If you can't explain it simply, then you don't know it well enough. — Albert Einstein

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How underground cable locator works?

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Before locating a fault in an underground cable, it is necessary to know the route of the cable. How can you locate a fault when you don't know where the cable is? Sometimes you may find maps of the cable routes in the documentation (and that is why a good documentation is necessary for every work). But, when you don't have the map of the cable route, you need to locate and trace it. Locating underground cable is necessary not only for finding faults, but also for undertaking any excavation possibly near a cable route. This article will explain the types of underground cable locators and how they work.
Basically, there are two main cable location methods:
Active Location: This method involves locating a specific line/cable by means of injecting specific frequency signal using a transmitter and the tracing it with a receiver.
Passive Location: This method involves looking for unknown lines by detecting electromagnetic field with the help of a detector or receiver. Though the passive location method does not allow us to distinguish between the type of line, it is necessary prior to an excavation work. Some underground cables may already be energized and, hence, can be located by scanning for their electromagnetic fields. Non-energized underground utilities or cables may be located by scanning re-radition of signals induced on the utility by a radio transmitter.

How cable locator works?

A cable locator generally consists of two modules - a transmitter and a receiver.
A transmitter is basically an AC generator which supplies the signal current to the underground cable to be traced. The signal can be usually set to a specific frequency by the transmitter. This signal creates an electromagnetic field of the same frequency around the cable which can be then detected by a receiver.
Signal can be transmitted into the cable either by direct connection method or by induction method. The direct connection method is the most efficient way of applying a signal to the cable and should be used wherever possible. In this method, the transmitter is connected directly to the cable at one end while the other end is grounded to complete the circuit. While in induction method, there is no need to make any physical connection to the cable. It transmits the signal by inducing it electromagnetically from above the ground.
how cable locator works

A receiver detects the electromagnetic field created by the transmitted signal around the cable. The electromagnetic field can be detected above the ground at the top of the cable. A receiver can also be set to detect electromagnetic field of already energized cables (as in passive locating).
There are basically two types of cable locators depending upon frequency of signal - single frequency and multiple frequency locators.
Single frequency locators, as their name implies, are able to produce a signal at only one specific high frequency. These locators are well effective in noncongested areas where very cables or utilities present under the ground and are distinct from each other. Since high frequency tends to induce into everything that is conductive under the ground, it may produce distorted signals for the receiver. In other words, a high-frequency signal may get induced into a nearby gas pipe or communication line, making it difficult to distinguish the cable.
Copper, aluminum, iron, each metal respond better to a different frequency. A multi-frequency locator helps you to tune the transmitter frequency according to the material of the line. This makes it easy to distinguish between whats needed and whats not. Multi-frequency locator also helps in making the task easier. Higher frequencies are better for picking up more signals while lower frequencies make it easier to follow a particular line.
Many tracers or locators are available that detect 60 Hz or 50 Hz frequencies. These locators allow tracing of energized cables.
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Locating Underground Cable Faults

Finding the type of a fault in underground cables using a megger should not be a difficult task. But, finding the exact location of the cable fault needs special techniques. Two of the popular techniques are the Murray and Varley loop tests for locating faults in underground cables. This article explains about few other popular techniques for locating faults in underground cables - viz. (i) Cable thumping, (ii) TDR, (iii) High voltage radar methods

Cable thumping for locating underground cable faults

A cable thumper is basically a portable high voltage surge generator. It is used to inject a high voltage DC surge (about 25 kV) into the faulty cable. If you supply a sufficiently high voltage to the faulty cable, the open-circuit fault will break down creating a high-current arc. This high current arc makes a characteristic thumping sound at the exact location of the fault.
To find the location of cable fault using the thumping method, a thumper is set to thump repeatedly and then walking along the cable route to hear the thumping sound. The higher the dc voltage applied, the louder will be the resulting thump. This method is useful for relatively shorter cables. For longer cables, the thumping method becomes impracticable (imagine walking along a cable that runs several kilometers to hear the thump).

Advantages and disadvantages of cable thumping

A major advantage of cable thumping is that it can locate open circuit faults very accurately. Also, this method is easy to apply as well as easy to learn.
Though the thumping method provides very accurate fault location, it has its own drawbacks. Applying this method for longer cables is extremely time-consuming. It may take hours or even days to walk along the cable to locate the fault. Moreover, during that time, the cable is exposed to high voltage surges. So while the existing fault is located, the high voltage surges may weaken the insulation of the cable. If you are proficient in cable thumping, you can limit the damage to the cable insulation by reducing the power sent through the cable to the minimum required to conduct the test. While moderate thumping may not cause noticeable damage, frequent thumping may degrade the cable insulation to an unacceptable condition. Also, this technique can not find faults that do not arc-over (i.e. short circuit faults).

Time Domain Reflectometer (TDR)

time domain reflectometer TDR
Megger Time Domain Reflectometer
Source: Wikipedia
A Time Domain Reflectometer (TDR) sends a short-duration low energy signal (of about 50 V) at a high repetition rate into the cable. This signal reflects back from the point of change in impedance in the cable (such as a fault). TDR works on the similar principle as that of a RADAR. A TDR measures the time taken by the signal to reflect back from the point of change in impedance (or the point of fault). The reflections are traced on a graphical display with amplitude on y-axis and the elapsed time on x-axis. The elapsed time is directly related to the distance to the fault location. If the injected signal encounters an open circuit (high impedance), it results in high amplitude upward deflection on the trace. While in case of a short-circuit fault, the trace will show a high amplitude negative deflection.
signal trace on TDR - time domain reflectometer
Signal transmitted through and reflected back from a fault

Advantages and disadvantages of TDR

As a TDR sends a low energy signal into the cable, it causes no degradation of the cable insulation. This is a major advantage of using TDR to find the location of a fault in an underground cable. A TDR works well for open-circuit faults as well as conductor to conductor shorts.
A weakness of TDR is that it cannot pinpoint the exact location of faults. It gives an approximate distance to the location of fault. Sometimes, this information alone is sufficient and other times it only serves to allow more precise thumping. When the TDR sends a test pulse, reflections that may occur during the time of outgoing test pulse may be obscured from the user. This can happen with the faults at near end and called as blind spots. Also, a TDR can not see high resistance (generally above 200 Ohms) ground fault. If there is surrounding electrical noise, it may interfere with the TDR signal.

[Also read: Types of underground cables]

High voltage radar methods

As the low-voltage TDR is unable to identify high resistance ground faults, its effectiveness in finding underground cable faults is limited. To overcome this limitation of TDR, following are some popular high voltage radar methods. (i) Arc reflection method, (ii) surge pulse reflection method and (iii) voltage decay reflection method.

Arc reflection method

The arc reflection method uses a TDR with a filter and thumper. The thumper (or surge generator) is used to create an arc across the shunt fault which creates a momentary short-circuit so that the TDR can show a downward deflection effectively. The arc reflection filter protects the TDR from high voltage surge generated by the thumper and routes the low-voltage signal down the cable.

[Also read: Grading of underground cables]

Surge pulse reflection method

This method uses a current coupler, a thumper and a storage oscilloscope (analyzer). This method is used for long run cables and on faults that are difficult to arc over which do not show up using arc reflection method. In this method, a thumper is directly connected to the cable without a filter which can limit both the voltage and current applied to the fault. The thumper injects a high voltage pulse into the cable creating an arc at the fault, which subsequently causes a reflection of energy back to the thumper. The reflection repeats back and forth between the fault and the thumper till its energy gets depleted. The current coupler senses the surge reflections which are then captured and displayed by the storage oscilloscope.

Voltage decay reflection method

This method uses a voltage coupler, a dielectric test set (high-voltage dc test set or proof tester) and a storage oscilloscope (analyzer). This method is used for transmission class cables when the generation of arc at the fault requires breakdown voltage greater than that a typical thumper or surge generator can provide. Here, the voltage coupler senses the reflections produced by the flashover of dc voltage at the fault and the analyzer captures and displays them.

Reference: http://www.cablejoints.co.uk/upload/Megger_Cable_Fault_Finding_Solutions.pdf
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Difference between Potentiometer and Rheostat

Potentiometer and Rheostat are two terms that are associated with the variable resistors. Technically both these terms represent the two different configurations provided by the same components. After reading this post you'll be able to develop a crystal clear concept regarding both terms.

Introduction to Variable Resistor (VR)

A variable resistor is a three terminal device. It provides a variable value of resistance across electrical circuits. For example, a 9 kΩ V.R will provide resistance ranging between 0-9k.
The most common type of V.R is shown below. It has three terminals a, b, c (We will drive in the details later on). The circular knob can be rotated to achieve variation in output resistance.

variable resistor

As previously mentioned the above type of variable resistor is most common. Meanwhile, it is the oldest one too.
Today's variable resistors are packaged as trimpots (the latest version) with a small bolt on one side. A screw tightener can be used with trimpots for operations.
[Also read: Diodes, Transistors and GTO]

Potentiometer

Let's reconsider the original variable resistor. The potentiometer configuration utilizes all the three terminals in working.
The left side of image displays the circuit diagram for config and right side displays the practical look.

potentiometer

Two blue wires connect to an external circuit for providing a variable voltage to the output. And that is the reason why potentiometer is named so.

Rheostat

This arrangement employs two terminals of a variable resistor in its working. Terminal a connects with the power source, b connects in series with the external circuit and c is left open. The purpose is to achieve a constant value of 'R' so as to achieve a variable current in the connect circuit/device. The left side of image displays the circuit diagram for Rheostat configuration and the right side provides a practical connection for this config.

rheostat

Potentiometer vs. Rheostat : Practical applications

A potentiometer provides variance in voltage at output terminals and is employed in Power industry for controlling the speed of DC Machines. It also finds its application in sound equipment for controlling the audio. The frequency matching on old radio sets utilised repeated principles of both these configurations.
Closing the above discussion, in a nutshell, we can summarise the results:
The potentiometer and rheostat are the two configurations that can be used in electronic circuits and components to achieve a variable voltage and current values.

Author: Guzel Sans completed his bachelor in Power Electrical Engineering. His areas of interest are HF Modelling, Power Systems Protection, and Electronics design Engineering. He loves Programming JS, CSS and playing with HTML5 in leisure hours. He is the founder of online tool Electrical Calculators. Favourite software: MATLAB.
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Loop Tests for Locating Faults in Underground Cables

We have seen the types of faults in underground cables and how to detect them using a megger in the previous article. This article explains how to locate the exact place where the fault has occurred.
There are various methods for locating the faults in underground cables. Following are some popular methods explained.

Murray loop test for location of faults in underground cables

Murray loop test is the most common and accurate method for locating earth faults and short-circuit faults. However, to perform the Murray loop test, it is necessary that a sound (good) cable runs along the faulty cable.This test employs the principle of Wheatstone bridge for fault location.
To perform the Murray loop test, the alongside sound cable and the faulty cable are shorted with a jumper conductor at the far end. The test side end is connected through a pair of resistors to a voltage source. Also, a null detector or galvanometer is connected between the two conductors at the test end. The circuit diagram is as shown in the image below.
murray loop test for location of faults in underground cables
Once the connections are made as shown in the above circuit, adjust the values of R1 and R2 so the null detector/galvanometer shows zero reading. That is, bring the bridge to the balance. Now, in the balanced position of Wheatstone bridge, we have,
murray loop test equation 1
Now, if r is the resistance of each cable,
then, Rx + Ry + Rg = 2r
Putting this in the above equation,
murray loop test equation 2
We know, the value of resistance is proportional to the length of the cable. Therefore the value of Rx is proportional to the length Lx. Therefore,
murray loop test fault distance location formula
Where L is the total length of the cable under test. (The value of L is proportional to the value of Rg.)

Varley loop test

Varley loop test is also for locating short-circuit and earth faults in underground cables. This test also employs the principle of the wheatstone bridge. However, the difference between Murray loop test and Varley loop test is that, in Varley loop test resistances R1 and R2 are fixed, and a variable resistor is inserted in the faulted leg. If the fault resistance is high, the sensitivity of Murray loop test is reduced and Varley loop test may be more suitable.
Varley loop test for location of underground cable faults
To perform Varley loop test, connections are done as shown in the circuit diagram above. Resistors, R1 and R2 are fixed and the resistor S is variable. In this test, the switch K if first thrown to the position 1. Then the variable resistor S is varied till the galvanometer shows zero deflection (i.e. bridge is balanced). Lets say, the bridge is balanced for the value of S equal to S1 Then,
varley loop test equation 1
Now, the switch K is thrown to the position 2 and the bridge is balanced by varying the resistor S. Say, the bridge is balanced at the value of resistor S is equal to S2. Then,
varley loop test equation 2
Now, putting the result of eq.(ii) in eq.(i),
varley loop test equation 3
Since the values of R1, R2, S1 and S2 are known, Rx can be calculated. When Rx is known, the distance from the test end to the fault point Lx can be calculated as,
Lx = Rx/r
Where, r = resistance of the cable per meter.

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Faults in Underground Cables : Types and Detection

One of the major limitations of underground cables is the fault detection. Since the cables are laid under the surface (directly or inside pressurized ducts), the visual methods of inspection don’t work effectively. This is not the case in Overhead Lines. In order to identify the faults in the cable, we need to develop special methods, which will be discussed in this article.
Before we discuss fault detection methods, we shall study the various types of faults occurring in Underground cables and their causes. The faults occurring in cables are:
  • Open circuit fault
  • Short circuit fault
  • Earth faults

Causes of Faults 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. The lead sheath is also damaged frequently, usually due to the actions of atmospheric agents, soil and water or sometimes due to the mechanical damage and crystallization of lead through vibration.
We shall study various faults and how to detect them.

Open Circuit Fault

As the name suggests, this fault involves an open circuit in the conductors. When one or more cable conductors (cores) break, it leads to discontinuity. This discontinuity also occurs when the cable comes out of its joint due to mechanical stress. This is known as Open circuit fault.

Fault detection

An open circuit is characterized by infinite resistance. This is utilized in fault detection. The conductors at the far end are bunched together (shorted) and earthed. Then the resistance between each conductor and the earth is measured using a megger.

Observations

  • If there’s no fault, megger will read nearly zero.
  • If there’s an open circuit in a conductor, the will read infinite when connected between that conductor and the earth.

Short Circuit Fault

It occurs only in multi-cored cables. When two or more conductors of the same cable come in contact with each other, then this is called a short circuit fault. It is impossible to detect visually without taking the cable apart. A short-circuit fault occurs when the individual insulation of the cables is damaged. It can also be detected using a megger.

Fault detection

A short-circuit is characterized by zero resistance. This is utilized in fault detection. The resistance between any two conductors is measured using a megger. This is done for all the conductors, two at a time.

Observations

  • If the megger reads zero, it indicates that a short-circuit fault has occurred between those two conductors.

Earth Fault

When any of the conductors of the cable comes in contact with the earth, it is called an earth fault. This usually occurs when the outer sheath is damaged due to chemical reactions with soil or due to vibrations and mechanical crystallization. It is somewhat similar to a short circuit fault as the current again takes the least resistive path and flows through the earth. This too can be detected using a megger.

Fault detection

The megger is connected between the conductor and the ground and megger reading is noted. This is repeated for all the conductors of the cable.

Observations

  • If an earth fault is present, the megger will show nearly zero reading.

Hence we can detect faults in underground cables using a megger.
detection of underground cable faults using a megger
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