Electricity distribution takes place through power cables that are placed underground. These cables are present inside ducts that insulate and protect them against extreme weather conditions. However, with age, even the best-insulated power cables can fall apart. When this happens, there will be a decrease in their conduction efficiency. It is challenging to locate where exactly the fault lies as the wires are present beneath the ground. You can learn more about fault location here. Some of the common types of underground cables faults are: 

Most Common Faults in Underground Cables

Open Circuit Faults

Open circuit fault is the most common issue with underground cables, which happen when its conductor material breaks. You need to use a megger, a fault locator device, to figure out if there is an open circuit fault in your cabling system. For this, connect the three conductors at the end of your cable to the ground after shortening them. You can read the resistance between the earth and all the conductors using the megger. There is no fault if the resistance is zero. If your cable is faulty, the megger will show infinite resistance. In such a case, it is best to get the cable replaced. 

The Short Circuit Fault

Short circuit fault is another common issue with underground wiring. In a multicore cable, there could be a short circuit fault if there is frequent contact between two conductors. You can test this fault using a megger too. For this, you will have to connect two conductors to the merger’s two terminals. There is a fault in the wiring if there is no reading between the fault detector’s electric conductors. 

The Earth Fault

The Earth Fault is also common with underground power cables. This fault occurs when the ground and the conductors come into contact, and the current passing through the conductor is earthed. You need to connect the earth and the conductor to your megger’s terminals to detect this issue. An earth fault exists if there is a zero reading on your megger.

Locating and Fixing Faults

Before repairing the cables, you need to detect where the fault lies. You will require proper equipment to find the faults. An experienced and trained electric underground wiring expert will know how to use these tools and manage the whole fault detection process to come up with an effective solution. An electrical engineer or electrician uses unique methods to locate underground cable faults:


By physically splicing and cutting the cable, you can reduce its reliability. You need to divide the cable into small sections to reduce its reliability and find the fault. For instance, if you have a 500-ft length cable, you will have to cut it first into two sections of 250-ft length each. After this, you will need a high-voltage insulation resistance (IR) tester or an Ohmmeter to measure the reading in both ways. Your cable is defective if the IR tester shows a ‘low’ reading. Until you reach a short section, you will have to repeat this procedure.


You can also detect faulty cable wires by using noise. For this, you will have to supply a high voltage to a faulted cable. The high-current arc will make a loud noise which you will be able to hear. As compared to Sectionalizing, this method is relatively easier, but it has a few drawbacks. 

At a voltage of 25kV, a high current amp will produce an underground noise. Thumping requires the voltage to be high, or else the sound might not be loud enough to be heard over the ground. Also, the cable insulation could get ruined as the high electric current usually gets very hot. You can limit the damage to the cable if you reduce the power sent through it. Nevertheless, there is a minimum power required to conduct this test. You also need to have the skills to carry out the test. 

Moderate testing might not be able to produce a loud sound that you can hear. If you test frequently, the cable insulators can degrade to a poor condition. However, electrical expert suppliers may accept a certain degree of damage to the cable insulators as there is no existing technology that can replace the thumping method. Nevertheless, newer methods, such as the Time Domain Reflectometry (TDR), use a combination of complex technology to locate cable faults. If you are looking for experts who can detect and repair cable faults, visit HV Diagnostics.