Pulser fault locators: operation principle and technology for cable troubleshooting

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Unlike bridges, which can be used to measure the distance to the defect along the cable route, pulser fault locators search for the cable fault location, i.e., determine where exactly the defect is located. At the same time, cable parameters or temperature do not affect its localization accuracy.

The primary purpose of pulser fault locators is to localize defects in the protective insulating sheath of the cable, due to which the shield and/or conductors are shorted to the ground. The signal from the generator is applied between the defective conductor and the earth to find such locations. As a result, a closed circuit is formed (generator-conductor-defect-soil-generator).

Current flowing in the ground is concentrated in the area of the earth electrode and fault. However, the current flows at different depths along various paths with the lowest resistance between these points. A sensitive voltmeter can detect the potential difference in any area of the ground due to current flow. It is such a device that is used in a pulser fault locator as a receiver. The signal is picked up by two probes located a small distance from each other (usually mounted on an A-frame). The receiver probes are stuck into the ground to reduce transient resistance during signal acquisition.

Depending on the condition of the ground, pulser fault locators detect defects with a resistance of 0.5-2 megohms. For successful localization, it is necessary that as much of the current as possible flows through the fault into the ground and through the ground to the earth electrode. Therefore, the generator should be connected to a high-quality earthing system.

Photo from website RIDGID ® (www.ridgid.com/us/en/a-frame-fault-locator)

AC voltage or DC voltage pulses are used as the test signal. In the first case, the receiver only measures the signal strength between the two probes (potential difference). In the second case, in addition to the signal strength, the polarity of the signal can also be used to determine the position of the receiver's probes relative to the defect (before or after the fault). But the AC signal also has an attractive feature - it allows you to trace the cable.

In any case, the signal strength is minimized when the probes are positioned strictly above the defect and symmetrical about it: if the fault is located exactly below the probe (midway between the two probes of the A-frame), the signal strength will be zero. It is this fact that is used to localize the defect.

For error-free fault location and refinement of the cable trace, the probes should be placed in a plane perpendicular to the trace and again find the point where the signal will be minimized - accuracy can be as low as 0.1 meters.

The signal levels are maximized at these locations since the current is concentrated near the earth electrode and the defect. If the distance between the earth electrode and the fault is large, the signal may not be detected because it is too weak. The distance to the defect should be estimated using a bridge or TDR to reduce the search time beforehand. This will allow you to start the search close to the fault instead of walking along the trace from its beginning.

It is essential to remember that the signal's amplitude at a certain distance from the earth electrode is equal to the amplitude at the same distance from the defect. This can significantly accelerate the search - using a receiver calibrated in this way allows not to react to minor signal fluctuations on the trace. Using a point located on the other side of the earth electrode at the same distance from the cable as the earth electrode as a reference is usually recommended.

Suppose any conductors (pipelines, armored cables, etc.) run close to the cable and parallel to it. In that case, the return current, choosing the path of least resistance, will flow through them rather than through the ground. This significantly reduces the signal level and makes it difficult to localize the defect.

The same problem occurs if the localizable defect is located in one of several cables from the same point. In such a case, the capacitance of the shield-to-ground circuit can be quite significant since the cable shields are usually connected. The large capacitance of the shield-to-ground circuit leads to the fact that the leakage of the test signal through the capacitance of the shields into the ground becomes comparable to the leakage of the test signal through the defect resistance.

For the same reason, all conductors in the cable under investigation that are grounded under normal circumstances and are therefore capable - instead of grounding - of carrying current back must be disconnected from grounding.

The generator earth electrode should be installed as far as possible from the cable route in a perpendicular direction to eliminate the influence of parasitic circuits running parallel to the cable. The grounding must be as good as possible (with the lowest possible resistance).

Suppose several defects are detected in the cable. In that case, their magnitude is estimated by the leakage current value of each of them - if the cable has deviated perpendicularly from the cable route when taking measurements, the more significant the defect, the greater the distance at which it will be detected. It should be noted that in the presence of several defects located close to each other (which often happens when the cable sheath is damaged or it gets wet), strong defects can "mask" weak ones. Therefore, after the localization of the fault and its elimination, the cable should be rechecked.

When the cable is under a hard surface (asphalt, concrete), to reduce the transient resistance between the probes and the surface, the surface should be moistened by placing pieces of foam rubber soaked in slightly salted water on both probes. However, this complicated method is only used when the size of the hard coating is sufficiently large.

The situation is more manageable if the cable is across a small, hard-surfaced area (e.g., crossing a road). To locate the cable, you can "extend" the probes so that they are outside the pavement. An additional earth electrode is connected to one of the probes with a wire, placed on one side of the pavement, and the other probe is inserted into the ground on the other side to do this. The next step is to find the position of the probe where there is no signal and locate the fault in the middle of the segment between the probe and the second earth electrode.

If the cable is installed along a hard-surfaced area (e.g., under a roadway), a strip of soil along the roadside can be used for the feeler gauge.

It is impossible to localize the defect if the cable does not lie in the ground but in a non-conductive conduit. The current will flow along the inner surface of the conduit to the seam or crack, where it will leak into the ground. This location will be identified by the pulser fault locator receiver as the location of the defect.

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Kevin Gibbs

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