Saturday, February 28, 2015

Cable Fault Tone Traccing


Equipment Construction

Tone tracing equipment includes a tone generator and a signal tracer. Tone Generator A tone generator, sometimes called a transmitter, is illustrated in Figure 1. A tone generator is a suitcase-size test set that is powered by 120 volts. It has controls that adjust the magnitude of high-voltage DC output and
the frequency of its oscillator output

Signal Tracer

Figure 1 also contains an illustration of a signal tracer. The signal tracer consists of a hand-held detector and an insulated-shaft probe. The signal tracer is sensitive to those frequencies that the tone generator produces, and it filters all other audio-frequency signals.
Equipment Operational Principles

Tone Generator

The tone generator has a built-in sine-wave oscillator that injects an audio-frequency current into the core conductor of a faulted cable. This audio frequency current is driven by a solid-state amplifier that is typically rated at 2.5 watts of output power. The output is usually a single frequency, but some models have more than one selectable frequency. Audio frequencies are in the range of 10 Hz to 10,000 Hz depending on the specific model and manufacturer of the tone generator.

The tone generator also has a built-in high-voltage DC source that can be used to break down a high-resistance shunt fault for the purpose of providing a low-resistance path for the audio-frequency current.

Signal Tracer

The signal tracer has an inductive pickup built into its probe. The output signal of this probe and the sound it he headphones become stronger as the probe is brought closer to a cable.
Basic Localization Techniques Methods

The basic method of locating a shunt fault is to adjust the high-voltage DC output of the tone generator to a magnitude that will make the damaged insulation in the cable flash over. The audio-frequency current allows the technician to trace the path of the buried cable. The location of the fault is found by sweeping the probe over the surface of the earth above the cable and listening for the characteristic sound of flashover in the headphones.

The basic method of locating an open-circuit fault is to adjust the high-voltage DC output to a minimum and adjust the audio-frequency current to a maximum. The location of the fault is found by sweeping the probe over the surface of the earth above the cable and listening for the audio tone.

Maximum Signal - For a shunt fault, the characteristic sound of a flashover will be loudest in the detector’s headphones when the probe is oriented directly above the fault.

For an open circuit fault, the audio tone heard in the detector’s headphones will fade and become inaudible when the technician sweeps the probe past the location of the open circuit.

Null Signal - The probe’s inductive pickup has a directional characteristic such that whenever the probe is oriented in parallel with a cable that is carrying an audio tone, its output signal strength will become near zero (null signal). When the probe is oriented perpendicular to a cable carrying an audio tone, its output signal will reach a maximum

Minimizing Interference and Crosstalk

Electromagnetic interference is produced by any cable that is energized with normal voltage. In order to minimize interference, nearby cables should be de-energized to whatever extent is possible without disrupting electrical service.

The audio-frequency current that is intentionally injected into one power cable can unintentionally induce an audio-frequency current in another cable that is buried nearby. This unintentional induced current, called crosstalk, can mislead the technician who is tracing the path of a cable. In order to minimize crosstalk, deenergized cables should be connected to ground on both ends.

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