Underground Cable Faults in Lagos: How We Find the Fault Without Digging Up the Whole Road

When an underground cable fails, the supply to a building, estate or factory can go off for days. The old approach was to dig along the cable route until the damage was found. In a city like Lagos, where cables run under roads, paved compounds and other people’s property, that is slow, expensive and disruptive. Modern fault location finds the fault to within a short distance before anyone picks up a shovel.

Why underground cables fail

• Mechanical damage: excavation for roads, drains, telecom ducts and construction is the most common cause.
• Failed joints and terminations: poor workmanship, moisture ingress or the wrong jointing kit.
• Water ingress: damaged sheaths let water in, especially in low-lying and flood-prone areas during the rainy season.
• Overloading and ageing: cables that run hot for years lose insulation strength.
• Ground movement: settlement and heavy traffic can stress cables and joints.

How we locate a cable fault

1. Make safe and identify the cable

The faulty cable is isolated, proved dead and earthed, under a permit-to-work. Where several cables share a route, the correct cable is identified before any testing.

2. Test to understand the fault

Insulation resistance tests between cores and to earth show what type of fault it is: a short circuit, an earth fault, an open circuit or a high-resistance fault. The type of fault decides which locating method will work.

3. Pre-location: measure the distance

Instruments such as a time-domain reflectometer (TDR) send a pulse down the cable and measure the reflection from the fault. High-resistance faults often need a surge generator combined with arc reflection methods. The result is an estimated distance to the fault from the end of the cable.

4. Trace the route

Cable routes are rarely recorded accurately. A cable locator traces the actual path on the ground so the measured distance can be marked in the right place.

5. Pinpoint the exact spot

A surge generator sends repeated pulses into the cable. At the fault, each pulse makes a small flashover that can be heard and detected through the ground with acoustic and electromagnetic receivers. This narrows the fault down to a small area, often within a metre or so.

6. Excavate, repair and joint

Only a small pit is dug at the fault. The damaged section is cut out and replaced with a properly made joint, using a heat-shrink or cold-applied kit rated for the cable’s voltage.

7. Test and recommission

The repaired cable is tested, for example with insulation resistance and VLF or DC withstand testing, before it is put back into service.

How to reduce cable faults

• Keep accurate route drawings and mark routes on site
• Supervise any excavation near your cables
• Use qualified jointers and the correct jointing kits
• Test critical cables periodically, so deterioration is found before failure

Learn how commissioning tests protect new installations in our guide to planning HV switchgear installations.

Talk to Corustar

Corustar Technological Services is an electrical engineering contractor based in Ikeja, Lagos, working across Nigeria. See our Cable Jointing & Fault Location service, request a quote, or call +234 816 976 9672. For outages and faults, call our 24/7 emergency line on 0705 739 9297.