Locating Theory - SaskPower 4 Book 1/6. LOCATING... · 2005. 3. 29. · 2. One requirement for an...

March 29, 2005

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

Locating Theory

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: • None.

Objectives: Explain the principles of how cable locating equipment is used to induce and receive signals.

Rationale: You must understand these principles so that various circumstances can be understood when operating cable locating equipment.

Learning Objectives• List the requirements for an electromagnetic field to exist on a

conductor.• List three requirements to induce a current in a conductor.• Distinguish between open and closed circuits.• Explain seven terms used by cable locating personnel.

Learning Methods• 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

STUDENT RESOURCES• None.

L O C A T I N G T H E O R Y 3


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: Field StrengthLearning Objective:List the requirements for an electromagnetic field to exist on a

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

Introduction

Many companies and utilities use underground systems out of necessityor convenience. Naturally, utilities must cross each other’s “right ofway” and also maintain systems as they deteriorate. These activitiesrequire that companies have the capability to locate their undergroundapparatus in an accurate way.

The technology for locating underground cables consists of radar, infra-red imaging, and electromagnetic field tracing. Electromagnetic fieldtracing has all the advantages in that the equipment is easily portable,easily utilized, and of low cost technology.

Cable locators, in actual practice, do not trace the path of a cable. Rather,they are designed to trace and locate electromagnetic fields. Therefore,the intent of the operator of such equipment is to put an electromagneticfield onto the conductor being targeted or, perhaps, trace any naturallyexisting magnetic field on the cable.

There are many different magnetic fields that exist in the earth becauseof the many signals that are sent via radios, telephones, TV, andmicrowaves, etc. These foreign fields can create interference that canbe misleading to cable locators.

Magnetic Fields

As is already known, iron magnets have magnetic fields. These fieldsare invisible to the naked eye, but their patterns can be identified bypouring iron filings around the magnetic field. Magnetic fields cannot beshielded by any substance.



Figure 1. Flux Lines

Electromagnetic Fields

For an electromagnetic field to exist on a conductor, there must be:

• a closed (complete) circuit• a current flowing in the circuit



The field strength of the conductor can be strengthened by increasing thecurrent in the conductor. When clip-on ammeters are used to measurethe amperage on a wire, the device is actually measuring the strength ofthe wire’s magnetic field and converting the field strength to a scalereading in amps.

As previously stated, a current carrying conductor has a magnetic fieldaround it. If the same current carrying conductor is wrapped around ametal core, the magnetic fields of the individual wraps combine toproduce a stronger magnetic field between the poles of the coil.



The strength of the coil’s field can be increased by:

• increasing the number of wraps of wire• using a more permeable core• increasing the value of current through the coil

Coils are used as components in cable locating equipment. These coilshave the capability of inducing magnetic fields on undergroundconductors, as well as receiving magnetic fields from conductors.



Lesson 2: Mutual InductionLearning Objective:List three requirements to induce a current in a conductor.Learning Method:Self-learning + On-the-jobEvaluation Method:Knowledge Test

Introduction

It was previously stated that cable locators can operate under theprinciple of inducing magnetic fields onto conductors and then tracingthat induced field. This inducement is called mutual induction.

There are three requirements necessary to induce a magnetic field:

• a conductor in a closed circuit• a magnetic field• relative motion between the field and the closed circuit

An illustration of this is where a current carrying conductor induces acurrent into a grounded conductor running parallel to it. We know thatthe magnetic field of the live conductor cuts the grounded conductorbecause the magnetic field expands and collapses in unison with therising and falling value of alternating current. The expanding andcollapsing field is the relative motion.



In a like manner, a magnetic field produced by a coil in a transmitter canbe made to induce a magnetic field known as a signal on an undergroundconductor. When using a transmitter to induce a current onto aconductor, it is critical that its coil be placed properly so that themagnetic field properly cuts the cable to be located.



When a transmitter puts a signal on a conductor, then, a receiver’s coilcan be used to trace the magnetic field. Magnetic fields will travelthrough any substance. Therefore, it is possible that your transmittermay induce fields onto unwanted targets (cables). This is undesirableand may mislead the operator.



Lesson 3: Open and Closed CircuitsLearning Objective:Distinguish between open and closed circuits.Learning Method:Self-learning + On-the-jobEvaluation Method:Knowledge Test

Introduction

It has been stated that a magnetic field cannot exist unless there is acurrent flow and, further, that there can be no current flow unless thereis a closed circuit. Therefore, without a closed circuit, signals cannot beinduced on the conductor.

What constitutes a closed circuit? A closed circuit must provide acomplete path for current flow. Remember that a capacitor appears to bean open circuit due to the insulation between its two plates. However,current will still flow in a capacitive circuit because the plates have theability to store and discharge current.



Current continues to flow in the above circuit despite the fact that thereis insulation between the plates of the capacitor. Electrons flow fromone plate to the other and back again in an alternating fashion.Therefore, the above is a closed circuit and the conductors will emit amagnetic field.

Likewise, an insulated cable and the ground can combine to form acapacitor. The ground acts as one plate, and the conductor acts as theother plate.

The above indicates that, when an unloaded underground cable isenergized, there is a current flow due to capacitance. Therefore, therewill also be a magnetic field around the conductor.

Cable locating equipment can be used to create a magnetic field on aconductor. The field can then be traced.



Lesson 4: TermsLearning Objective:Explain seven terms used by cable locating personnel.Learning Method:Self-learning + On-the-jobEvaluation Method:Knowledge Test

Introduction

There are a number of terms that cable locator personnel must becomefamiliar with. Locators are becoming more sophisticated, and there areseveral options that can be used to get a locate done.

Frequency

Frequency is the number of complete cycles of AC voltage that occur inone second. The North American system produces electricity at 60 Hz.Most locators have a variety of frequencies at which magnetic fields canbe induced onto conductors.

Target

The target is the conductor that is intended to be traced by cable locatingtechniques.

Passive Modes

The passive modes are selections on a receiver which will allow it toreceive fields normally existing due to 60Hz power lines, or radiosignals in the lower frequency (



to other nearby lines.

Sheath

The sheath is the covering or outer jacket surrounding a cable. Sheathscan be metal, rubber-like, or thermoplastic. Sheaths are used formechanical protection of the cable. The purpose may be to keepunwanted chemicals and weather out. The sheath may act as a returncurrent path.

Shield

The cable shield is a conductive layer (not necessarily metal)surrounding the insulation of an energized cable. Its purpose is toequalize the distribution of voltage stress lines.



Summary

To summarize this module, you have learned:

• The requirements for an electromagnetic field to exist on a cable.• How to increase the field strength of a coil.• Three requirements for induction.• To recognize open and closed circuits and capacitance.• Seven terms used by cable locating personnel.

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. One condition for an electromagnetic field to exist on a conductor is:(a) Air around the conductor.(b) Lead around the conductor.(c) A ground strap on the conductor.(d) A conductor in a closed circuit.

2. One requirement for an electromagnetic field to exist is:(a) An insulated conductor.(b) A current on a conductor.(c) A coil of copper wire.(d) An open circuit.

T / F 3. Cable locators are designed to locate and trace electromagnetic fields.

T / F 4. Radio, telephone, and TV signals do not create interference when locating cables.

T / F 5. Magnetic fields can be seen with the naked eye.

T / F 6. There is no way to increase the electromagnetic field strength of a conductor.

7. Increasing the field strength of a coil can be accomplished by:(a) Increasing the current flow in the coil.(b) Adding coil wraps.(c) Use a more permeable core.(d) All of these

T / F 8. A conductor in a closed circuit is necessary to induce a current.

T / F 9. A circuit must be grounded to induce a current.

T / F 10. Current cannot be induced unless there is a magnetic field cutting a conductor.



T / F 11. A capacitor connected in series with a 120V source is an open circuit.

T / F 12. An open circuit is capable of carrying current.

T / F 13. A 1000 meter primary cable energized from a live 14.4kV line is part of an open circuit.

T / F 14. The term “active mode” implies that the operator has actually induced a signal on the cable.

T / F 15. Coupling is a term used to describe that voltage has been induced onto a conductor.

T / F 16. Coupling is a term used to describe that a signal has been induced onto a conductor.

T / F 17. The shield is a conductive layer surrounding the insulation of an electrical power cable.

T / F 18. Frequency is the number of complete cycles of AC voltage that occur in one minute.



Review Question Solutions

1. A conductor in a closed circuit.

2. A current on a conductor.

3. T

4. F

5. F

6. F

7. All of these

8. T

9. F

10. T

11. F

12. F

13. F

14. T

15. F

16. T

17. T

18. F

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