Electricity: conductors and insulators – Conduits – cables or conductors – Insulated
Reexamination Certificate
2002-11-27
2004-08-31
Mayo, III, William H. (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Insulated
C174S036000
Reexamination Certificate
active
06784371
ABSTRACT:
RELATED APPLICATION
Applicant claims the benefit of European Application No. 01 127 478.4, filed Nov. 28, 2001.
FIELD OF THE INVENTION
The present invention relates to a cable like a power cable, a copper telecoms cable, a fibre optical cable or the like and in particular, the present invention relates to a cable, which comprises a cable core, an inner cable sheath, an outer sheath and a sensor, which is extending along the cable for detecting a detectable substance like water inside the cable.
In such a cable conditions can occur in which substances like water intrude through the partially damaged outer sheath and such substance travels between the inner cable sheath and outer cable sheath in longitudinal direction leading to damage of the cable by chemical and electrochemical effects in a much larger cable section than the section of the partial damage of the outer sheath and inaccurate substance intrusion measurements. The invention particularly addresses these problems in order to limit damaged areas and to increase the measurement accuracy and lifetime of the cable.
BACKGROUND OF THE INVENTION
Cables like power cables, copper telecoms cables, fibre optical cables or the like very often comprise a cable core, an inner cable sheath, and an outer cable sheath around that inner cable sheath. Typically, a cable core of copper telecoms cable includes at least two copper wires insulated against each other and commonly insulated in addition. Furthermore, depending on the type of cable, a screen might build in. Typically, a cable core of fibre optical cables is generally similarly constructed as the copper cable but including optical fibres instead of copper wires. Typically, a cable core of a high voltage transmission cable comprises a conductor and a screen surrounding that conductor. Furthermore, independent of the type of cable, such cables very often are provided with an insulating and water-proof outer coating. In order to avoid the entry of water or water vapours through the outer coating to the cable core the outer coating preferably consists of metal or a metal sheet as an inner sheath in combination with an outer layer of PE or PVC as an outer sheath (layer coating).
Cables are used to distribute communication data or electric power, which both have to be transported to the end users. Because of limited space, especially in the cities cables are often subject to third party damage during installation work on site. In many cases the damage only happens to the cable sheath without effecting the electrical function of the cable core. If this damage remains undetected, water (or other substance) ingress to the sheath will cause an electrical damage by the time. Cables with metal laminated plastic sheaths can be fitted with water sensors, which are integrated in the screen area of the cable inside the inner sheath of the cable. These sensors stay dry as long the inner cable sheath is tight. In case of a damage of the outer and inner sheath the sensors get wet and change their electrical resistance regarding the screen. By monitoring the insulation resistance of the sensors the water ingress can be detected and located. In cases of cables with a tube-type metal sheath—in contrary to a metal foil, as mentioned above —it is very often of a major interest to protect the metal sheath as this is the main barrier to the cable core against water. Such cables can have a dry-type plastic insulation or be paper insulated and oil filled. With respect to oil filled cables a tight metal sheath is of special interest because such cables are under internal pressure and will fail immediately under decreasing oil pressure and pollute the environment in case of a damage. The protection of the metal sheaths mostly are performed by plastic sheaths. In case of mechanical damage of this plastic sheath, the metal sheath can be subject of corrosion and can fail later. The avoidance of, for example, water intrusion is of particular significance, since intruded water accelerates the aging processes of the insulating materials of the cable made of cross-linked polyethylene and thus leads to an early damage or failure of the cable.
Water sensors and a monitoring system already known can prevent this chain of failures by detecting the presence of water on the metal sheath. Water monitoring systems are used in order to detect the water intrusion into the cable and to determine the location of the water intrusion. Typically, one or more water sensing wires are provided in the cable core screen. The water sensing wire extends in the screen either parallel to the cable core or is wound around the cable core. A measuring circuit performs voltage drop measurements at the water sensing wire or water sensing wires because a water intrusion into the cable will cause a change of the isolation resistance of the water sensing wire and thus different voltage drops at the water sensing wire. In order to perform the voltage drop measurements typically the measuring circuit injects a measuring current into the water sensing wire. Such a monitoring system can not detect a damage of the outer sheath before the inner sheath will be damaged additionally.
To avoid travelling of water along the cable between the inner cable sheath and the outer sheath it is also known to use swellable compounds in the form of tapes applied between the cable core and a sheath in order to prevent ingress of water into the respective interstice. Such swellable tapes are typically made out of water blocking swelling nonwoven fabric, capable to increase its size in contact with water. Such tapes can avoid the intrusion of water for some time but are not a long term solution. Therefore information regarding water intrusion is still necessary.
DESCRIPTION OF THE PRIOR ART
In the German patent applications DE 195 44 391 A1 and DE 195 27 972 Pirelli describes a water monitoring system for a three-phase power transmission system. Each of the three cables has a respective water sensing wire and the three sensing wires are connected at the cable beginning and the cable end through serially connected measurement and termination resistors. At the cable end the common connection node is connected to ground via a load resistance. At the cable beginning a DC voltage source is connected between the common connection point at the cable beginning and ground. Also the screens are connected to ground. The DC currents flowing in the water sensing wires constitute a measure for the presence or absence of a water intrusion and also a measure for the location of the water intrusion.
The cable shown in DE 195 27 972 has the sensor wire applied within the screen inside the inner metal sheath. With such arrangement water intrusion inside the inner cable sheath can be detected.
SUMMARY OF THE INVENTION
As explained above, in a cable conditions can occur in which substances like water intrude through the partially damaged outer sheath and possibly travels between the inner cable sheath and outer cable sheath in longitudinal direction leading to later damage of the inner sheath over a longer distance and leading to inaccurate substance intrusion measurements and damage of the cable by chemical and electrochemical effects in a much larger cable section than the section of the partial damage of the outer sheath.
It is therefore a primary object of the present invention, to provide a cable which meets the requirements of detecting water in the interstice between outer sheath i.e., plastic) and inner sheath (i.e., metal or plastic).
This object is solved by a cable comprising a cable core, an inner cable sheath, an outer sheath and a sensor, which is extending along the cable for detecting a detectable substance inside the cable.
According to the invention, (claim
1
) the sensor is arranged between the inner cable sheath and the outer sheath for detecting such substance outside the inner cable sheath.
Such cable according to the present invention, for example may be a power cable, a copper telecom cable, and a fibre optical cable.
Such arrangement of the sensor or sensors betw
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Mayo III William H.
Pirelli Kabel und Systeme GmbH & Co. KG
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