Cable testing
Cable fault location in three steps: analyse, prelocate, pinpoint
When a cable trips out of service, the pressure is to start digging. Experienced crews do the opposite: they spend the first hour measuring, because every metre of trench dug in the wrong place costs more than the test. A reliable workflow has three steps — analyse the fault, prelocate it to within a few metres, then pinpoint the exact spot from the surface.
Before you connect anything: make the cable safe
Isolate the cable at both ends, confirm it is dead, discharge it and apply working earths. Long cables hold a significant charge and can recharge from induced voltage on parallel circuits. Keep the earths on until the test leads are connected, and discharge again after every high-voltage test.
Step 1 — Analyse: what kind of fault is it?
The fault type decides which prelocation method will work, so classify it first.
- Insulation resistance between each core and earth, and between cores, with an insulation tester. This separates low-resistance faults from high-resistance ones.
- Continuity of each core, with the far end shorted, to find open circuits and broken conductors.
- Sheath resistance to earth on cables with an insulating outer sheath, because sheath faults need their own method (see below).
| Fault type | Typical reading | Prelocation method that usually works |
|---|---|---|
| Low-resistance (short) | Below roughly 100 Ω | TDR (pulse echo) |
| Open circuit / broken conductor | Core continuity lost | TDR |
| High-resistance | kΩ to MΩ | Arc reflection (ARM/SIM), impulse current, or bridge |
| Intermittent / flashing | Breaks down only above a certain voltage | Impulse current or decay method |
| Outer sheath fault | Low sheath-to-earth resistance | Murray bridge, then step-voltage pinpointing |
The 100 Ω boundary is a rule of thumb rather than a standard: it is roughly where a fault stops producing a clear reflection on a TDR, because the fault impedance is no longer small compared with the cable's characteristic impedance.
Step 2 — Prelocate: get within a few metres
Time domain reflectometry (TDR)
A TDR sends a low-voltage pulse into the cable and times the echo from any change in impedance. Shorts reflect with inverted polarity, opens with the same polarity. Distance is time multiplied by the pulse's propagation speed, so the result is only as good as the velocity factor you enter. Use the manufacturer's value for the cable type, or calibrate it on a known length or a known joint.
Arc reflection (ARM / SIM)
High-resistance faults are invisible to a plain TDR. A surge generator fires a high-voltage pulse that breaks down the fault, and while the arc is burning it behaves like a short circuit. The TDR records a trace during the arc and compares it with a trace taken at low voltage; the point where the two traces diverge is the fault.
Impulse current and decay methods
For faults that only break down at higher voltage, the instrument records the current transient travelling between the surge generator and the fault. The time between reflections gives the distance. This works on long cables and on faults that will not hold an arc long enough for arc reflection.
Bridge methods
A Murray loop bridge measures the resistance ratio either side of the fault, using a healthy conductor as the return path. It is the method of choice for cable sheath faults and for high-resistance faults on short cables where reflections are hard to read. A high-voltage bridge with a constant-current source can drive enough current through a high-resistance fault to balance the bridge without first burning the fault down.
Step 3 — Pinpoint: confirm the spot before digging
Prelocation gives a distance along the cable, not a position on the ground. Route records, snaking and slack loops can put the real fault several metres from where a tape measure says.
- Route tracing. Inject an audio-frequency signal and walk the route with a receiver to confirm where the cable actually runs and how deep it is.
- Acoustic pinpointing. Repeated surges make the fault discharge with an audible thump. A ground microphone combined with an electromagnetic pickup measures the delay between the magnetic pulse and the sound; the delay is shortest directly above the fault.
- Step voltage. For sheath faults, DC pulses applied to the sheath create a voltage gradient in the soil around the fault. An A-frame with two earth spikes shows the direction to the fault and nulls directly above it.
Common mistakes that cost time
- Using a default velocity factor on an unfamiliar cable type.
- Surging at higher voltage than the cable's test rating allows. Start low and increase only as far as needed for breakdown.
- Skipping route tracing because the drawings "should be right".
- Forgetting that joints, tees and cable type changes also produce reflections — compare with a trace from a healthy phase.
Choosing equipment for your crews
A complete kit covers analysis (insulation tester), prelocation (TDR with arc reflection, a surge generator and, for sheathed HV cables, a bridge) and pinpointing (acoustic receiver and A-frame). Many utilities standardise on one van-mounted system for distribution networks and add a portable bridge for transmission cable sheath testing. Tell us the cable types and voltages you maintain and our engineers will suggest a configuration.