Cable Line Tracing: Choosing the Operation Frequency. Passive Location

How does a generator's signal frequency affect the range of the cable location signal? What cable identification frequency is more efficiently? Why can cable identification at low frequency be more reliable? And when is passive location used?

Cable tracing devices consist of a generator and a receiver. Some generator types have the option to choose a frequency (usually in the range of 200 Hz - 130 kHz). Moreover, choosing the correct frequency is incredibly important. The value of the frequency affects the working distance of the cable location signal. This is the distance at which the receiver "picks up" the signal from the generator.

The impedance of a long cable is predominantly capacitive. As the frequency increases, the signal leakage from the traced cable into the ground also increases. The power of the current also decreases faster along the length of the cable. Consequently, the distance where the cable location signal can be detected also decreases.

By the way, this explains why the cable (or pipeline) diameter affects the signal detection distance. The larger the surface area of the cable shielding or pipeline, the greater the increase of current leakage to the ground that one can observe. This leads to a decrease in the signal strength over the length of the power line. Therefore, when a signal is sent over a power line with a smaller diameter, it can be detected at a greater distance from the transmitter. However, it is only true up to a certain point. To close the circuit (and increase the strength of the current), it is necessary to ground the far end of the routed power line.

Circuit impedance also depends on the soil's conductivity. The soil structure (loose or dense) and moisture content affects two things: the return current conditions and its leakage into parallel lines. The former is simple and easy to understand. The impedance of moist and dense soil is lower than that of dry and loose soil. If the soil is moist and dense, the strength of the current in the circuit will be higher. However, leakage will increase as well. Generally speaking, a thin cable that's laid in the desert can be detected at a much greater distance from transmitter than a thick cable laid that's in a swamp.

Let's imagine that a cable and a metal pipeline are laid side by side in soil that has low conductivity. If you route a cable in such soil, return current will flow through the pipeline. The impedance of the circuit will decrease and the current will increase. But now the pipeline with the return current flow will also emit a signal. In such case, tracing the cable will most likely not become easier.

The frequency of the signal also affects the difficulty of routed line tracing when there are several power lines running in parallel. The higher the frequency, the greater the signal that's induced in the parallel lines. This makes it more difficult to identify the necessary cable. Let's assume that a parallel line is routed at shallower depth than the one being traced. Thus, the signal detected from this parallel line can be stronger than the one coming from the line that's connected to the generator.

So, a reduction in frequency leads to increase in detection range of the cable location signal. It also makes it easier identify a traced line among several parallel lines.

Cable tracing at high frequency

Further, let's consider the operation of a cable locator at a high signal frequency. There are four main reasons to use a high-frequency signal during cable tracing.

Firstly, a high-frequency signal is required for the proper operation of an inductive antenna and an inductive transmitter. These devices are used to generate a signal to the line being traced without a direct connection. The induction method cannot be used to transfer a low-frequency signal at the necessary distance.

Secondly, the higher the frequency, the higher the current induced in the line being traced. This is true for both cables that have a small diameter and length, as well as in conditions of low-conductivity soil (dry sandy soil).

Thirdly, a high-frequency signal means low noise levels from the surrounding power supply lines. This is because the fundamental harmonics of their signals are within the range of 60 Hz to 3 kHz.

Fourthly, high-frequency currents can easily penetrate loose connections of metal pipes in cable duct systems or the insulation joints (pads) in pipelines.

Operating ranges (L1, L2, L3) for different frequencies (f1, f2, f3) for cable tracing depend on parameters, such as:

• signal circuit characteristics. These include the physical parameters of the power supply line and surrounding soil as well as the grounding method and quality.
• signal frequency value;
• transmitter power output;
• difference between the transmitter output impedance and signal circuit impedance;
• receiver sensitivity and selectivity;
• environmental noise in the area of the line being traced.

Considering this, we can derive a simple rule: Always start performing cable tracing with the lowest frequency. If the frequency does not provide the required signal level, switch to a higher frequency.

Passive Location

Above, we considered the signals fed into the being traced line with the cable locator generator. However, it should be noted that cable locators can operate without a transmitter. This method is called passive location. In this case, the cable locator receiver detects the signal emitted by an energized (live) cable. For example, when tracing power supply lines that are under load. Note that the magnetic field (which is detected by the receiver) is generated by the current that's running through the power supply line. There won't be any problems with tracing if the line is operational and there's current flowing through it. However, if a live cable not connected to a load, it is nearly impossible to trace it using passive location.

Other limitations are also applicable to passive location. It is easy enough to trace a single-phase AC line that's under load. But when it comes to three-phase lines, tracing becomes more challenging. Power supply lines are designed to provide nearly equal loads at all phases. This means that a current of nearly equal values flows through all phases. The fields generated by these currents compensate for each other. Therefore, it is difficult to trace multiphase lines. This nuance may lead to serious mistakes when tracing. Imagine that you have easily identified a single-phase line (e.g. a lighting system cable). However, you missed a high-voltage 3-phase line that runs in parallel to the identified line. This mistake may cause very dire consequences.

During passive location, you can also trace pipelines connected to cathodic protection systems. In order to ensure protection, rectified voltage is provided to the pipeline from the industrial power supply system. Thus, the current in traced power supply line will pulse at 120 Hz. Phone cables are the most difficult lines to identify during passive location. This is because all the alternating currents that flow through them are low and unpredictable.

Any lengthy metal objects located underground can also be identified with passive location in the low-frequency range of 60 Hz to 3 kHz. It is not limited to power supply cables. The fact of the matter is, return currents between the grounding points of transformers in electrical substations and line loads in power supply networks flow underground. If a cable with a metal sheath or a metal pipeline is located there, the return current will flow through it. This is all because the metal object's resistance is lower than the resistance of the soil. This current will create a signal that will allow you to identify the presence of the metal object with the cable locator during passive location. Since such grounding points are everywhere, the currents are also everywhere.

Moreover, long-wave radio signals can also be present in the immediate environment. They penetrate the soil and induce currents in the underground metal objects. Such objects function as antennas that re-emit signals. Then, the signals can be detected using a receiver that's configured for the respective frequency range. The signal level depends on the distance from the transmitting stations, as well as the power supply line and soil parameters. Generally, it is easier to identify a long-wave signal (140 to 300 kHz) if both ends of the power supply line being traced are well-grounded, and the line is long (i.e. the capacitor coupling is high).

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