#cablelocator

In some cases, a successful result can only be reached with excellent and proper grounding if the probing signal is applied relative to the ground. An incorrectly selected generator grounding location will prevent the probing current from flowing in the search direction if the line to be traced is branched. The transmitter ground point should be changed so the signal is fed in the right direction.  

The circuit impedance to which the signal is applied (cable-ground) depends on the ground condition (its type and humidity), the physical parameters of the traced line (size, insulation, etc.), and the characteristics of the grounding. Some generators can match the output impedance to the signal circuit parameters to increase the signal amplitude.

The grounding quality is essential for low frequencies. The better the grounding of the oscillator, the higher the signal amplitude. The ground resistance should be less than 1000 ohms. So a 5-gallon water canister will be helpful - water can always be poured into the grounding point and reduce the grounding resistance. A small metal plate is placed on the surface and watered if you encounter asphalt or concrete pavement. You do not need to use nearby metal structures (water pipes, fences) for grounding because they will emit a signal. You need to remember: if a parallel cable or pipe runs next to the cable to be traced, the reverse current will flow through them, and they will also emit an electromagnetic field therefore. It may take several attempts to experimentally find the best grounding point as a result.

The technique of routing pipelines and cable channels has its own features.

The generator is connected directly to them if they are made of conductive material. The pipeline must be disconnected from the ground at the point where the generator is connected. Otherwise, the signal will take a shortcut. It is worth using inspection pits for direct connection to a metal pipeline or cable channel in the middle of the route. If there are no inspection pits, the pipeline's location must be determined in advance by active or passive search. The connection quality of the generator (low contact transient resistance) to the metal cable channels is estimated by the change in signal level - the signal changes its tone when the load appears in most generators. When you trace cable channels and pipelines, you should always remember that the connection of their segments in some places can be made with non-conductive seals. It is advisable to connect such places galvanically so that you do not have to deal with each piece individually.

Tracing of non-metallic channels without internal conductive elements is done in two ways.

First, a metal CPS (Cable Pulling System) rod can be inserted into the channel, with the generator connected to both the rod and the ground.

Second, an active probe, a coil-loaded transmitter placed in a tiny capsule, can be inserted into the channel. The probe is attached to the CPS and pushed into the canal, and the active probe coil signal is detected by a receiver equipped with a coil sensor.

The first method is more convenient for tracing, and the second - is for finding specific locations (for example, to determine the location of a clog in the channel).

The signal can be fed to any two unterminated conductors of the cable or between one of the conductors and the building ground bus if you are using the sensor as an antenna.

Since the sensor-antenna "senses" the electric field, the signal is applied in such a way as to ensure its maximum (the circuit at the remote end must be open).

This must be remembered when the signal is applied to a pair of cores that has a defect in the form of a short circuit (reduced insulation) or is loaded (closed) at the other end since the electric field strength (its value is determined by the potential difference) will fall as you approach the point of short circuit.

Suppose a signal is applied to one of the twisted pairs of a multi-pair cable. In that case, when tracing (moving the antenna along the twisted pair), it will be maximal when one of the wires is near the probe and minimal at the point where the wires cross. Thus, unlike tracing using a coil probe, the signal level along the traced line will vary periodically. This effect is also possible when the signal is applied between the core of one of the pairs and the ground bus (the core position inside the cable changes all the time).

When identifying the pair to which the signal is applied, the antenna should be brought to one of the conductors, not placed between them - the electric field strength is maximum near the signal-carrying conductors and equal to zero strictly in the middle.

To verify correct identification, short-circuit the pair. The disappearance of the signal will indicate that the pair is correctly identified. Another way to check (by zero signal) follows from the above. Suppose you spread the conductors of the pair in the shape of a V and place the probe antenna in the center. In that case, the correctly identified pair generates a zero signal. It should be particularly noted that a negative result from both tests may indicate not only that the pair is incorrectly identified but also that there is a fault in the correctly identified pair. It can be a breakage or increased resistance of the cores, a symmetry violation (connection errors when two cores from different pairs are used - the so-called "unpairing"), or a reduced insulation resistance of one of the cores.

Because of the many nuances, identifying and tracing a cable with an antenna sensor requires excellent care and attention: carefully observing the antenna's position relative to the wires and not allowing your hand to be near or touch the antenna. Suppose the antenna is mounted on the body of the receiver. In that case, it should be kept away from the wire bundle to reduce the pickup noise on the receiver circuits.

Particular care must be taken when selecting how the signal is fed for tracing. When working with twisted pair cables (especially high-frequency and structured cables), the generator should not be connected to one pair but to the cores of two different pairs or to the core of a pair and the shielding sheath. Remember that the electric field, unlike the magnetic one, is almost entirely blocked by the cable's shielding (foil or braid). For the same reason, it isn't easy to work with wet wires. Therefore, it is better to connect the generator for tracing to one of the cable cores or its shielding and the grounding busbar of the building.

When using a receiver with two switchable sensors (coil and antenna), the signal must always be fed so that the return path does not go through a conductor in the same cable or run close and parallel. Failure to meet this condition (say, the signal is fed to cores in the same cable) will make the sensor-coil operation impossible - the magnetic fields of the conductors will be subtracted as the current flows in them in different directions.

How is the generator connected? How to choose the right type of receiver probe? How to provide a probing current in the line being traced? What affects the range of the locator?

A thorough knowledge of all the techniques of working with the device, of course, does not guarantee error-free results (statistics show that even a very experienced operator during the day can make 10 to 20% of the errors in the total amount of work), but it can reduce the number of miscalculations.

The main tasks that can be solved by the cable locator :

Determination of the cable route (cable bundles) and cable ducts;

Identification of cable lines in a bundle or cable ducts, among others running in parallel;

Identification of cable terminations;

Cable core identification.

Solving each of these problems may require an individual approach - it all depends on the type of line being traced (twisted pair cable, coaxial, power, cable duct, pipeline), its characteristics (cross-section, frequency properties, length), its location (on the surface, in a cavity or in the body of a monolithic building structure, in the ground), as well as the characteristics of the generator and receiver available. Therefore, giving a set of direct and unambiguous instructions is impossible. A system of preferences, however, can be described.

First, you must determine the most effective sensor type for the case in question (coil or capacitive antenna) and the method of signal delivery (direct connection or inductive method).

Direct connection involves physically connecting the transmitter to the line you are looking for, which requires access to either the cable strands or the surface of the conduit (conduit), and provides a rich selection of signal circuitry. In addition, it provides the most powerful signal applied precisely to the desired circuit. Therefore, if direct connection is possible, it should be preferred. The exception to this is when the line is energized. Although the transmitter output is normally protected against AC voltage up to 120 V, it is not advisable to connect it to live cable conductors. In this situation, an inductive coupler or inductive antenna is better suited. An inductive sensor offers much greater capabilities than a capacitive sensor. However, to use a coil, the signal must be applied to a closed circuit through which sufficient current can flow for the available receiver sensitivity.

If we are talking about two conductors of a cable (a pair of cores or a core and a shield), there are only two cases where such a circuit can be arranged. First, if the remote end of the circuit is accessible and the conductors to which the signal is applied can be shorted. Second, if it is loaded (for example, the conductors are connected to a load or to the input circuits of some equipment). This also applies to pairs of conductors shorted due to reduced insulation resistance.

It is crucial to understand that the range over which the receiver will be able to detect the signal depends to a large extent on the purpose and design of the cable. Since one of the goals of most cable designs (especially high-frequency cables) is to minimize the emission of the transmitted signal, selecting the conductors to which the signal will be fed must be made with great care. Tracing cables in a shield or protective covering (sleeve, wire, or foil braid) is possible because they do not eliminate the magnetic field completely. The signal is much more attenuated in cables armored with a steel tape or wire. It is almost entirely blocked by cable ducts made of steel or aluminum tubes. Therefore, such lines can be traced only by applying a signal to the channels themselves or by accessing the cable in the transition and junction boxes.

The last option to feed a signal into a traced cable by direct connection is to connect the generator to a conductor of the buried cable. In this case, the circuit can be shorted through capacitive leaks to the ground since AC voltage is used as the signal. There is no need to ground the remote end. However, connecting the transmitter to a conductor inside the cable (wire, shield, metal armor, or a central carrier) minimizes the effect of current leakage or signal transfer to nearby objects. For example, coaxial cable with an insulated shield and multipair telephone cable in polyethylene sheath is traced to a greater range than a twisted power cable and multipair telephone cable in the lead. The range is also affected by water in the conduits where the traced cable passes. Remember that a short cable section may not have the capacitive leakage present to provide enough current for the available receiver sensitivity. Then there is only one solution: ground the remote end of the line or switch to a high-frequency signal.

Grounding is also necessary if the cable comes out of the ground (e.g. when arranging an entry into a building). Suppose such a cable section is not grounded at the remote end, starting from where it left the ground. In that case, it will not be possible to trace the line - the current will not flow further because the capacitive connection with the ground will disappear.

This post contains a brief introduction to cable location principles. What does a typical locator consist of? Active/passive location methods. Cable connection methods.

Determining the route of an underground cable or conduit is a common challenge. This challenge can be overcome with the help of special measuring devices. Although they have a similar principles of operation, they can have different names, such as cable locator, cable avoidance tool (CAT), underground wire locator, or cable tracer. Using these devices, it is possible to successfully determine the route of all sorts of cables (even fiber optical cables, given that they are equipped with a metal cordage or jacket). Moreover, these devices can sometimes detect the location of cable faults, such as cable damage or a short circuit.

Locators that were used to search for metal objects underground were first introduced 40 years ago. Initially, they were only used to detect water, gas, or sewer pipes. Today this issue has become significantly more complicated. In addition to metal pipes, an enormous amount of power and telecommunications cables have been laid underground. Over the years, cable locators have become more accurate and now come with new features. However, they still use the same fundamental technology as the very first models. This technology is based on electromagnetic field detection.

How do the aforementioned devices work? The cable locator consists of two parts: a signal generator (transmitter) and a receiver (detector). The former sends a signal to the cable line, while the latter detects it. It can be said that the receiver serves as the “heart” of the cable locator. Firstly, its characteristics and features determine the aggregate capabilities of the generator-receiver pair. Secondly, in some use cases, a generator may not even be required.

The cable locator receiver needs at least one sensor to “pick up” the electromagnetic field. This can come in the form of a whip antenna (capacitive sensor) or a coil (inductive sensor). Both have specific advantages and disadvantages. Therefore, some devices have one or even several (two or even three) replaceable sensors. The signal that they receive is amplified and processed. The resulting processed signal is presented to the operator.

The signal can come from either a voltage or variable frequency current generator (200 Hz — 130 kHz). Some generators have a single fixed frequency. Others have multiple range of operating frequencies (up to 4) that can be chosen based on the use case. Some generators can generate a signal of several frequencies at the same time. And in some cases two frequencies may alternate.

A detection method is considered active if both receiver and generator are utilized. However, a sensor can also detect electromagnetic fields that are generated by other signal sources. This allows the device to detect and trace the routes of loaded power lines (50 Hz and higher harmonics of up to 3 kHz), cathodic protected pipelines (100 Hz), telephone cables via alarm signals (2–18 kHz), wired broadcasting networks (300 Hz — 130 kHz), as well as any conductive objects that have external radio transmitters which can induce a signal in long wave range (140 kHz — 300 kHz), and others. In such cases, the tracing can be performed in passive mode. In other words, it means that the use of a generator is not necessary.

In active mode, the signal can be sent to the cable via several different methods. The generator’s signal can be sent directly to the cable (direct connection), via an inductive antenna (remote antenna) or through an inductive coupler (signal clamp).

The direct connection method is self-explanatory. The transmitter must be physically connected to the cable that’s being traced. However, if this is impossible, one of the other two methods can be implemented.

An inductive antenna is a coil that receives a signal from the generator. The antenna is placed directly above the cable and induces a signal in it. Of course, such a signal is considerably less powerful as compared to one generated by a direct connection. But the upside is that there is no need to connect to the cable.

Remember that the inductive antenna signal is induced in all utilities at the location where it is placed. This can lead to errors if the signal is not only transmitted through the intended cable but also through other nearby utilities. However, the convenience of this method often outweighs its disadvantages.

Let’s assume that you decide to use a direct generator connection. If cable is long and the generator is not powerful enough, the signal will become undetectable at a certain point. To continue tracing in this scenario, the only option is to place an antenna where the signal intensity is strong enough to accurately determine the cable’s location. This procedure can be repeated several times, along the entire length of the cable.

An inductive antenna also provides other unique use case options. Remember that the antenna induces a signal in any conductor within its operating range. Let’s imagine that there are 2 people in this scenario. The first one has the transmitter with an antenna and the other has the receiver. Then they walk in parallel to one another at a distance of, for example, 65 ft. Thus, they can detect all conductive utilities that cross their path. To gain a more accurate assessment, the same procedure must be repeated several times in different directions.