Locating buried utilities is a process of tracing out the invisible (buried utilities), and coming to conclusions on the position of those utilities without ever having seen them.  This requires a combination of science, intertwined with logic and the knowledge of the utilities.   The most common geophysical instrument for locating buried utilities is the Pipe & Cable Locator.

A Pipe & Cable Locator is not at all the most advanced geophysical instrument for tracing out buried utilities.  Yet, it is the backbone of utility detection and mapping.  The reason for this is in the way that they function which makes them the only geophysical instrument that can trace out a single pipe or cable with confidence.  They can also do this using 4 separate methods: direct connection, induction, coupler induction, and passive.

This section covers the most important elements about the instrument and how it functions.  It is not meant to replace an actual training course, but only to ensure that the fundamental functions of the instrument have been covered for those who have never received any solid training on the instrument.

The distance that the EMF can reach is not specific.  It depends on the power output of the transmitter and other factors.  However, it helps to know that a typical EMF field produced by a 10 watt transmitter is only detectable at roughly 15 feet (4.5 meters) from the source of the current.

That will never be an exact measurement.  One utility may only be detectable at 12 feet, while another under certain circumstances may be detectable at closer to 18 feet (5.5 meters).  But 15 feet is a good rule of thumb for knowing how far the receiver can be from the utility line, and still be able to detect that utility.

A P&C Locator can be used in 4 completely different methods: direct connection, standard induction, clamp/coupler induction, and passive.

Direct Connection requires using the red and black lead wires; the red wire is attached to the utility, and the black wire is attached to a ground connection.  This applies current directly to the utility line or to the tracer wire that is placed next to the utility line.

With Standard Induction the transmitter can be placed on the ground where the utility line is suspected to be.  Simply make sure that neither the lead wires or the clamp/coupler are plugged in, and the transmitter will automatically switch to an internal induction coil which will produce a dual field, both up and down.

The upward portion of the field is only a side affect of induction, and does not typically help in locating.  It is the downward field that of course applies EMF down on to the utility line below.  Induction can be very ineffective if the utility line is deep, because it can be much deeper than the EMF can reach.

Most P&C Locators come with an attachment commonly called a Clamp.  This actually functions as a coupler because it is used to couple the utility line, but “clamp” is the more common word.  Plug the cable of the clamp into the transmitter and then place the clamp around the utility.

This is a form of induction because it induces current onto the utility from the clamp.  Induction current behaves differently than direct current, though you have to get into more involved details of what is happening to illustrate that point.

Clamps are most commonly used on cables along pole lines, and cables in manholes and handholes.  Clamps are only rarely used on pipes, not because they would not work, but because access to short sections of a pipe are far less common than access to short sections of a cable.  However, a clamp can certainly be used on a pipe exposed in a vacuum test hole or anywhere else.

The fourth method of using a P&C Locator is in Passive Mode.  In this case only the receiver is used.  In Passive Mode you can take advantage of the currents which are already present on buried utilities.

Those currents can be originating in two ways: those which are being applied to the utility by the utility owner, such as electric power on power cables; or they can be stray currents coming from a wide variety of sources.  We live in a world of electricity, radio broadcasts, TV broadcasts, wi-fi, Bluetooth, CB radios, NOAA weather broadcasts, and hundreds more.  Either way, there is not a single buried utility in the world which is not carrying at least some amount of stray current.

The two most commonly used Passive frequencies are Power Passive (50Hz to 60Hz), and Radio Passive (16 kHz to 19.5 kHz).

Power Passive is usually set to the 50Hz to 60Hz band in order to detect any electric power being generated in any country.  The United States, Canada, and South American countries generate electricity at 60Hz, while the rest of the world generates electricity at 50Hz.  Setting the instrument at 50-60 Hz covers the electric power transmissions no matter which country you are in.

The most critical issues here are that: some electric power, especially 3-phase electric power can eliminate its own field.  In other words, you can be standing directly above high voltage 3-phase transmission power lines – and yet not detect anything.

Another important point here is that all coaxial cable carries electric power as well as its communication signals.  This is injected into the Cable TV system at the Power Supplys and is mandatory for the operation of Cable TV.  So, coaxial cable in the United States contains 60Hz power which can be detected just as easily as any true power line.

Then there is the issue that any other buried conductor can attract stray power current.  This will be most common with any ferrous metal (iron or steel) pipe, and with any cable which has a steel sheath and is well insulated to retain that current (telephone cables).

So, Power Passive can detect buried power lines – but it will also be detecting buried coaxial cable, telephone cables, probably some large steel gas pipes, and other possibilities.  And if that is not enough, Power Passive often does not detect the most high profile power lines – 3-phase high voltage.

Radio Passive detects dying radio waves on buried utilities.  However, those radio waves are not simply from “radio broadcasts”, but from any type of broadcast in the radio wave format.  This includes radio broadcasts, TV broadcasts, Bluetooth, wi-fi, cellular telephone, and right down to the truck driver using a CB radio.

These radio waves are easily attracted to very long conductors without connections.  In other words, if you have two utility lines, both of them running for two blocks.  One is a large steel gas pipe and the other is only a tracer wire placed along side a fiber optic cable.  The gas pipe has regular connections (service attachments), but the fiber cable is only bypassing the neighborhood.  Which one will be most likely to attract radio waves?  The long tracer wire, not the large steel gas pipe.

The third most common Passive frequency is CPS, but that is covered in the Cathodic Protection text under MISC INFO.

Some P&C Locators have only one frequency, usually a very high frequency, but most of the instruments today come with a wide variety of frequencies.

All of these frequencies are within the range of about 250 Hz to 480 kHz.  This is a frequency range which is not typically used for any other purpose.  The reason for that is because this is a frequency range which is above electric current – but below radio broadcasts.  In other words, the advantage of this frequency range is that the current being applied has elements of both current and radio waves.  Any of these frequencies can apply current to a buried conductor, yet, the higher you go in frequency – the more the current can move from one conductor to another.  This is the good and bad side of high frequency.  We can get the current where we need it, but that high frequency current is also just below AM radio waves and can behave as much like a broadcast wave as it does an applied current.

While direct connection on low frequency will provide the most accurate and secure locate, it is not always possible.  In fact, the medium frequencies and even high frequencies can be very useful in many circumstances, and sometimes they make the difference in whether it is possible to locate out the line or not.

Low frequency can be considered to be anything up to 1 kHz.  The reason is that induction of any kind begins at roughly 1 kHz.  This is why the low frequencies are not available when using either standard induction or clamp induction.  The low frequencies do not work when trying to induce.  However, this also means that with direct connection they remain on the conductor where you applied it.  If you have direct access to the conductive portion of the utility, and you want the current to remain on that same utility – then use low frequency.  It could be 512 Hz, or 760 Hz, or 940 Hz – anything below 1 kHz.

Medium Frequency is a limited area starting at 1 kHz and going up to about 15 kHz.  This is the most common frequency range for induction of any kind while still keeping the frequency at a reasonable level.  For the longest time 8 kHz was the only frequency available in this range, and even today 9.8 kHz is usually the only other mid frequency available.  However, these are excellent frequencies for using the clamp/coupler, and even for most standard “drop box” induction.  This is a high enough frequency range to produce induction, but not so high that the current will indiscriminately jump to another utility.

Manufacturers do not produce P&C Locators which offer a frequency anywhere in the range of 15 kHz to about 21 kHz.  This is the area of dying radio waves, the bandwidth used in Radio Passive.  To be purposely producing a signal on buried utilities in this range would only create a lot of unnecessary confusion.

High Frequency is anything from about 25 kHz and above.  It could also be said that anything above 100 kHz or so is very high frequency.

Though high frequencies should never be a first choice with most utility lines, they do provide a great benefit.  Many utilities may need to be induced and yet mid-range is not high enough to produce the needed results.

High frequency, and for that matter, very high frequency, is very useful in sweeping an area to see if there are any unmarked utilities.  In this case use the highest frequency you have available.

To help visualize frequency better, think of the frequency as the speed of a car.  The faster you go the more likely you are to go flying off the road.  However, it is not the speed of the current that is different.  All current travels at the same speed – 186,282 miles per second – the same as the speed of light.

Always keep in mind that what you are detecting is a round ball of energy.  If you were standing five feet off to the side of the utility you can still detect it just fine.  All you have to do is to point the receiver towards the utility – and the EMF will register on the internal coils.

In other words, since the EMF can be detected far to the side of the utility, the only way to determine the actual horizontal position of the utility is to keep the receiver pointed downwards at all times.  Knowing the utility is directly below you is only possible if you are pointing the receiver downward, and moving it back and forth.  It is that highest point of the ball of energy that will be directly above the utility line.  That is the apex of the EMF.

Many locators, including those with years of experience, have never been able to understand that swinging the receiver back and forth like a pendulum will result in marks that are several feet off from the true location.

Each time you connect to a utility you are searching for a new utility line.  You probably have a good idea of which direction that utility is going from your connection point – but that is not at all good enough.

The purpose of locating/designating is not to put paint down where the utility is supposed to be – but to put paint down where the utility line actually is.  It is up to the operator of the instrument to determine the location of the utility, not to just follow the utility prints, to follow past marks on the ground, or to assume that what ever signal is first detected is the target utility line.

Anytime you apply current into the ground you will be applying current to every single conductor in the ground to some degree.  The target line, the utility you applied current to, should have good current, but every other buried utility will have at least some current as well.

Never assume that just because you have a signal, that it is the target signal.  You must sweep the area around your connection in order to determine which is the strongest signal.  It is never just signal that you should be looking for but the strongest signal.  That will almost always be the target utility line.