Discuss the following method of locating underground fault in cable
- Blaviers test
- Muray loop
- Valey loop
Fault Location Techniques for Underground Cables
Underground cables are susceptible to various types of faults, including earth faults, short circuits, and open circuits. Accurate fault location is crucial for efficient repair and restoration of service. Here are three common methods used to locate faults in underground cables:
- Blavier Test
The Blavier test is a simple method used to locate earth faults in a single cable. It involves measuring the resistance between the faulty cable and earth at one end while the other end is open-circuited. Then, the far end is grounded, and the resistance is measured again. By comparing these two resistance values, the distance to the fault can be calculated using a specific formula.
Key Points:
- Simplicity: Easy to implement with basic equipment.
- Limitations: Accuracy can be affected by factors like cable insulation resistance and ground resistance.
- Murray Loop Test
The Murray loop test is a more precise method that requires a healthy cable to be looped with the faulty one. A Wheatstone bridge circuit is used to balance the resistances of the two cables, and the position of the balance point indicates the location of the fault.
Kepy Points:
- Accuracy: Provides accurate fault location.
- Requirement: Requires a healthy cable for looping.
- Varley Loop Test
The Varley loop test is similar to the Murray loop test, but it uses a variable resistance instead of a fixed resistor. This allows for more precise adjustment of the bridge circuit, leading to more accurate fault location.
Key Points:
- Precision: Offers higher accuracy compared to the Murray loop test.
- Complexity: Requires more complex equipment and calculations.
Choosing the Right Method
The choice of method depends on various factors, including the type of fault, the availability of a healthy cable, and the desired level of accuracy.
Additional Considerations: - Time-Domain Reflectometry (TDR): A modern technique that uses a pulse signal to measure the time taken for the pulse to reflect back from the fault location. It provides accurate fault location and can identify multiple faults.
- Frequency Domain Reflectometry (FDR): Similar to TDR, but it uses a continuous wave signal to analyze the impedance variations along the cable.
- Cable Fault Locators: Specialized instruments designed to locate faults in cables, often using a combination of techniques.
By understanding these methods and the factors influencing their effectiveness, engineers can accurately locate faults in underground cables, minimizing downtime and ensuring reliable power distribution.
other methods include
4. Sectionalizing: This procedure risks reducing
cable reliability, because it depends on physically
cutting and splicing the cable. Dividing the cable
into successively smaller sections and measuring
both ways with an ohmmeter or high_voltage
insulation resistance (IR) tester enable to narrow
down search for a fault. This laborious procedure
normally involves repeated cable excavation.
5. Time domain reflectometry (TDR): The TDR
sends a low_energy signal through the cable,
causing no insulation degradation. A theoretically
perfect cable returns that signal in a known time
and in a known profile. Impedance variations in a
“real_world” cable alter both the time and profile,
which the TDR screen or printout graphically
represents. One weakness of TDR is that it does not
pinpoint fault
TYPES OF UNDERGROUND CABLE FAULTS
Cables are generally laid directly in the ground or in
ducts in the underground distribution system. For this
reason, there are little chances of faults in underground
cables. However, if a fault does occur, it is difficult to
locate and repair the fault because conductors are not
visible. Nevertheless, the following are the faults most
likely to occur in underground cables:
- Open_circuit fault
- Short_circuit fault
- Earth fault
- Open_circuit fault: When there is a break in the
conductor of a cable, it is called open_circuit fault.
The open_circuit fault can be checked by a
megger. For this purpose, the three conductors of
the 3_core cable at the far end are shorted and
earthed. Then resistance between each conductor
and earth is measured by a megger. The megger
will indicate zero resistance in the circuit of the
conductor that is not broken. However, if the
conductor is broken, the megger will indicate
infinite resistance in its circuit. - Short_circuit fault: When two conductors of a
multi_core cable come in electrical contact with
each other due to insulation failure, it is called a
short_circuit fault. Again, we can seek the help of
a megger to check this fault. For this purpose, the
two terminals of the megger are connected to any
two conductors. If the megger gives zero reading,
it indicates short_circuit fault between these
conductors. The same step is repeated for othe
conductors taking two at a time. - Earth fault: When the conductor of a cable comes
in contact with earth, it is called earth fault or
ground fault. To identify this fault, one terminal of
the megger is connected to the conductor and the
other terminal connected to the earth. If the megger
indicates zero reading, it means the conductor is
earthed. The same procedure is repeated for other
conductors of the cable.
