Pinpointing marks on the ground the spot where to dig. A surge generator discharges high-voltage impulses into the cable; at each discharge the fault breaks down with an arc that produces a bang and an electromagnetic pulse. A receiver on the surface picks up both: the electromagnetic pulse arrives almost instantly and the sound later, and that time difference shrinks as you approach the fault. It works with faults that arc; bolted short circuits and sheath faults are located with other methods.
What pinpointing is
Pre-location gives a distance measured along the cable from one end. But a buried cable does not run straight: it has bends, slack in manholes, crossings and changes of depth, and the propagation velocity has its own uncertainty. That is why BAUR insists that pre-location does not detect deviations of the route and that the exact point is a stage of its own. Digging with only the reflectometer distance usually ends in trenches longer than necessary and in more splices than the repair required.
Pinpointing is done on the surface, over the cable route and around the pre-located distance. For high-resistance and flashover faults, the most widely used method is the surge wave with acoustic listening. The full four-stage process is summarized in the guide on underground cable fault location, and the other cable tests in cable diagnostic tests.
How it works
- The surge generator (also called a thumper) charges a capacitor and discharges it onto the faulted conductor at regular intervals.
- The fault breaks down. Each discharge produces an arc at the fault point, with a bang that travels through the ground and a current pulse that creates a magnetic field along the route.
- The surface receiver has an acoustic sensor (geophone) and a coil for the magnetic field. It records the arrival of the electromagnetic pulse, practically instantaneous, and of the sound, which travels much more slowly through the ground.
- The time difference between the two signals depends on the distance from the sensor to the fault. Walking along the route toward the fault, that difference decreases; directly above the fault it is at its minimum.
Megger describes its pinpoint locators with this logic: they show the acoustic intensity, the delay between the acoustic and magnetic signals, the relative distance and the direction to the fault. BAUR presents it as the most common method for high-resistance faults and flashover faults.
What conditions affect listening
| Factor | Effect | What to do |
|---|---|---|
| Depth and soil type | They attenuate sound; soft soil absorbs it | Longer listening time, sensor well seated |
| Ambient noise (traffic, machinery) | Masks the bang | Use the time difference, not only the ear |
| Cable in a duct | Sound travels along the duct and may emerge at another point | Confirm with the magnetic signal and the time difference |
| Pavement or slabs | Transmit sound away from the point | Measure at several points until the minimum delay is found |
| Poorly known route | Listening away from the cable | Trace the route before listening |
When it does not work
- Bolted short circuit: if the fault does not arc, there is no bang to hear. For short circuits in certain cable types BAUR points to magnetic field distortion methods (twist or minimum distortion).
- Sheath fault: the damage is in the outer sheath, not in the main insulation. It is located with a measuring bridge and step voltage.
- Fault that does not break down at the available voltage: first the pre-location is worked with DC or decay methods, and it is assessed whether the fault can be made to break down repeatably.
The risk of surge energy
The energy of each discharge does not act only at the fault: it travels through the whole cable. In aged extruded-insulation cables with water trees, repeated impulses can weaken points that were healthy. IEEE 1234 frames the whole location process with that criterion: find the fault with the least risk of damaging cables, accessories and equipment that remain healthy. That is why correct practice is to arrive with an accurate pre-location, a well-traced route and energy adjusted to what the fault requires, so that listening lasts as little as possible. We do not publish voltages or energies per case: they depend on the cable, the fault and the equipment, and are defined by the testing company’s procedure.
Safety in the field
- The cable under surge is at high voltage along its whole length: remote ends must be isolated, marked and watched.
- The energy stored in the capacitor and in the cable capacitance is discharged to earth before any contact.
- Manholes and switchgear where other circuits terminate are checked before starting, and the circuit is isolated and locked out as in any lockout and tagout.
- Before cutting, the cable is identified in the trench: the exact point says where to dig, not which of the uncovered cables is the faulted one.
What not to do
- Listen without having traced the route.
- Raise the energy to “hear better” instead of moving closer with the receiver.
- Leave the generator thumping for long periods while the crew walks the route.
- Consider a point located just by the loudness of the bang, without the minimum of the time difference.
Cable testing with Tiza Group
Our team works with test equipment from recognized manufacturers (OMICRON, Doble, Baur, among others), in Colombia or elsewhere on request. If you have a buried circuit out of service, we help you plan and run the tests of your cable network so you dig only where the damage is. See the cable fault location service or write to us.
Related guides
- Power cable diagnostic tests
- Medium-voltage splices and terminations
- Power cables
- Medium-voltage collector network of a solar plant
- Underground cable fault location: methods
- Route tracing and identification of buried cables
- High-resistance cable faults: ARM, ICE and decay
- Sheath faults: location by step voltage
References
- IEEE Std 1234-2019, Guide for Fault-Locating Techniques on Shielded Power Cable Systems.
- BAUR GmbH, Cable fault location: process steps and methods (acoustic location, twist and minimum distortion methods).
- Megger, datasheets of the MPP1000 and MPP2000 pinpoint locators.
Frequently asked questions
Why does the receiver measure the time difference and not just the loudness of the bang?
Because loudness misleads: traffic noise, soil type and depth change what is heard. The difference between the arrival of the electromagnetic pulse and of the sound depends on the distance to the fault, so it shrinks on approach and reaches its minimum directly above it.
Does the surge wave work for any fault?
No. It needs the fault to break down and form an arc. A bolted short circuit between conductor and screen does not arc, and a sheath fault is not in the main insulation. For those cases there are other pinpointing methods, such as magnetic field distortion or step voltage.
How much surge energy should be used?
The amount needed for the fault to break down repeatably and be heard, and no more. The energy of each impulse also stresses the healthy insulation, and in aged cables repeated impulses can create new weak points.