Faults in Underground Cables: Types and Detection
Faults in Underground Cables: Types and Detection
What is the fault in underground cables?
Most of the faults occur when moisture enters the insulation. The paper insulation provided inside the cable is hygroscopic in nature. Other causes include mechanical injury during transportation, laying process or due to various stresses encountered by the cable during its working life. In underground electrical distribution systems, the cables used are mounted in the ground or in some sort of ducts. It makes the cables solid, and there are very few chances of faults in them. If there is a flaw in these cables, it is impossible to find and patch the flaw because the conductors are not visible.
How to find fault in underground cables?
Open-circuit fault
A break in the conductor of a cable is called open-circuit fault. This type of fault is checked with the help of a device called ‘megger.’ In this type of fault, the 3 conductors of the 3-core cable at the far end are shortened and then connected to the ground. The megger is then used to read the resistance between each conductor and the ground. If the megger indicates 0 resistances in the circuit of the conductor, it means it is not broken. But if the megger measures infinite resistance, it means that the conductor is broken which needs to be replaced.
Short-Circuit Fault
When an insulator fails, it is due to the 2 conductors of a multi-core cable coming in contact with each other electrically which indicates short-circuit failure. For this again, a megger is used. In this type, the 2 terminals of the megger are connected to any 2 conductors. The fault is indicated when the megger gives zero reading between the electricity conductors. The same process can be repeated by taking another 2 conductors at a time.
Earth Fault
If a cable’s conductor comes in contact with the earth (ground), then it is called an earth fault. In order to identify this fault, the two terminals of the megger are connected to the conductor and to the earth, respectively. Earth fault can be studied if the megger indicates zero reading. The same procedure is applied to the cable’s other conductors.
Fault identification
Prior to locating a fault, it is necessary to determine the nature of the fault. Isolate the faulty cable and test each core of the cable for earth fault. Check the insulation resistance between the conductors. Short and earth the three cores of cable at one end. Check the resistance between the cores and earth, between individual cores (at the other end) to check open circuit faults. In case there is any fault, the insulation test of individual cores with sheath or armour and between the cores is essential. The test should also be done by reversing the polarity of the insulation resistance tester (megger). In case of any difference in readings. The presence of moisture in the cable insulation is confirmed. The moisture in the cable forms a voltage cell between the lead sheath and conductor because of the difference in the conductivity of these metals and the impregnating compound forms an organic acid when water enters it.
Testing of faulty cable
The cables are tested as per the following test for finding fault. 1. Murray loop test 2. DC charge and discharge test for open circuit fault location 3. Phase to phase fault test for short circuit fault location 4. Fall of potential test for earth fault location 5. Capacity test 6. Induction test 7. Impulse wave echo test 8. Time domain reflectometry test
Methods to Locate Faults
An electrician or electrical engineer uses two methods to locate an underground cable fault.
Sectionalizing
It involves physically cutting and splicing the cable, which can reduce the cable’s reliability. In order to reduce the cable’s reliability, the cable needs to be divided into small sections which enable us to find the fault. E.g. – On a 500-ft length cable, the cable is cut into 250-ft length sections each, and reading is measured in both ways with the help of Ohmmeter or high-voltage insulation resistance (IR) tester. If the reading on the IR tester shows low then it is defective. One has to repeat this procedure until reaching a short section which in turn will allow repairing the fault.
Thumping
This procedure requires noise to detect the fault. When a high voltage is supplied to a faulted cable, the high-current arc makes a loud noise enough to be heard. This method is relatively easier compared to Sectionalizing, but it has its own weaknesses. Thumping requires a high current amp at a voltage as high as 25kV to produce an underground noise, loud enough to be heard above ground. The high electricity current gets really hot, which ruins the cable insulation. Damage can be limited by reducing the power sent through the cable to a minimum required to conduct the test if you are skilled enough to carry out this test. On the other hand, moderate testing may not even produce sound loud enough to be heard. But frequent testing may cause the cable insulators to degrade to an unacceptable condition. Many electrical expert suppliers accept some level of damage to the insulators for 2 main reasons. Firstly, if the thumping method is used for a short period, the cable insulation is damaged. Secondly, there is no existing technology which can replace this method.
Underground cable fault location methods
Time Domain Reflectometry (TDR)
TDR uses low-energy signals through the cable to locate faults which cause no insulation degradation. When the signal is sent, a perfect cable returns the signal in a proper time and profile manner. While an imperfect cable will alter the time and profile, which can be seen on the TDR screen. The graph (called trace) gives the user approximate distances to “landmarks” such as opens, cuts, Y-taps, transformers and water ingression. The TDR method is not flawless. One weakness is that it does not pinpoint faults as this method is accurate only up to 1% of testing range and the information is also not sufficient enough. On other times, it allows more precise thumping which reduces time and cost. Another drawback of this method is that reflectometers become blind to detect faults-to-ground if the resistance is greater than 200 ohms.
High-voltage Radar Method – There are 3 basic high-voltage radar methods.
Arc Reflection
This method uses TDR along with a filter and thumper. The filter limits the surge current and voltage that can reach the cable while running tests, causing minimal stress to the cable. Hence, arc reflection provides an appropriate distance to the fault.
Surge Pulse Reflection Method
This method requires the use of the current coupler and a storage oscilloscope with a thumper. The main advantage of this method is its ability to detect difficult and distant faults. Its disadvantage is its high output surge can damage the cables and reading the trace requires more skill compared to other methods.
The Voltage Pulse Reflection Method
This method uses a voltage coupler and a proof tester. This method finds faults which occur above the maximum thumper voltage of 25kV.
Conclusion
The methods mentioned above help to locate the underground faults. It has become more efficient, less time consuming to fix the issues and has minimized the possibility of programming the electricity cable for additional faults while finding the present fault.