Locating a fault in an underground cable is a staged process: first the fault is analyzed (which phase, what type and what resistance), then the distance from one end is prelocated with reflectometry or surge methods, then the real route of the cable is traced, and finally the exact point is marked on the ground with a surge wave or step voltage. Done this way, you dig only where the damage is and repair with a splice, instead of opening the whole trench or replacing the entire section. IEEE 1234 is the reference guide for shielded cables.
What cable fault location is
When a buried cable fails, the protection opens the circuit and the cable stays out of service until the damage is found. The problem is where: the cable can be hundreds of meters or several kilometers underground, with joints, crossings and sections in duct. Digging blind costs time, permits and road repair; replacing the whole section costs more.
Fault location is the set of measurements that leads from “the cable failed” to “the damage is here, under this point”. The reference guide is IEEE 1234-2019, Guide for Fault-Locating Techniques on Shielded Power Cable Systems, applicable to all voltage classes of shielded insulated cables. Its stated purpose is to locate the fault quickly and safely, with the least risk of damaging the cables, accessories and equipment that remain healthy, and it includes a help table for choosing the technique according to the fault.
The four stages
The manufacturer BAUR describes the process in four steps, which match field practice:
- Circuit tripped and isolated following the safety rules
- Fault analysis. Faulted phase, type and fault resistance
- Prelocation. Distance from one end: TDR, arc reflection, surge current, decay or bridge
- Route tracing. Real path of the cable on the ground
- Pinpointing. Point at the surface by acoustic surge wave or by step voltage
- Cable identification. Confirm which cable in the trench is the faulted one before cutting it
- Targeted excavation and repair
1. Fault analysis. With an insulation resistance measurement the faulted phase and the type of fault are identified. With a voltage test the insulation strength is checked. With the sheath test it is seen whether the damage is in the outer sheath. The result defines the next method.
2. Prelocation. The distance to the fault is measured from one end. Reflectometry (TDR) works for low-resistance faults and opens. High-resistance and intermittent faults need high-voltage methods: arc reflection (SIM/MIM or ARM), surge current (ICM) or decay. Sheath faults are prelocated with a measuring bridge.
3. Route tracing. Prelocation gives a cable length, not a position on the ground. If there are no reliable route drawings, the route is traced with a transmitter and a receiver.
4. Pinpointing and identification. Along the route, near the prelocated distance, the exact point is marked: with a surge wave generator and acoustic listening for faults that arc, or with the step voltage method for sheath faults. When the cables are uncovered, the faulted one is identified, because a trench usually holds several.
Fault types and the method that usually works
| Fault type | How it shows in the analysis | Usual prelocation | Pinpoint |
|---|---|---|---|
| Low resistance or short circuit | Low resistance between phase and shield or between phases | TDR | Twist or field distortion method, depending on the cable; the surge wave does not work if the fault does not arc |
| High resistance | High resistance, but the cable does not hold the voltage | Arc reflection (SIM/MIM), ICM | Acoustic surge wave |
| Intermittent or voltage dependent | It breaks down only above a certain voltage | Decay, DC-SIM/MIM, conditioning | Acoustic surge wave |
| Conductor open | Open continuity | TDR | Depends on the case |
| Sheath fault | Sheath test with high leakage current | Measuring bridge (Murray or Glaser) | Step voltage |
The details of each method are in the guides on cable diagnostic testing and in those of each technique in this series.
Why it avoids replacing the whole cable
A medium-voltage cable that fails at one point is almost never damaged along its whole length. If the cause is local (third-party excavation, a badly made joint, a stone that punctured the sheath), locating precisely allows a small pit to be dug, the damaged section to be cut out and one or two splices to be installed. If the cable is aged along its whole length, the decision changes, and for that it is complemented with an insulation diagnostic such as VLF tan delta; the criteria are in splice or replace a medium-voltage cable. What is not advisable is deciding without knowing where or why it failed.
What changes at medium voltage, low voltage and DC
- Shielded medium voltage: this is the case that IEEE 1234 covers. The shield gives a defined return path for the pulses and for the high-voltage surge. In collector networks of solar plants and in industry it is the most common scenario; see the guide on the medium-voltage collector network.
- High voltage: the circuits are longer and of higher capacitance. BAUR recommends surge current (ICM) for long cables, where the TDR pulse arrives too attenuated.
- Low-voltage and DC cables of solar plants: almost always unshielded and with insulation that does not accept the same surge voltages. The principles hold, but the technique is adapted according to the equipment and cable manual; see faults in buried low-voltage and DC cables.
Safety before measuring
Fault location works with surge voltages of several kilovolts and with energy stored in the cable capacitance. Before connecting any equipment the circuit is disconnected, locked out, verified de-energized and grounded, as in any lockout and tagout. IEEE 1234 devotes part of its content to grounding and sectionalizing for that reason. Adjacent energized cables and terminations in neighboring switchgear are risks that are resolved in the work plan, not at the moment.
What not to do
- Dig with the reflectometer distance alone, without tracing the route.
- Apply repeated, uncontrolled surge waves to an aged cable: the surge energy also stresses the healthy insulation.
- Cut a cable in a shared trench without identifying it first.
- Replace the whole section without knowing the cause: if the problem is the sheath or an accessory, the new cable can repeat the fault.
Cable testing with Tiza Group
Our team works in substations, industry and solar plants with test equipment from recognized manufacturers. If you have an underground circuit out of service or a cable network to accept, we help you plan and carry out the tests on your network. Field work is done in Colombia, or elsewhere on request. See the cable fault location service or write to us with the length, the cable type and what the protection saw.
Related guides
- Power cable diagnostic tests
- Power cables: parts, types and selection
- Medium-voltage joints and terminations
- Medium-voltage collector network of a solar plant
- TDR reflectometry on cables: how to read the trace
- High-resistance faults in cables: ARM, ICE and decay
- Measuring bridge for cable faults: Murray and Glaser
- Route tracing and identification of buried cables
- Pinpointing faults: surge wave and acoustic
- Sheath faults: step voltage location
- Cable sheath test: DC voltage and leakage current
- Splice or replace an MV cable after a fault?
- Faults in buried low-voltage and DC cables
References
- IEEE Std 1234-2019, Guide for Fault-Locating Techniques on Shielded Power Cable Systems.
- BAUR GmbH, Cable fault location: process steps and methods (manufacturer technical page).
- Megger, technical documentation of pinpoint locators (acoustic and electromagnetic detection).
Frequently asked questions
Why is the distance given by the reflectometer not enough to dig?
Because the reflectometer measures the electrical length of the cable, not the position on the ground. The cable does not run straight: it has bends, slack and changes of depth, and the propagation velocity has uncertainty. That is why, after prelocation, the route is traced and the exact point is marked before digging.
What do I do first if a medium-voltage circuit tripped and will not re-energize?
Isolate and ground the circuit following the safety rules, identify the faulted phase with an insulation measurement and characterize the fault: whether it is low resistance, high resistance, intermittent or a sheath fault. That characterization decides which prelocation method works.
Can a fault be located in a low-voltage or DC cable of a solar plant?
Yes, with the same principles, but the conditions change: low-voltage cables usually have no shield and accept less surge voltage. IEEE 1234 is written for shielded cables, so at low voltage the methods are adapted according to the equipment manual.