How an Underground Cable Fault Is Actually Found

The expensive part of an underground fault is never the repair. It is the excavation, the driveway, and the reinstatement. Which means the entire value of doing this properly is arriving at a hole in the right place, first time.

This is the sequence a fault actually gets narrowed down by, and why "we will dig and have a look" is the most expensive method available.

Why underground faults behave the way they do

Buried faults have two habits that shape everything about finding them.

They are usually not where the damage was. A cable nicked by a shovel, a post hole or an excavator bucket rarely fails on the spot. The insulation is compromised, moisture gets in over weeks or months, the fault develops, and by the time the power drops out everyone has forgotten about the digging. So the customer's account of when it started and the location of the cause are often unrelated.

They are frequently intermittent before they are permanent. Water ingress faults come and go with rain and with load. A fault that clears when the cable dries out or cools is a fault that will not present itself while somebody is standing there, which is why symptoms get dismissed.

Typical causes, in rough order of how often they turn up: excavation damage, water ingress at a joint or a pit, tree roots and ground movement, rodent damage in conduit, and age-related insulation breakdown.

Step one: prove it is actually the cable

Before any locating equipment comes out, the fault gets bounded. This step gets skipped by people in a hurry, and skipping it is how a perfectly good cable ends up dug up.

The supply is isolated, the cable is disconnected at both ends, and insulation resistance testing is done between each conductor and between conductors and earth. That produces a number, and the number tells you what kind of fault you have:

• A very low reading to earth means a solid earth fault
• A reading that is low but not zero means a resistive fault, usually moisture
• Open circuit on a conductor means a break rather than a leak
• A good reading across the board means the cable is fine and the problem is at a termination, in the board, or at the supply

That last outcome is common enough to matter. A lot of "underground faults" are terminations at the pillar or in the meter box.

Step two: how far along is it

With the fault type established, the next question is distance.

TDR, time domain reflectometry. A pulse is sent down the cable, and the equipment measures what comes back and when. Discontinuities reflect the pulse, so an open circuit, a short or a joint each produce a signature at a measurable distance. It is a radar trace for a cable.

TDR is very good at breaks and dead shorts. It is much weaker on high-resistance faults, which is exactly the type water ingress produces, because a damp fault does not reflect a low-voltage pulse cleanly.

Surge or high-voltage methods, for the resistive ones. Where TDR cannot see the fault, the fault has to be made to announce itself. A controlled high-voltage impulse is applied so the fault breaks down momentarily, which either produces a reflection the equipment can read, or an audible and detectable discharge underground.

This is where it stops being a plug-in-a-meter job. It is also why a general electrician with a multimeter genuinely cannot do this work, and why the honest answer to "can you just find it" is sometimes "not with what is in the van".

Step three: where is the cable, exactly

Distance along a cable is not much use without knowing the cable's route, because buried services rarely run in straight lines and almost never run where the plans say.

Cable tracing. A signal is injected onto the cable and followed from the surface with a receiver, which gives the route and the depth. Done properly it also finds the things you do not want to hit: water, gas, comms and other electrical services sharing the trench or crossing it.

This is also the step where dial-before-you-dig information gets checked against reality rather than trusted.

Step four: pinpointing

Distance plus route gets you to within a few metres. That is still too big a hole in a driveway.

Acoustic pinpointing. With a surge applied, the fault discharges and makes a noise. Ground microphones pick it up, and the loudest point is the fault. It is a slow, patient process and it is what turns a few metres into a metre.

Earth gradient or step voltage. For earth faults, current flowing into the ground at the fault creates a measurable voltage gradient at the surface. Walking a pair of probes across the ground and watching where the reading reverses puts you on top of it.

Thermal imaging, where the fault is shallow or in a conduit that is accessible. A fault under load runs hot, and a camera sees it. More often useful on terminations and joints than on a buried run, but it costs nothing to look.

Then, and only then, excavation

By this point the hole should be a metre or so of trench in a known spot, clear of other services, with the repair method already decided.

The repair itself is a joint or a section replacement, done to the applicable rules, tested, and certified. Where the faulted cable is the service line between the network and the property, that is Level 2 ASP work, because it is on the connection.

Sometimes the right answer at this point is not a repair. If the cable is old, the fault is one of several, or the route is being dug up anyway, replacing the run is better value than patching it. And if the fault is in a driveway that has to come up regardless, that is the moment to consider whether the run should be rerouted.

What this costs you if it is done badly

The failure modes are worth naming, because you may be being quoted one of them:

• Digging on a guess. One hole in the wrong place costs more than the entire locating exercise.
• Replacing the whole run without locating. Sometimes the right answer, but it should be a decision, not a default.
• Repairing the symptom. A termination remade at the board, on a cable with a developing fault 20 metres away, buys a few weeks.
• Not checking for other services first. Hitting a water main or a gas service turns an electrical job into a much worse day.

Frequently asked questions

My power drops out and comes back, and nothing has tripped. Is that an underground fault? It is one of the likely candidates, along with a degrading termination and a supply-side neutral problem. The common thread is that all three are upstream of your board, which is why nothing on the board reacts. The wider symptom list is here.

Can you find a fault without cutting the power? The locating work needs the cable isolated and disconnected, so there will be an outage. It is normally a few hours rather than a day.

Someone dug in my yard months ago. Could that be it? Very commonly, yes, and the delay is typical rather than suspicious. Excavation damage that lets moisture in fails weeks or months later.

How accurate is the location? Done through the full sequence, pinpointing to around a metre is a realistic expectation on a domestic service. Distance-only methods like TDR alone get you to a zone, not a spot.

Is it cheaper to just replace the cable? Sometimes. On a short run with easy ground, replacement can beat locating plus repair. On a long run under finished surfaces, locating almost always wins. The assessment should tell you which situation you are in rather than assuming.

Would converting to underground avoid this? Different question. Underground supply trades a visible failure mode for an invisible one; it is not maintenance-free, it just fails differently. The conversion decision has its own trade-offs.

If your power is dropping out with nothing to see

Call 0485 824 714 and describe the pattern: when it happens, whether it correlates with rain or load, and whether anything has been dug near the property in the last year. That pattern narrows the likely fault type before anyone arrives.

We are accredited Level 2 ASPs under the NSW Government's ASP Scheme, authorised to work on the distribution network, with 15 years on connection-side work and a Lifetime Labour Warranty on what we do. Free quotes, across Sydney and the Blue Mountains on both the Ausgrid and Endeavour Energy networks.