Electricity: measuring and testing – Conductor identification or location – Inaccessible
Reexamination Certificate
1998-06-08
2001-02-27
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Conductor identification or location
Inaccessible
Reexamination Certificate
active
06194889
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to cable location using a locating tone impressed on the cable.
BACKGROUND
Telephone, cable television (CATV) and other communication and control cables are often direct buried or placed in underground duct structures. For this type of cable placement, the most significant cause of cable outages is caused by dig ups by contractors. In an effort to minimize inadvertent dig ups “call before you dig” programs are heavily promoted. The operating company must then be able to quickly and accurately locate and mark the buried cable.
Methods have been developed and are in commercial use which place a locating tone on the cable sheath via the armour or shield and a special receiver with magnetic field detecting coils is used to sense the tone current travelling along the cable. The strength of the received signal is directly proportional to the magnitude of the tone current in the cable sheath directly below the receiver.
The transmission circuit for the tone signals is formed by a metal armour or shield of the cable, insulated by a plastic cable jacket from earth, which forms the return conductor. The circuit is basically a form of coaxial transmission path with the cable shield or armour forming the inner conductor, the plastic cable jacket the insulator and the surrounding earth forming the outer conductor.
As with any transmission line with resistive elements, the line is lossy, and the loss increases with increasing frequency. In an attempt to reach longer and longer distances, the tone frequency is selected as low as is practical and the input current and power level set as high as is reasonable and safe. Fibre optic cables are often installed in long lengths of up to 75 miles (120 km) or more. Even with the use of low frequencies and high power the signal is often weak or unusable at these long distances.
The situation may be further complicated by a requirement to transmit high frequency tones, which are often used for inductively coupled short distance location applications.
This application relates to the enhancement of the long distance low frequency tones. In preferred embodiments this will not impede the use of high frequency tones for inductively coupled short distance applications.
SUMMARY
According to the present invention there is provided a transmission enhancement system for low frequency cable locating signals on a metallic shield of an underground cable, said system comprising at least one equalization circuit including an inductor connected in series in the shield.
The equalization circuits may be placed on the cable armour or shield at regular intervals, for example at splice locations. This significantly reduces the rate of attenuation of the low frequency tone signal and greatly enhances the strength of the signal at distances where the signal would otherwise be very weak or unusable.
In preferred embodiments of the invention, the equalization circuit includes a capacitor connected in parallel with each inductor to allow local application of high frequency tones for local location purposes without undue attenuation.
The attenuation of the shield with the equalization circuits is preferably no more than 1 dB per mile (0.6 dB per km) at the frequency of the low frequency locating signal. Where the capacitance is included, the inductance and capacitance are selected to provide a resonant circuit with a resonant frequency between the low frequency locating signal and the high frequency local locating signals. The currently preferred circuit acts as a band stop filter with a resonant frequency between 600 Hz and 2500 Hz, to enhance signals at about 500 Hz and allow higher frequency local locating signals, for example at about 8 kHz.
The preferred embodiments of the equalization circuit include a surge arrestor coupled in parallel with the inductor and capacitor.
REFERENCES:
patent: 4549039 (1985-10-01), Charlebois et al.
patent: 4862088 (1989-08-01), Etienne et al.
patent: 2 300 267 (1996-10-01), None
Aikins, MacAulay & Thorvaldson
Norscan Inc.
Snow Walter E.
Thrift Murray E.
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