When an underground HT cable fails, the instinct is to start digging where the cable was last jointed. Sometimes that works. Usually it does not, and every wrong trench costs a day of downtime and a repair bill of its own.
A cable fault can be found in hours instead of days. The method has three steps, and none of them begins with a shovel.
Why guessing fails
A buried cable gives no sign of where it has failed. The feeder trips, the insulation test shows a fault, and that is all the information the site has. The route may run for hundreds of metres under roads, floors and other services. Old drawings are often wrong, and joints made years ago are not marked.
So the crew digs at the most likely place, finds a healthy cable, and digs again. Each trench means breaking and restoring a surface, and each one risks damaging the cable or another service.
Step 1: Test and identify the fault
Before locating anything, we test the cable to learn what kind of fault it is: a short between cores, a fault to earth, or an open circuit. The type of fault decides the method that will find it. Skipping this step is the most common reason a location attempt fails.
Step 2: Pre-locate with a TDR
A time-domain reflectometer sends a pulse down the faulted core and reads the reflection. The trace shows the distance to the fault as a share of the cable length, before anyone digs. For a high-resistance fault that a TDR alone cannot see, we use a surge generator with the TDR to make the fault show itself.
Pre-location narrows the search from the whole route to a short section.
Step 3: Pinpoint with a surge generator
A surge generator, often called a thumper, discharges into the cable. At the fault, the discharge makes a small sound and a magnetic pulse underground. Our engineer walks the pre-located section with an acoustic receiver and marks the point where the sound is strongest, usually within about a metre.
That gives one excavation, one repair and one proof test, instead of five trenches dug on guesswork.
Repair, joint and prove
Finding the fault is half the job. After the damaged section is cut out and jointed, we hipot test the repaired cable, so the feeder goes back on load on evidence. The report gives the fault location, the cause where it can be determined, and the test values.
What you can do before we arrive
- Keep the feeder isolated and earthed. Do not keep re-closing on to a faulted cable.
- Find the route drawing, even an old one.
- Note where joints were made and when.
- Tell us the cable size, voltage and length.
These four things shorten the job more than anything else.
What causes most faults
- Digging by other contractors
- Water entering a badly made joint
- Old joints that were never tested after they were made
- Overloading that ages the insulation
Most of these are preventable. A hipot test after every joint, and a marked route, prevent the second fault.
Need a cable fault found?
Tajalli Tech locates underground HT and LT cable faults in Lucknow, Kanpur, Varanasi, Bareilly and across Uttar Pradesh. Call +91 76070 78408 or write to info@tajallitech.com.
Related: Underground cable fault locator service in Lucknow · Cable fault locator in Kanpur · Cable laying, jointing and termination · Contact us