Cable fault monitoring system

Details Cable fault monitoring system Cable fault monitoring system

1999-02-17

2001-05-22

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

For fault location

C324S552000

Reexamination Certificate

active

06236217

ABSTRACT:

The present invention relates to a cable fault monitoring system. It is concerned particularly, but not exclusively, with the monitoring of sheaths of underground cables where damage to the outer insulation gives rise to a conduction path from an inner metallic element to ground.
It is known that where the insulation of an underground cable is damaged, the short between the metallic element of the cable and ground causes corrosion to occur if left unprotected.
Known techniques for location of faults from the end(s) of the cable fail when there are multiple faults present in the cable. Faults may be divided into primary faults, with low impedance, and secondary faults with higher impedance. Where there is a primary and a secondary fault on the line, the accuracy of location of the primary fault will be affected by the existence of the secondary fault. If there are multiple primary faults, it is not possible to identify them separately, and hence locate them. This problem is particularly acute where DC measurements are made from each end of the cable, since in that case it becomes impossible to resolve more than one fault. In general, DC measurement techniques are preferable, because of the attenuation of high frequency signals over long distances, due to signal current leaking to ground through capacitive coupling.
It would be possible to resolve multiple faults by use of time domain reflectometry. However, it has been found that the impedance and attenuation of typical cable sheaths generally precludes the use of time domain reflectometry techniques. This is particularly true where signals are to be applied to the conductive sheath of a fibre optic cable.
It is also possible to provide a plurality of sensors along the cable, each of which is capable of detecting voltage (and possibly current) values at each location along the cable and transmitting those values to an analysis station. That analysis station then analyses the values along the length of the cable, to determine the location of the or each fault by extrapolation between the values generated by the sensors.
JP-A-55-562364 discloses an arrangement in which currents at each end splice along a cable are used to trigger respective links between auxiliary lines running along the cable, to enable the distance to the fault to be determined at a base station to which the auxiliary lines are connected.
The present invention operates on a third principle. It proposes that a plurality of sensors are provided along the cable, each of which is capable of measuring the cable-to-ground resistance of a section of the sheath of the cable, using a resistance measurement device in the sensor. The results of the resistance measurements may then be transmitted to an analysis station.
If a section of cable on which a measurement is made is intact, its cable-to-ground conductance will be low (in the order of micro-siemens/km). However, if the cable is damaged, and there is a conduction path from the sheath to ground, the resistance measured will be much less.
In order to lay a long line of cable, sections of optical fibre, with a length of about 2 km, are connected by splice joints. It is therefore convenient to locate the sensors at such splices, since the sections of the sheath are accessible there for electrical connection.
Preferably, each sensor contains a switch, eg. a remotely activatable software-controlled electronic switch, which connects or disconnects the sheath of two separate sections of the cable. When the switches at each end of a section are open, that section is isolated from the rest of the cable and appropriate resistance measurements can be taken. If the sensor then signals back to a base station, and this process is repeated for each cable section, it can be determined if any section contains a fault. It can be noted that this permits multiple faults to be detected since measurements are carried out on each section separately. Preferably, the values determined by each sensor are transmitted to the analysis station via the cable itself, but telecommunication transmission systems are also possible.


REFERENCES:
patent: 4110683 (1978-08-01), Cason et al.
patent: 4947469 (1990-08-01), Vokey et al.
patent: 5990686 (1999-11-01), Vokey et al.
Japan Patent abstract (55062369).

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