Cable Fault Location: Pre-location and Pinpointing Methods

Cable Fault Location: Pre-location and Pinpointing Methods is the two-stage discipline of finding a fault in an underground power cable: first narrowing it to a short section along the route (pre-location), then marking the exact spot for excavation (pinpointing). Underground cables cannot be inspected visually, so the workflow relies on measurements from the cable ends, then walking the route with a receiver.

Why a two-stage approach

Distance measurements from a cable end are accurate to within a few percent of the length, which on a long feeder can still mean tens of metres of trench, far too coarse to open the road once. Pre-location narrows the fault to the right manhole or section; pinpointing then resolves it to within roughly a metre using surface detection. Skip either stage and you waste a day or dig on a flawed estimate.

Identifying the fault type first

The correct method depends entirely on the fault, so insulation-resistance and continuity tests at the outset tell you what you are dealing with before you choose a technique.

• Open circuit: a broken conductor with no continuity.
• Short circuit or low-resistance fault: conductor-to-conductor or conductor-to-sheath contact with low fault resistance, often below a few hundred ohms.
• High-resistance fault: insulation breakdown that only conducts at high voltage; resistance can be many kilohms to megohms and is invisible to low-voltage instruments.
• Sheath fault: damage to the outer jacket letting current leak to earth, located by voltage-gradient methods rather than reflectometry.

These simple checks classify the fault and steer the whole job. For a refresher on the insulation side, see our note on motor insulation classes.

Pre-location: time-domain reflectometry (TDR)

A TDR, or cable radar, injects a low-voltage pulse and times the reflections. It converts the round-trip time to distance using the cable velocity of propagation (VOP), commonly around half the speed of light for power cables. An open circuit reflects the pulse with the same polarity; a short or low-resistance fault reflects it inverted. TDR is fast and non-destructive but cannot see a high-resistance fault on its own, because a small pulse will not break down the insulation to create a reflection.

Pre-location for high-resistance faults: arc reflection and surge methods

Most in-service faults are high-resistance, so two high-voltage techniques dominate:

• Arc reflection (ARM): a surge generator fires a high-voltage impulse that ionises the fault into a temporary arc, and a TDR captures the reflection during it. Overlaying the healthy and arc traces shows where they diverge, which is the fault distance.
• Impulse current method (ICE): the surge generator repeatedly discharges into the cable while a coupler records the transient current wave reflecting between the fault and the cable end. This suits high-resistance, intermittent and flashing faults, and long cables where arc reflection struggles.

The Murray loop bridge, a Wheatstone-bridge resistance ratio, still helps when a healthy parallel conductor is available.

Pinpointing: the surge generator (thumper) and acoustic detection

At the estimated spot, a surge generator, commonly called a thumper, discharges a high-voltage capacitor into the cable every few seconds. At the fault the discharge arcs across, producing a mechanical thump and an electromagnetic transient. The operator walks with two sensors:

• Acoustic ground microphone: hears the thump through the soil; loudest directly above the fault.
• Electromagnetic (surge) receiver: detects the magnetic pulse, and the delay between it and the later acoustic signal shrinks toward zero as you stand over the fault.

Combining both signals resolves the fault to within about a metre. For jacket faults, an earth-gradient (step-voltage) survey traces the leakage current to the breach instead of listening for an arc.

Fault type versus best method

Supporting insulation and diagnostic tests

Locating the fault is half the job; you also need to know whether the rest of the cable is fit to re-energise. Two diagnostics are standard on medium-voltage cable:

• Very Low Frequency (VLF) testing: applies AC at 0.1 Hz to withstand-test or provoke weak points without the bulky sources a mains-frequency test would need. It is widely used to prove a repaired cable before return to service.
• Tan delta (dissipation factor): measures dielectric loss to grade overall insulation ageing and water-tree damage, spotting a cable that is degrading globally rather than at one point.

These fit into a broader condition-monitoring routine; see our overview of partial discharge testing for the online side of cable diagnostics. Fabrico holds the test records, VLF and tan delta results, and repair history against each cable asset so trends stay visible. Book a Fabrico demo to see how the asset history fits together.

Frequently Asked Questions

Why can a TDR not find a high-resistance fault on its own?

A standard TDR pulse is too weak to break down a high-resistance fault, so it produces no reflection there. Arc reflection solves this by using a surge to ionise the fault into a temporary arc the TDR can then see.

Is a thumper safe to use on any cable?

The cable must be de-energised, isolated and earthed, with the surge energy matched to its rating. Too much thumping energy can enlarge the fault or damage sound insulation, so operators use the minimum that gives a clear signal.

When should VLF and tan delta be run?

Run them after a repair to prove the cable before re-energising, and periodically as condition monitoring. VLF provides a withstand or diagnostic voltage at 0.1 Hz, while tan delta grades dielectric loss to reveal ageing no single fault-location shot would catch.