7 Step: How Find Cable Fault Location

How to Find Underground Electrical Energy Transmission Lines Cable Fault?

Finding the faulty point of an underground cable doesn't have to be like finding a lost item on the street. There are many faults locating methods depending on who finds the fault, and how he wants to find it -detection technologies are now much easier to find the cable fault, also important is the technique applied that makes the task much easier and less time-consuming. However, we can say that there is no single method or combination of methods that is universal. Skilled technique and proper tool selection is the major key point to finding the cable fault more easily and within a minimum time.

Types of Cable Faults

When electrical energy is generated in the generation stations or power stations, it is distributed to different loads like- cities, towns, and villages for consumption of electricity. The cables are mainly used to distribute the electrical energy, the cables are may either insulated or uninsulated. The choice of underground cables mostly comes into play when energy is to be transferred in the underground installation process.

Commonly Found types of Cable Faults in the underground Cables transmission line system are as below.

1.   Open-Circuit Fault means there will be no flow of electrical current at all as the conductor is broken. An open circuit fault is a kind of fault that occurs as a result of the conductor breaking or the conductor being pulled out of its joint

2.   Short-circuit or cross fault means the current will flow directly from one cable to another or from one core or multi-core cable to the others.  This kind of fault occurs when the cable insulation between two cables or between two multi-core cables gets damaged.

      Ground or earth faults mean current will not flow through the load but rather ground or earth. This kind of fault occurs when the insulation of the cable gets damaged.

There are two basic methods of locating an underground cable fault.

Sectionalizing means cutting and splicing the cable, dividing the cable into successively smaller sections that will enable finding the faulty point.

Thumping means supplying a high voltage to a faulty cable whose high-current arc makes a noise loud enough for you to hear above ground.

General methods to find the cable fault step by step as below:

Step 1 - Cable Isolation & Safety Procedures: During a cable fault it is always a permanent fault which means the protection devices either in both or one end tripped and isolate the faulty part from the live part of the transmission network. But the faulty cable is still not safe to work on it until is isolated from the system by earthing or grounding both or at least one end. Only after the appropriate procedures have been carried out can any testing personnel be allowed to approach the cable and prepare for testing. 

Step 2 - Cable Identification: There are multiple cables are exit either outside the substation or inside the substation, cable identification testing will identify the correct cable to work on otherwise it may cause much longer outages for the connected customers due to fatal mistakes.

Step 3 - Cable tracing: Cable tracking is important before starting the cable fault finding, generally underground cable laying is brought in a straight line, but rather meanders in depth and direction. Cable tracing is done to determine that the route of the cable is following the expected path. 

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Step 4 - Fault identification: First need to identify the faulty phase from multiple cables and then find the resistance of the fault. Typically, if the fault is found to be below 100 Ohms, a low voltage pulse (e.g., 40 V) from a TDR (time domain reflectometer) can be used. If the fault is a higher resistance (> 100 Ohm), a low voltage pulse will likely not see it, and needs an Impulse generator (shock discharge).

Step 5 – Fault pre-location: A reliable and precise pre-location method is necessary to locate a cable fault quickly and efficiently.

a)   If it is a low resistance fault, pre-location was likely to be the only means necessary for location.

b)   For high resistance faults, ARM (arc reflection) or ICE (impulse current) techniques on an SWG (surge wave generator) need to be used.

Step 6 - Pinpointing:  In most cases, shock discharge generators are used for pinpointing in conjunction with acoustic methods. The discharge creates a loud noise, which is pinpointed precisely using an acoustic pinpointing device.

Step 7 - Re-energization of the cable: After completing the fault finding and all testing and repairs successfully, the cable is then handed back to the appropriate operators to reinstate the cable and re-energize the loads on the newly repaired cable. 

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Causes of Cable Faults

Faults in cables are mostly caused by dampness in the paper insulation cables, Lead or Almunium sheathedXLPE cable can be damaged in many ways. Most of the faults may due to the chemical action of soil on the lead when buried, mechanical damage, and crystallization of the sheath through vibration.