Electrical Cable Fault Locating
Electrical Cable Fault Locating
Electrical Cable Fault Locating
TYPES OF FAULTS
b Many types of faults
To Earth, Other Conductor, Sheath Primary Cable, Secondary, L.V. Persistent, HV (Flashing) Intermittent (random) Many Combinations of the above
A-FRAME BASICS
b Must be earth fault.
A-FRAME BASICS
b b
Ducted cable usually prevents proper location. The A-frame basically turns the locator into a very sensitive micro-voltmeter. The flow of current through the resistance of the earth creates a small voltage. The polarity of the voltage gives the direction to the fault. For this reason, transmitter now sends DC square wave (shift) with AC locate tone. Dry topsoil, gravel, concrete and asphalt all reduce the voltage that can be measured on the ground. Good, Remote ground stake is CRITICAL !
A-FRAME BASICS
b
If cable under road, A-frame along the grass median and draw perpendicular line out from it. Scrape dry earth / rocks from under A-frame. Bring water jug, wet down earth / concrete TDR first (coming up) to reduce walking with A-frame. Trust the equipment, it is normal to lose Fwd/Bkwd lock in the middle of long spans, if you havent walked over it, keep going. Prelocate cable path to save time.
Impedance = Series and Parallel Resistance and Complex Inductive and Capacitive Elements
b
When the relationship between the insulating materials is constant, there is no change in impedance and the waveform will be constant. Here, the relationship has changed slightly with the untwisting of the pairs. The resulting waveform shows a partial open caused by a raising of impedance. .Here, the relationship has changed significantly with the untwisting and separation of the pairs. The resulting waveform shows a larger raising of impedance.
b b
Small relative changes in impedance will not show up Rule of thumb is impedance change must be under 8x to detect Often HV cables faults are 1k-100kohms and make too small of a reflection to be detected when HV cables characteristic impedance is ~25 ohms Concentric gives best traces, braided the worst Heat Trace Cable very good application
Open circuit
A 100 nanosecond pulse waveform on a straight run of street lighting cable. There is a complete open circuit at 280 meters. A 100 nanosecond pulse waveform on a straight run of street lighting cable. There is a complete short circuit at 280 meters.
Two 25 nanosecond waveforms, taken on a low-voltage network simulation cable. One waveform is of a good phase and the other shows the same run of cable with a short circuit at ~40 meters (just ahead of the cursor). The two are displayed together with the overlay adjusted to zero offset. This is a difference waveform, using the 1 microsecond pulse, taken on a low-voltage network simulation cable. It shows the difference between a good phase and a partial open circuit at 125 meters.
b b
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Perfect trace, cable features are easily visible Since we know cable is open circuit at next transformer, we know that is at 292m Solid cursor is at splice at 199m (93.5 meters back) Features past open are the same reflections just bouncing back and forth (distances are all multiples) Sometimes the end isnt what you think it is, have helper short far end This was used with A-frame, we stated looking from the far end and found splice had failed.
A reflection with the same polarity indicates a fault with OPEN (high impedance) tendencies. The reflection shown at the 2nd cursor is a COMPLETE OPEN.
A reflection with the opposite polarity indicates a fault with SHORT (low impedance) tendencies. The reflection shown at the 2nd cursor is a DEAD SHORT.
A properly TERMINATED cable will absorb the TDR signal, resulting in no reflection. Faults prior to the termination will appear as reflections along the waveform.
Testing through to an antenna usually results in an S shaped reflection, although reflections can vary greatly depending on the antenna.
Knowing Velocity Of Propagation (VOP / PVF) of cable is easier but there are ways around that.
Even without VOP we can use TDRs just as well ! ! (sample or ratio methods)
b b
Have a distance from the TDR However, cables do not run in straight lines Use a cable locator and measure wheel / GPS
(even C$200 GPS are 3-5m accurate now)
Make notes while wheeling off (i.e.: 65m post) Cable pairs in multicore cables are laid up in spirals inside the sheath The pairs in the middle are shorter than the pairs closer to the sheath (only 1-2%)
Count all wire distances (including test leads) Use a cable locator to find true path Watch for funny locates showing possible slack loops
minus
Fault position
b b b b b
Sometime due to loose wraps on cable such as streetlights, braided and triplex cable Multiple faults, and/or Ts and splices (sometime undocumented) Remove other features Move towards faults Use filtering (60 Hz. and/or averaging) Use 2 line mode to subtract all the noise Find the end first, have helper short far end
PC TDR Software
The software allows for information stored in the instrument to be uploaded to a computer. Waveform information can be archived, adjusted, compared to original benchmark, or analyzed on your PC while your TDR goes back into the field to work for you.
Why Thump ?
b
Unless shorted or burned open, a typical fault on a high voltage cable does not present enough of an impedance change for a TDR to detect it If there is a path to ground - A-frame is possible With a dielectric breakdown, it looks and is perfect at low voltages, we need to make it flash
b b
b b
b
Some faults require higher then rated voltage to show up faults on demand (not waiting for right combination of circumstances) Subjecting the cable under test to repeated stresses of high voltage reduces its life expectancy Statistics prove older cables that get thumped fail much faster then non-thumped cables
Do what you can with low ( <50) voltage methods (TDR/A-frame)
b b
TDR or A-frame costs C$6-8,000 compared to C$40-80,000 If you have to thump, use methods to reduce the number of thumps required with visual and/or pre-locating.
Fine Locator
whumph underground l Also monitor the electromagnetic pulse from the current flow (no current flow past fault, hence no EM pulse)
Thumping Notes
b If the breakdown voltage of the cable is
above the rating of your Thumper, it is common to burn down the fault, creating more damage at the fault so that it will fail at a lower voltage
l l
Finds faults up to 200 M.Ohms Locates faults between conductors or between conductors and sheath, neutral or earth High sensitivity Compliment to TDR for high resistance faults
Two sections of the faulty conductor, one on each side of the fault, together with a good conductor if necessary, comprise the two external arms of the bridge. The other two arms of the bridge are contained within the instrument. High resistance faults in dielectrics such as rubber and polyethylene can be located with an accuracy well within 0.5% of the loop length and typically 0.1%, although this may be limited by the non-uniformity of the conductor.
Very simple device Typically no processors, just amplifiers, resistors and meter Use: null out meter on cable, add external battery to create fault current and adjust 0-1000 count resistor to re-zero meter, read % of distance.
Running continually, the IFD mode captures any change in the impedance, whether an open or a short. Make sure the TDR is on line power when possible
TDR Resistance Fault Locator Multi Meter Insulation Resistance Meter Pair Balance and Noise Measurement Convenient and cheaper than several test units Simplifies technicians job, especially when climbing or working in confined spaces (vaults) Training & familiarity on one unit Sometimes have lower specs than individual units
DUCT NOTES
The A-frame fault location may not be where the cable damage is due to the current migrating along the inside of the duct. Empty ducts can be located with sondes / Omni
Training
b
Even the wizziest equipment will give poor results of users can not use it effectively. b Insist on complete initial and future training. b Reasonably local support will make life easier.