Electrical wiring and cabling is used in wide variety of applications. A cable comprises of multiple conductors and wires that are held and covered together with a covering. It is used to transmit electric power and they are generally buried underground. Underground electric cable are subjects to faults due to vibrations, environmental changes etc. Each cable has a limit for handling the power supply but when it goes beyond to that it sparks. The spark may be minor or major depending upon the situation.
To detect and fix these issues, cable fault detector is used. Neither all the cables of same type nor every fault is same. So, different types of techniques and equipments are required to find and correct the issues. These detectors or cable locators work in very systematic way. They first detect the cable where the fault has occurred, then the locator searches for the exact location of fault to fix it. Fixing a cable is tedious and laborious. There are ample of techniques used to identify and locate the cable faults. Three most commonly used techniques are:
Cable Thumping
Thumper or high-voltage surge generator is a portable device that injects a high voltage DC pulse at the surface of termination of the cable in which the fault has occurred. If there is high voltage which causes underground fault to break down, thumper creates an arc which results into a thumping sound at the exact location of the fault.
Fault location is generally carried out by different measuring techniques and by setting the thumper to thump repeatedly until the thump could be heard. Higher the DC voltage is applied, louder the resulting thump and easier to locate a fault. This technique is not suitable for longer cables.
SIM/MIM (Secondary/Multiple Impulse Method)
It is considered to be the most efficient method of cable testing and fault location. Most of the faults can be easily located by using SIM/MIM. The method locates the fault on the basis of a logical consideration that most frequently structural faults should be looked firstly. It reduces the time required to trace the fault.
It is most preferred due to its different advantages like universal applicability, simple handling, and easy interpretation of the echograms.. High-resistance faults are generated by a surge voltage pulse at the fault. During a single discharge, the IRG echometer check the fault distance five times, which has proven to be ideal in practice. The measurement results are saved automatically.
TDR or Time Domain Reflection Method
TDR is used to determine cable breaks, total length of the cable, and short circuits and their distance. It uses a pulse echo-range finding technique to measure the distance to changes in the cable structure. Its working involves transmitting short duration low voltage pulses at a high repetition rate into the cable. Then it measures the time taken for them to revert back from the areas where the cable has low impendence, known as fault. These reflections are traced on a graphical display.
Generally, a cable in right condition (without any fault) will not cause any reflection until the very end. Low voltage TDR or time domain reflection method is perfect for locating open circuit faults and conductor-to-conductor shorts. It is not used in general practice because the majority of faults in underground cables are of high-resistance, typically measured in millions of ohms.
In addition to these three, other electric cable fault finding techniques are impulse current method (ICM) and differential impulse current method (DICM), decay method and differential decay method, bridge method for cable sheath faults, acoustic pinpointing of faults, and many others. So, use a right technique to find and fix the underground cable fault.
