The two stages exist because they operate at different resolutions and use different physics. Pre-location measures a distance from one end of the circuit. Pinpointing detects the fault from the surface directly above it. One produces a number in metres from a known point; the other produces a position within an excavation.

Why location is a two-stage method

Pre-location methods work from the end of the cable. They send a signal along the cable and interpret what comes back, deriving a distance from the time the reflection takes to return or from the behaviour of the signal as the fault is approached. The distance they produce is accurate relative to the propagation velocity assumed for the cable, and that velocity depends on the insulation type and on the construction.

The inherent uncertainty in that distance comes from several sources. The propagation velocity is a nominal value for a cable type rather than a measurement of the specific drum length. The route length recorded on the drawing may not match the cable length installed, because cable follows a route in three dimensions including vertical drops, slack coils and deviations. The time measurement itself has a resolution limit.

The consequence is that pre-location gives a search window rather than a position. On a short circuit that window may be a few metres. On a long transmission cable it can be tens of metres, which is more than enough to require a second resolution. Pinpointing is the stage that converts the window into a position.

Pre-location: time domain and its limits

Time domain reflectometry sends a pulse along the cable and records the reflections that return. A low-resistance fault between conductors produces a clean reflection at the fault, and the distance follows from the time between the outgoing pulse and the reflection, using the cable’s propagation velocity.

The method is quick, requires no high voltage, and is the first technique to apply where the fault is a solid short. Its limits appear as soon as the fault is not a clean one. A high-resistance fault reflects only a small proportion of the pulse, and the reflection may not be distinguishable from the reflections produced by joints, water trees in the insulation or the far end of the cable.

The second limit is a fault that only appears under voltage. An intermittent fault that is open circuit at low voltage presents no reflection at all, because it is not conducting. The method that suits this type is a surge generator used in combination with the reflectometer, where the surge causes an arc at the fault and the arc provides the reflection that the pulse alone could not produce.

Pinpointing methods and when each applies

Pinpointing methods detect a signal at the surface above the cable. The three in common use are the acoustic method, the electromagnetic method and the combination of the two, and each suits a different fault condition.

The acoustic method uses a surge generator to produce a discharge at the fault, which generates a sound wave that travels through the ground to the surface. A technician with a listening device detects the strongest signal and marks the position. The method works well on faults that break down with a definite discharge, and it is most effective at modest depths where the sound reaches the surface with enough amplitude.

The electromagnetic method detects the magnetic field produced by the surge current as it travels along the cable and into the fault. The signal is strongest directly above the cable and changes character at the fault, where the current path leaves the cable. The method suits faults that are less audible and routes where the cable runs close to the surface.

Using both methods together is standard practice on difficult faults, because they respond to different aspects of the same discharge. Agreement between an acoustic peak and an electromagnetic change is much stronger evidence than either alone, and the combination also helps where one method is affected by a hostile environment such as a busy road surface.

Fault type and resistance decide the technique

The table reflects a practical rule: choose the method from the fault condition rather than from the equipment available. A surge-based pre-location applied to a clean low-resistance fault introduces stress the cable does not need, and reflectometry applied to an intermittent fault produces a trace with no useful information.

Grounding, safety and access at the fault point

Fault location equipment includes a surge generator capable of delivering a substantial energy to a fault that may be close to the surface, in a trench that may be shared with other services. The safety arrangements are those of a high-voltage test, with the addition that the discharge point is unknown and may be under a road, a footpath or a garden.

The cable must be isolated and earthed at both ends before work begins, and the earthing arrangement established for the duration. Where the fault location involves applying a surge, the area above the likely position should be controlled so that no one is standing over the cable when the surge is applied, because a fault near the surface can produce a discharge at ground level.

Access matters as much as safety for the practical outcome. Pinpointing requires the equipment to be positioned along the route, which means access along a road, a verge or a private property. Where access is restricted, the equipment has to be carried or the search has to be conducted in stages, and either way the time required increases. Knowing the access conditions before the team arrives is part of the planning.

What field data shortens the search

The inputs that shorten a fault search are all records that should already exist. A route trace showing the cable position in plan and section, with the positions of joints, terminations and any known deviations, allows the search to begin at the pre-located distance and proceed efficiently rather than by following the route from one end.

The cable type and its propagation velocity establish the constant used to convert a time measurement into a distance. Where the velocity is not recorded, an estimate has to be used, and the resulting distance carries an additional uncertainty that could have been avoided.

The history of previous faults and repairs is the third input. A route that has failed before is statistically likely to fail again at a joint that was repaired or at a section that was replaced, and knowing where those are allows the search to concentrate. The record produced by the previous fault is therefore an input to the current one.

Recording and confirming the located point

The located point should be confirmed before excavation. The confirmation is a repetition of the pinpointing at the marked position, and where the two methods are in use, agreement between them. Marking the position with a survey reference rather than only with paint allows the position to be found again if the excavation is delayed.

The record should state the pre-location distance, the method used, the propagation velocity assumed, the pinpointed position with its distance from the nearest known joint or marker, the depth if established, and the date. After excavation, the fault type found should be added, along with the repair performed.

That record does two things. It makes the next fault on the same route faster to locate, and it allows the pattern of failures on a route to be reviewed. A route that fails repeatedly at joints is a different problem from one that fails in the cable body, and the distinction only becomes visible when the positions are recorded systematically. The cable types and their construction are defined in IEC 60502-2 for medium voltage extruded cables and in IEC 60840 for higher voltage classes, with the highest extruded classes covered in IEC 62067. Cable system documentation from the manufacturers, such as the material published by Prysmian and Nexans, carries the construction details that determine the propagation velocity, and field experience with cable diagnosis is coordinated through CIGRE study committees.

Common errors that double the search time

The most expensive error is excavating on the pre-location distance without pinpointing. It converts an uncertainty of tens of metres into an open trench of tens of metres, and it delays the repair while the actual fault remains undiscovered.

The second is using the wrong method for the fault condition, which produces either no result or a misleading one. The third is failing to control the surge during acoustic pinpointing, so that the discharge occurs at a poor connection in the test lead rather than at the fault, producing a signal at the wrong position.

The fourth is an incomplete record of the previous work on the route. Where earlier repairs are not plotted, the pinpointed position may correspond to a joint that was known to be a weak point and does not appear on any drawing. The fifth is time pressure: a search conducted at the end of a long shift produces a rougher result than the same search conducted with the route data available and the equipment set up properly, and the difference is usually measured in excavation.

Deploying the right equipment for each fault type

A complete fault location set covers pre-location, surge generation and pinpointing in one arrangement, because the method has to follow the fault. Sets that provide only one of the three functions make the choice of method a consequence of the equipment rather than of the fault, which is the wrong direction.

The equipment should be specified against the voltage class and the fault types the team expects, with the surge energy matched to the cable’s capability and the pinpointing capability matched to the likely depths and surface conditions. The cable acceptance and withstand testing that follows the repair is covered in the accompanying article on cable withstand testing after repair, and the cable-specific fault and system testing range is grouped on the cable fault testing hub.