Cable locator: features of cable tracing and identification
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.
Generally speaking, tracing with an antenna sensor is not very convenient and is suitable only when working in buildings and with suspended cables. This device is convenient for the identification of cable pairs (separation of pairs at the connection of cables or cross) and cable terminations.
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Source: Website
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