Fault Locating Applications 4 Book 1/8... · connection to the earth. The resistance of a fault can be measured in ohms by use of a meggar or ohm meter. High resistance faults do

March 29, 2005

S T U D E N T M A N U A L

Fault Locating Applications

Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved

2 S T U D E N T T R A I N I N G M A N U A L

Prerequisites: • Locating Theory module• Cable Locating Applications module

Objectives: Explain how to locate a cable sheath fault.

Rationale: Faults that cannot be located may require entire lengths of cable to be replaced. If faults can be located, the effort and disruption to private property can be lessened when restoring service.

Learning Objectives• Explain high and low resistance faults.• List the equipment necessary to find a cable sheath fault under soil.• Explain the theory of how the A-frame indicates a fault position.• Explain how to prepare a cable for fault finding.• Explain how to find a cable fault in soil.• Explain how to find a cable fault under a concrete surface.

Learning Methods• Self-learning + On-the-job• Self-learning + On-the-job• Self-learning + On-the-job• Self-learning + On-the-job• Self-learning + On-the-job• Self-learning + On-the-job

EVALUATION METHODS

• Knowledge Test• Knowledge Test• Knowledge Test• Knowledge Test• Knowledge Test• Knowledge Test

F A U L T L O C A T I N G A P P L I C A T I O N S 3


STUDENT RESOURCES

• Locating Theory module• Cable Locating Applications module• Your Fault Locator Owner’s Manual

Learning Steps1. Read the Learning Guide.2. Follow the steps outlined in the Learning Guide.3. Clarify any questions or concerns you may have.4. Complete the Practice and Feedback.5. Complete the Evaluation.



Lesson 1: Cable FaultsLearning Objective:Explain high and low resistance faults.Learning Method:Self-learning + On-the-jobEvaluation Method:Knowledge Test

Fault Current

Fault current is the current that flows to ground when a live cable isgrounded. This happens when there is a breakdown in the insulationsurrounding a conductor. An energized, faulted cable may have large,none or little current flowing to ground at the point of the insulationbreakdown. Respectively, the resistance can be low, high, or somewherein between. The amount of fault current will depend (in part) on theresistance between the conductor and the earth at the fault location.

High Resistance Faults

A high resistance fault means that a conductor does not have a goodconnection to the earth. The resistance of a fault can be measured inohms by use of a meggar or ohm meter. High resistance faults do notgenerate any significant current flowing from the cable into the ground.It is therefore difficult to locate such a fault with standard cable/faultlocating equipment.

High resistance cable faults often do not affect the normal flow ofelectricity in an electrical circuit, and remain unnoticed in dry or sandysoil. These faults become noticeable after the cable goes through stagesof oxidization, heating and contamination. The end result is that theconductor turns into a high resistance powder and will not allow currentto flow to the customer or the earth.

Low Resistance Faults

Low resistance faults have a small opposition to current flowing into theground. Low resistance faults generally create high enough fault currentto cause overcurrent protection to trip or blow open. Operators becomeaware of faults when customers complain about the loss of power.

Low resistance faults are generally easy to find using low cost, faultfinding apparatus.



Lesson 2: Fault Locating EquipmentLearning Objective:List the equipment necessary to find a cable sheath fault under

soil.Learning Method:Self-learning + On-the-jobEvaluation Method:Knowledge Test

Introduction

Low cost fault locating equipment consists of a transmitter, receiver, A-frame, connecting cables and the ground probe.

The purpose of the transmitter is to put an alternating current onto theconductor.

The same transmitter is used for cable locating.

The receiver is used to provide a means whereby the operator can see thesignal results of being closer or further from the fault.

The receiver is connected directly to the A-frame.



The A-frame (or earth contact frame) is used to probe the ground alongthe cable path when searching for a fault.



Figure 1. Earth Contact Frame

Some models have an aerial built into the A-frame. Other modelsrequire the A-frame to be connected to the receiver’s aerial.

The connecting cables and the ground probe are necessary in order tomake the necessary connections as per the operator’s manual for faultlocating.



Figure 2. Direct Connect Transmitter Cable

Figure 3. Earth Frame Receiver Cable



Figure 4. Ground Rod or Probe



Lesson 3: Fault Locating TheoryLearning Objective:Explain the theory of how the A-frame indicates a fault position.Learning Method:Self-learning + On-the-jobEvaluation Method:Knowledge Test

Introduction

Sheath faults on electrical circuits become apparent when there is apermanent disruption in electrical service to the intended customer.

When current flows to ground from a cable fault, the current will flowoutward in all directions from the fault point. If a voltmeter’s probeswere put into the ground on the east side of the fault, there would be avoltage reading in either the positive or negative direction. Avoltmeter’s probes put in on the west side would produce the oppositedirection of voltage. We will see that the A-frame works in a like mannerto indicate the direction of a cable fault.

Consider the transmitter as a battery connected across a faulty conductorand ground. The path of the current will be along the cable to the fault.From the fault, the current will flow out in all directions and then backto the battery.



If an A-frame has it’s probes into the ground at a point along the cablepath, there will be (in theory) a voltage difference between the probes.The A-frame would also indicate the direction of voltage (positive ornegative). The direction of the voltage at the A-frame probes dependsupon what side of the fault the probes are on.



In this manner, the A-frame will indicate the direction of the cable fault.When the “display” indicates neither positive or negative, it means thatthe probes are equidistant from the fault.



Lesson 4: Preparing the CableLearning Objective:Explain how to prepare a cable for fault finding.Learning Method:Self-learning + On-the-jobEvaluation Method:Knowledge Test

Introduction

Before the fault locating apparatus is used, the cable must be prepared.The only ground contact to the cable should be at the point of the fault.The following must be accomplished:

• Isolate and de-energize the faulted cable.• Remove all grounds and ground bonds from the cable.



Lesson 5: Pinpointing Cable FaultsLearning Objective:Explain how to find a cable fault in soil.Learning Method:Self-learning + On-the-jobEvaluation Method:Knowledge Test

A-Frame Limitations

The A-frame will locate sheath faults on cables in permanent contactwith the soil. The A-frame will not detect faults on lead cables or metalarmoured cables unless the fault current can flow into the soil. Likewise,faulted cables in ducts cannot be found using the A-frame. There aremore sophisticated and costly methods of locating faults for thesesituations.

Fault Finding Procedure

• Prepare the cable.• Locate the cable route of the target cable.• Connect the transmitter directly to the cable and the receiver to the

A-frame.• Move the A-frame (and receiver) along the cable path and stop to

penetrate the probes in the soil every few paces.• Follow the display on the receiver until the fault is crossed.• Backtrack until the display indicates that the probes straddle the

fault.• Put one probe on the fault and rotate the other probe at various points

in a circular fashion. The display should always point to the probe

Reference

Your Locator’s Operating Manual has specifics aboutfrequency, setup, etc.

CAUTION!Never connect the transmitter, using the directmethod, to a live cable or a cable capable of deliveringenergy! (Some models may allow direct connect to240V.)



over the fault.



Lesson 6: Remedies for Concrete SurfacesLearning Objective:Explain how to find a cable fault under a concrete surface.Learning Method:Self-learning + On-the-jobEvaluation Method:Knowledge Test

Introduction

If a cable is buried under paving or concrete, the probes of the A-framecannot be made to get sufficient ground penetration for a good signal.The following techniques can be used and may work to overcome thesesituations.

Water• Pour water over the road or fasten damp sponges to the A-frame

spikes. This may be adequate to simulate good probe penetration.

Extension of Probe• Put one probe in the soil, on the cable path, and keep the other probe

free of the soil.• The free probe will be extended using a length of insulated wire

attached to it. The far end of the wire will reach well across thepavement and attach to a screwdriver, inserted in soil, over the cable.

• Move this arrangement back and forth along the cable path until thereceiver display indicates neither forward or back.

• The cable fault will be exactly half-way between the screwdriverand the A-frame probe penetrating the soil.



Triangular Method• Start with the A-frame probes parallel to the cable path and as close

as possible to the pavement. The display will show a fault directionif you’re not too far from the fault.

• Rotate the A-frame a few degrees at a time until the display needleflips to show the fault in the other direction.

• Draw a line perpendicular to the probes of the A-frame until itintersects the cable path. This should be the fault location.

• As a confirmation, move the A-frame several paces ahead and repeatthe procedure. This second perpendicular line should intersect thecable path where the first line crossed the cable path.

---Note---If your display needle reverses back and fourth when the A-frame probes are in the ground it may mean that you are too faraway from the fault. Move along the cable path for a distanceand try for a definite reading.



Summary

To summarize this module, you have learned:

• The meaning of high and low resistance faults.• The equipment necessary to find a cable sheath fault under the soil.• How the A-frame is used to locate faults.• How to prepare a cable for fault finding.• The procedure to find a cable fault in the soil and under a concrete

surface.

Practice Feedback

Review the lesson, ask any questions and complete the self test.

Evaluation

When you are ready, complete the final test. You are expected toachieve 100%.



Review Questions

1. High resistance faults on electrical services will have:(a) High fault current values.(b) Low fault current values.(c) Low opposition to current flow.(d) None of these

2. Low resistance faults on electrical services will have:(a) High fault current values.(b) Low fault current values.(c) Low opposition to current flow.(d) Both high fault current values and low opposition

to current flow.

T / F 3. Fault current is the current that flows to ground when a live cable is grounded.

T / F 4. A high resistance fault means that a conductor does not have a good connection to the earth.

T / F 5. Low resistance faults are harder to find than high resistance faults.

6. Low cost fault locating equipment necessary to find a sheath fault under soil is:(a) An A-frame.(b) A receiver.(c) A direct connect transmitter cable.(d) All of these

T / F 7. Fault current radiates in one direction from the point of an earth faulted cable and then back to the source.

T / F 8. The probes of an A-frame can be thought of as measuring the direction of the current as it leaves the fault site.

9. When fault current enters the ground:(a) It only flows north.



(b) It only flows down.(c) It flows outward in all directions.(d) It flows upward.

T / F 10. In preparing a cable for fault finding you must isolate and de-energize the fault cable, then remove all grounds and ground bonds from the cable.

T / F 11. A-frame fault locating in not possible if the cable fault is feeding directly into the soil.

T / F 12. It is permissible to connect a transmitter by the direct method to a live power cable.

13. The A-frame does not work well on concrete surfaces because:(a) The fault current won’t flow under concrete.(b) The probes fail to make good contact to the earth.(c) Sound waves won’t penetrate concrete.(d) Radar won’t penetrate concrete.

14. A good practice to make the A-frame work on concrete is:(a) The transmitter power is turned up.(b) Wet sponges are used on each probe.(c) A wet sponge is used on only one probe.(d) Gas is poured onto the concrete first.

15. The A-frame can work at the edge of concrete by:(a) Using the triangular method.(b) Using the circular method.(c) Using the square method.(d) Using the root mean square method.



Review Question Solutions

1. Low fault current values.

2. Both high fault current values and low opposition to cur-rent flow.

3. T

4. T

5. F

6. All of these

7. F

8. T

9. It flows outward in all directions.

10. T

11. F

12. F

13. The probes fail to make good contact to the earth.

14. Wet sponges are used on each probe.

15. Using the triangular method.

Documents

Fault Locating Applications 4 Book 1/8... · connection to the earth. The resistance of a fault can be measured in ohms by use of a meggar or ohm meter. High resistance faults do