Locating buried pipes and cables

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Locating buried pipes and cables

April 19, 2023 1


Radiodetection

ABC to XYZ of locating buried pipes and cables - For the beginner to the specialist

Radiodetection Corp.154 Portland RdBridgtonME, 04009877 247 3797


Cable locators do not find cables.....

?


...they find magnetic fields


Because fields do things that pipes and cables don’t do

Why does it matter?


Buried conductors don’t move, but the fields we’re tracing are subject to…


Affected by:

1. Method of signal application2. Grounding3. Peak or Null4. Congestion5. Frequency applied

...Distortion


What is a Magnetic Field ?

A magnetic field is radiated by a current carrying conductor


…Which can be detected by a receiving coil which is excited by the expansion and contraction of the field.

Aerial

Volts


What is Electromagnetic Induction?

If a bar magnet is inserted into a coil of wire, a voltmeter will show a deflection but only while the magnet is moving. When the magnet stops, the meter reads Zero. When the movement of the magnet is reversed, the meter deflection is reversed.

S

N

+ volts0


one a.c. cycle

Alternating current creates a moving and reversing magnetic field

What is an Alternating Magnetic Field?


What is Electromagnetic Induction ?

Direct current does not produce an electro - magnetic field detectable with a locator. DC Current builds to its voltage and produces a constant level of magnetism. There is no change in this voltage to excite an adjacent receiving coil.

Bulb will not light


Passive

Active

There are two methods of signal detection


Passive signals are a by-product of the 20th century. The proliferation of power and radio technology in this century has caused almost every underground metallic pipe or cable to emit detectable magnetic fields.

Passive


Detectable Radio signals come from high power, low frequency international communication and navigation systems.

Such signals are present nearly everywhere on earth.

Passive


FastEasyDoes not identify

Passive


However tracking lines to their source can also aid identification

Passive


Passive

Ensure a full grid search is done to detect conductors laid in different directions.


Passive

Active

There are two methods of signal detection


Increases Locator VersatilityDepth MeasurementPositive Identification

Active


Affected by:

1. Method of signal application

…Distortion


There are three main methods of Active Signal Application

Induction "Spill"

Connection "Clip"

Signal Clamp "Clamp"

Applying a signal


Connection

A successful connection to the metallic pipe or cable will be indicated by a change in the transmitter audio or visual indicator. Always make the best possible connection for reliable signal.

mA

mA


Requires three components:

1. A transmitter or signal source

2. A metallic conductor

3. A return path. Ideally, the earth.

Direct Connection Locating


Affected by:


2. Grounding

…Distortion


Connection

The ground return point is the "back door" that the outgoing signal returns to. What you ground to and where you place the ground can significantly affect your results.

Remember: Every milliamp of signal that is going out the metal to metal connection on the red lead has to come back in through a metal to dirt connection on the ground lead.


Ground stake too close to target conductor: Less range, some signal transfer.

Connection


Badly positioned remote ground causes more signal transfer

Where possible, place ground rod away from known adjacent utilities which may act as return paths.

Connection


Remote ground = better range, less signal transfer.

As a general rule, try to position the ground point at right angles to and 5-10' from the connection point and direction of conductor.

Connection


Grounding to a structure which is also grounded can produce multiple signals.

The signal returns on every conductor that shares the same ground

Connection


Connection

Positioning the ground stake in the direction you wish to locate will often encourage the signal to flow in that direction


Connection

When locating Tees, position ground stake in the direction you wish to locate.


Connection

For greater signal travel, an extension cable can be used to place the ground stake further away.


The Cable Reel


A double-ended connection does not use ground as part of the circuit and is more dependable. This is particularly true when trying to locate metallic pipes buried close together.

Connection


Metal fences may seem a convenient ground point, but might produce interfering signals.

Always use an independent ground such as a ground stake, large screwdriver or other isolated metallic object buried in the ground.

Connection


After making a connection, sweep a complete circle around a signal source; measure and mark all occurrences of signal. Repeat to assure accuracy.

Connection


At older buildings where the services are all metallic and may share a common ground, connecting close to multiple service bondings may produce multiple signals.

Connection


Energizing in the street and tracing toward the bonding point can be more reliable

Connection


The Signal clamp is a current transformer which induces or spills a locate signal onto a target line

Clamp


A clamp safely applies a signal to a live cable without interrupting the supply

Transmitter Clamps


The Signal Clamp is a means of applying a transmitter signal to an insulated pipe or cable, without physical connection. It's most useful for live electric cables.

Signal Clamp must be completely around line with jaws closed.

Clamp


When using a clamp, the applied signal moves away from the nearest ground to a distant ground. For this reason, it's a useful way to put a locate signal on a specific line.

Clamp


At higher frequencies - or with longer lines - capacitive leakage completes the circuit.

Clamp


Direct connection does not give reliable identification on multiple bonded lines

As an exception, connection to the bonding ribbon in a manhole will send tone in one direction, allowing the conduit run to be located.

Connection

6 12 14 6


Generally, clamping produces more reliable signal identification.

Excellent on headers. Especially with using current measurement and direction.

Clamp

6 22 10 6


Place the clamp below the earth bond when locating cables. Do not place above the earth bond.

If there is no bonding strap, the clamp may not transmit a signal.

Clamp


If using the clamp around a metallic pipe, any insulating flange must be bridged.

Clamp


Induction

Induction allows the user to quickly and easily apply a locate signal, by placing the transmitter in the vicinity of a known conductor or conductors


min. 10 paces

A minimum distance of 10 paces between receiver and transmitter is necessary to avoid the receiver detecting the signal directly from the transmitter. This is known as "Air coupling".

Induction


Often induction will energize every metallic conductor close to the transmitter. This is useful for checking an area for buried conductors, but not useful for finding specific conductors in congested areas.

Induction


The induced transmitter signal is usually strongest on the line directly below and in line with the transmitter

Induction


Aerial

The orientation of the aerial is important as seen with an AM radio


When the aerial is not in line with the magnetic field, no signal is detected. This is known as "Nulling".

Aerial


When the transmitter is turned onto its side so the coil axis is vertical, the induction signal is spread over a wider area; but this results in zero signal directly below the transmitter. Which can be useful...

Induction


Min. 10 paces

Using transmitter on its side to 'Null out''When the transmitter is over the target cable, minimum signal will be found by the receiver and the adjacent conductors will be energized strongly.

Induction


100 20 5.8 2.7

Induction

Estimated induction signal ratios


50 30 19 140

Estimated induction signal ratios

Induction


Induction Search (two person).Maintain minimum Rx/Tx separation.

Sweep 3 times ! +/- 30-40 degrees

Induction


Induction search, radial.

Maintain constant Rx/Tx separation

- Lazy Susan idea

Induction


Affected by:


2. Grounding

3. Peak or null

...Distortion


Aerial Responses

Different aerial orientations can be used for different responses

1 2 3

Signalresponse

1 2 3

Signalresponse


In Null, minimum signal is encountered exactly over the center of the buried conductor field

A maximum signal response will be detected on each side of the minimum. If both of the "shoulders" are symmetrical, the Null point is accurate.

Receiver Response


Null antenna gives a sharp response which gives the impression of good accuracy, but can be less accurate than Peak.

100%

5%

Receiver Response


Peak coil response is less sharp, but actually more accurate.

100%95%

Receiver Response


This is a typical peak signal response over a buried conductor. Peak response provides position and direction.

Note that the maximum signal response occurs directly over the center of the underground conductor

Receiver Response


Peak response is more reliable. It enables you to measure depth.

Peak response is more accurate in congested areas

Peak

Receiver Response


Null is useful for deep lines and rapid line tracing , but...

Null

d

Receiver Response


Null response in an area with congested plant is often useless and misleading

Null

Receiver Response


Peak and Null coils produce opposite types of error when tracing bends

PeakNull

Receiver Response


Affected by:


2. Grounding

3. Peak or Null

4. Congestion

...Distortion


Distortion

Current on one line may induce 'stray' signals onto nearby conductors


When the conductors are close together, the magnetic fields can interfere with each other, causing a distorted field which is no longer cylindrical. This can lead to poor locate results and inaccurate measurements.

Distortion


d

PeakNull

d

Distortion and Depth

When there is a discrepancy between the aerial responses the following must be observed.1. The Peak response will always be more accurate.2. Push button depth estimation should not be used until the two responses agree

Peak Null


Peak

Null

13

This distortion varies depending upon the direction and magnitude of the current flow, causing a discrepancy between the peak and null aerial responses.

Distortion


dd d

Because we know the field is likely distorted,it's not a good idea to rely on depth readings taken near a change of direction...

Depth


dd

d

or near a tee

Depth


dd

or near another line

Depth


dd d

or near a depth change

Depth


d dmin. 30 paces

or near an inducing transmitter

Depth


100%70%

d

d

Depth Triangulation Peak Aerials

Depth estimation can be verified using a triangulation technique. Move the receiver from 100% to 70% signal strength, on each side of conductor and measure the distance. This distance will be approximately equal to the depth.


Affected by:


2. Grounding

3. Peak or null

4. Congestion

5. Frequency applied

...Distortion


Which frequency should I use ?

Very LowLess than 1 kHzVery Long RangeNo InductionLittle "Spillage"Better IDent

Low1-10 kHzLong RangePoor InductionLess "Spillage"

High10-50 kHz.Easy InductionShorter RangeMore "Spillage"

Very High50kHz+Short RangeExcellent InductionSevere "Spillage"

Characteristics of different frequencies.

All of these characteristics get better or worse linearly with frequency. There is no signal that is magically better for one than another.


High frequency signal quickly escapes from target line and may return on an adjacent line Lines in close proximity will readily accept signal. This can lead to field distortion and poor locate information.

Which Frequency ?


Higher frequency = shorter range (greater capacitive signal loss to ground) This frequency is better for induction onto small or short length conductors, such as telephone drops, CATV cables or street light cables.

Which Frequency ?


Low frequency = long range (minimum capacitance signal loss to ground)

This frequency is better for connection and locating longer metal pipes or cables

Which Frequency?


High frequency may locate service drops which are not grounded at the termination point but range on main line is reduced and may induce onto crossing services

Which Frequency?


Low frequency tends to keep to main line, does not induce onto crossing lines and bypasses service drops which are not grounded.

Which Frequency ?


High frequency can use a capacitive ground connection through a metal plate

This provides a ground in areas where direct earth ground cannot be achieved

Which Frequency ?


General Locating

When pinpointing the signal, keep the bottom of the receiver blade close to and parallel to the ground.


Always pivot receiver to ensure maximum signal is received

This is particularly true prior to taking a depth reading

General Locating


General Locating

Searching for laterals

Use Peak and search 2 paces away from and parallel to the main line


Problems

If multiple signals are received at uniform distances apart, you are probably trying to locate through reinforcing bars.

Raise the receiver up to chest height. This should allow the main signal to be detected.


Problems

Lost signal must be either: End of line


Problems

OR: a T lateral


Problems

OR: a bend


Problems

OR: a depth increase

The field has simply become weaker at the coils because of the change of depth


Small Waterproof self contained transmitters enabling non metallic ducts, drains or sewers to be located and traced with a Receiver.

Transmitting Sondes


Ghost Ghost

Receiver response to a Sonde signal. Small ghost signals may be detected either side of main signal

Note: Receiver is held in line with Sonde

Sondes


Sondes

Location procedure for pinpointing Sondes


Using a Sonde to find a collapsed sewer

Sondes


Tracking an inspection camera with a Sonde

Sondes


Tracking a boring tool with built-in Sonde

Sondes


Sondes

A sonde in a vertical position gives a very precise "null" signal


Sondes

This can be used for very accurate positioning through a wall prior to drilling etc


Locating a leak in a non metallic water pipe using a Sonde inside a "Pig“

Need VLF for metal pipes

Sondes


A B

d=0.7AB

d

Triangulation depth method for verifying "push button" depth or estimating depth which is beyond the receiver's depth range

Sondes


Levels of Confirmation


Automatic Gain Control - AGC


m

%

mA

m

%

mA

Signal strength

Current reading

Depth

Signal strength

Current reading

Depth

Change of depth effects signal strength but not current reading

RD400 Digital Series


mA mA mA

Current losses returning to signal source

RD7/8000 Digital Series


mA mA

Current loss due to poor insulation

There is very specialized equipment for this



mA

mA

mA

Current loss due to a Tee connection



Expected current direction indications and return signal path

For locators equipped with current direction indication only

Target line always

identifiable by current direction



Possible current direction indication on conductors.

RD7/8000 CD Series


100%

60%40%

Modern equipment has many “Levels of Confirmation”



Current flow through an earth fault

FaultFind


Meter readings before, at and after the fault.

FaultFind


Distant fault – no indication

Line fault

ahead

Line fault

behind

FaultFind

Meter readings along a faulted cable


FaultFind

FaultPositionLocate

Faultline

Locate line

It is possible to detect a fault a reasonable distance away, and to the side of the cable

FaultFind


Pinpointing a fault under a roadway with two or more bearings (triangulation)

Fault line 1

Fault line 2

Faultposition

FaultFind

Locating buried pipes and cables

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