duct and
we are getting some current flow through that path as well, limiting our signal that leads us to the
fault. If the cable problem is entirely within a good duct (such as happens when a jacket gets
skinned while pulling it into the duct) all of our current flow can be inside the duct until a path to
ground is reached, (such as at a defective duct joint) and we can't find the fault. At best we can
find where the moisture inside the duct is coming into contact with the earth such as at a bad
joint.
Proper set up and use of an A-frame system requires that we prevent the fault locate current from
getting to earth anywhere other than at the fault(s). This insures there is a maximum amount of
signal to follow and it will lead us to the right position. As such, all other paths to ground,
including neutrals and ground wires, have to be disconnected. This may require that power
cables be taken completely out of service before testing for faults.
Use of a good earth ground is even more important when fault finding than it is with regular
cable locating. An extra spool of ground wire with a large clip on the end is a valuable tool,
allowing the use of distant, independent ground stakes. Stop signs, insulated anchors and
existing but isolated ground stakes will often improve the performance of locate equipment.
The practical limits of fault detection are around 0.5-2 MΩ of DC resistance depending on
ground conditions. Since the A-frame is measuring voltage, it needs to have an electrical contact
with the earth under it. Concrete, asphalt, dry or sandy soil are all high resistance paths to
ground and can limit the voltage we can detect through the ground. Sometimes, wetting down the
ground and even the pavement in the path of the target conductor will assist fault detection.
Better yet, if there is access to unpaved ground running parallel to the target conductor, the Aframe may be used even if it is several meters off to one side.
After we have determined it is an earth fault and there is sufficiently low resistance to detect its
position, we can start walking the path of the cable with the a-frame. There are two indications
on most a-frame systems; signal strength and fault direction. The signal strength is best shown
as a logarithmic value as there is a huge range
of signal that can be detected. Close to the
ground stake where there is a concentration of
current, the receiver should indicate a large
fault magnitude.
It is a good idea to
remember this value as a ‘reference number’.
The value of the detected current (reference
number) should be indicated at the fault
location as well.
Most manufacturers
recommend that the A-frame be placed in the
ground approximately the same distance away
from the ground stake (in the direction
opposite of the actual cable path) as the cable is deep, to get this initial fault magnitude
reference.
Between the fault and transmitter the magnitude will drop, often substantially, as the current has
spread out so far as thus become 'diluted'. The behavior of the arrows may also change. Near
the fault and the ground stake they will exhibit a good hard 'lock' on the fault direction but in the
middle of the cable span, the weaker signal can result in fluctuations or no direction indication.
The proper technique is to keep walking through the weak area and as we approach the fault, the
signal will increase and normal operation will resume.
A fault is detected when two things happen; the signal strength increases due to the increased
concentration of current as we approach the fault and the arrows suddenly reverse as we cross
over the fault. Turning the a-frame 90 degrees and crossing the cable path will add more
accuracy to the fault location. A final test of the fault location is to perform a 'pothole' or circle
of the fault. Leave the front leg of the a-frame in the ground, directly over where the fault is
believed to be and circle that point, placing the back leg in the ground in several points on the
circle. If all signal indications point towards the stationary leg, we have found the best location.
If the number in the receiver at this location does not reflect the reference number you saw back
at the transmitter’s ground stake, you have an indication there may be more faults on this cable
(or the cable depth is different than you expected).
If possible, excavate the fault, fix it or at least remove its contact with the earth and retest the
cable with the ohmmeter function. Removing all contact includes drying the outside of the cable
to prevent a current path through the moisture. If the cable now tests good, we can be confident
there are no other faults on the cable.
Unjacketed concentric neutrals can pose a problem as our fault location current sometimes finds
the neutral an easier path back to the area of the ground stake than the earth. This effect can be
minimized by placing the ground stake as far away from the cable as possible and using a very
good (low resistance) ground.
The A-frame is the most accurate but not necessarily the fastest tool to use as the operator has to
walk the length of the cable from the transmitter to the ground fault. Often an earth fault has let
in water and the conductors have also corroded open or short circuit. Using a Time Domain
Reflectometer to estimate the rough location of a fault prior to the A-frame to find the exact
point will often give the most efficient use of technicians' time.
Next month we will cover cable fault locating using a Time Domain Reflectometer (TDR).
If you have any cable or cable fault locating questions, please send your e-mail to me at the
address below and I will answer them. I will also try and use them in future articles as a case
study.
Gord Parker, C.E.T. is employed by Radiodetection as the western Canada Applications Specialist. Radiodetection
and divisions Pearpoint, TeleSpec, RiserBond, BicoTest, Dielectric, and Amprobe manufacture pipe and cable
locators, cable fault locating instruments, cable test and pressurization products, specialty Cathodic Protection
troubleshooting equipment and video camera inspection systems. He can be contacted at 403-281-1808 or
gord.parker@radiodetection.spx.com