GB2242418A - Protective cap for insulated electrical cable end - Google Patents

Abstract

A cap for protecting an end of an insulated electrical cable comprises a tubular portion 6 fittable over the end of the cable and closed at one end by a closure portion 7 which is provided with means 11 arranged to allow gas contained in the cable to escape from the cap and to prevent extraneous liquid from penetrating into the cap. Use of the cap enables degassing of a cable to which it is fitted. The means 11 may be a one-way valve including an elastic closure member (27 Figs. 2, 3) or may comprise a semi-permeable membrane. <IMAGE>

Description

PROTECTIVE CAP FOR INSULATED ELECTRICAL CABLE END The invention relates to a cap for protecting an end of an insulated electrical cable.

Such caps are mounted at the two ends of a cable wound on a storage drum inter alia for preventing extraneous liquid, especially water, from penetrating into the cable.

It is known that an insulated electrical cable is liable to release gases, depending on the nature of its internal components and cables usually include an external metal screen and/or an external protective sheath which are gas-tight or which at the very least have a low gas permeability. Since the end caps are themselves gas-tight or of low gas permeability gas generated inside the cable cannot escape. The disadvantages arising as a result of this are evident both during the storage of the cable and in use.

During storage, the gas pressure increasing within the cable tends to cause its sheath to swell and can also cause damage affecting the lifespan of the cable, particularly during storage in the open air where solar heating amplifies the swelling.

During the operation of cutting the cable before laying, and when the gas is inflammable, there is a risk of fire if a flame is used in the vicinity of the cable end.

In use, the swelling of the sheath of the cable can also occur as a result of the presence of gas trapped in the cable, this process being accelerated by the heating of the cable as a result of the Joule effect.

Within a junction chamber between two cables, the accumulation of an explosive gas can likewise lead to an explosion.

The invention relates more particularly to medium-voltage cables insulated with polyethylene cross-linked by peroxide. It is known that, during the cross-linking of cables insulated with crosslinked polyethylene, the decomposition of the peroxide gives rise to some by-products including methane. The amount of methane formed in the insulation depends on the type of peroxide, the concentration of peroxide and the decomposition rate of the peroxide. As an example, the amount of methane occurring as a result of the decomposition of dicumyl peroxide can amount to 700 to 1000 litres per km of 12/20 kV medium-voltage cable insulated with cross-linked polyethylene.

In an attempt to overcome the above-mentioned disadvantages, the cable insulation is degassed during manufacture of the cable before application of the external sheath over the insulation. Depending on the temperature at which the degassing is carried out, the duration of this operation can vary from a few days to more than a week.

An object of the invention is to allow gas sing of the cable during storage, whilst at the same time protecting the latter from the external environment.

According to the invention, there is provided a cap for protecting an end of an insulated electrical cable, comprising a tubular portion fittable over the end of the cable and closed at one end by a closure portion which is provided with means adapted to allow gas contained in the cable to escape from the cap and to prevent extraneous liquid from penetrating into the cap.

Thus, during storage of a cable fitted with an end cap as defined in the last preceding paragraph, the gas generated by the insulation can escape progressively to the outside of the cable, and at the same time corrosion of the cable attributable to ingress of water is prevented.

The invention also includes an insulated electrical cable having at least at one end, a cap as defined in the last but one preceding paragraph.

In one embodiment of the invention the abovementioned means comprises a valve including a gas outlet port, a closure element for closing said port, and means adapted to apply the closure element to the gas outlet port against a pressure exerted by the said gas and to allow outlet of gas through said port when the pressure exerted by the gas reaches a predetermined value, which for example may be 10 mbar.

Advantageously, the closure element may be elastic, and the means for applying the latter to the gas outlet port may comprise a protrusion bearing only on a central region of the closure element facing the said port. Such an arrangement is described in French Patent 2,209,062.

Furthermore, the valve may comprise a chamber which is delimited by a peripheral wall and two end walls arranged opposite each other, and in which is located said closure element, said gas outlet port being provided in one of the said end walls1 the said protrusion extending from the other said end wall for applying the element to said gas outlet port and a gas duct being provided in said other end wall.

In some cases, it may be desirable to prevent the closure element from assuming an unstable position of equilibrium corresponding to the situation in which the internal thrust exerted by the gas is equal to the force applying the closure element to the port, as indicated in the above-mentioned French Patent 2,209,062. This unstable position of equilibrium is conducive to an exchange of fluids between the interior and exterior of the cap and can be detrimental if the fluid so introduced into the cap is capable of damaging the cable.

In such circumstances, a viscous layer can be placed between the closure element and a surface delimiting the gas outlet port.

In another embodiment of the invention, the said means for allowing the escape of the gas comprises a semi-permeable member which is gas-permeable and liquid-tight and which extends across a port provided in said closure portion of the cap.

In order that the invention may be better understood, some embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section of a cap fitted to an end of an insulated electrical cable for protecting the same; Figure 2 is an enlarged axial section of a valve of the cap shown in a closed condition; and Figure 3 is a partial view of the same valve showing it in an open condition.

In Figure 1 there is illustrated a cable end 1 of an insulated electrical cable comprising a conductive core 2, insulation 3 over the core 2 and an outermost sheath or screen 4, which is water-tight and at least substantially gas-tight, over the insulation 3.

A cap 5 is fitted to and covers the cable end 1.

The cap comprises a tubular portion 6 of substantially circular cross-section which is closed at one end by means of a closure portion 7. The cap is formed of a dimensional recovery material, such as a heatshrinkable or elastic material. This material is water-tight and at least substantially gas-tight.

In the embodiment, an inner face of the tubular portion 6 of the cap 5 carries a layer 10 of hot-melt adhesive which makes a close connection with the sheath or screen 4 of the cable end.

The closure portion 7 of the cap 5 is provided with a valve 11 which has an internal structure substantially identical to that described in French Patent 2,209,062, but a different external structure.

The valve comprises a base 12 of general tubular shape which is of circular internal cross-section about an axis 15. A partition wall 13 extends transversely within the base 12 in an axially central region thereof.

A duct 14 passes through the partition wall 13 and extends parallel to the axis 15 of the base in the vicinity of an internal tubular wall 16 of the latter.

A conical protuberance 17 on the axis 15 extends from one face of the partition wall 13.

An external wall 20 of the base 12, carries an annular rim 21 arranged in the axially central region of the base and extending perpendicularly relative to the axis 15 of the base.

The external wall also carries, at an end furthest away from the conical protuberance 17, a frusto-conical annular rim 22 widening towards an opposite end of the base 12. The base 12 is provided with a frusto-conical, or dovetailed, internal annular groove 23 at its end remote from the frusto-conical annular rim 22.

A disc-shaped member 24 has a frusto-conical rim 25 adapted to be received in the annular groove 23 of the base 12, and a central port 26. A chamber 30 is delimited between a peripheral wall of the base 12, the partition wall 13 and member 24 forming respective opposed end walls of the chamber 30. Any suitable sealing means are used to provide a seal between the member 24 and the base 12.

The base 12 and the member 24 are formed of polyethylene.

The valve also comprises a disc-shaped closure element 27 formed of an elastic material, such as natural rubber, and arranged in the chamber 30 in such a way that it rests on the member 24, with the interposition of a viscous layer 31, such as a standard silicone oil, and closes the port 26.

Moreover, the conical protuberance 17 bears on the closure element 27.

The closure portion 7 of the cap 5 has a circular port 8, the diameter of which corresponds substantially to the external diameter of the valve between its two annular rims 21, 22. To fit the valve 11 to the closure portion 7 the end of the valve 11 having the frusto-conical annular rim 22 is pushed through this port 8, which in the illustrated embodiment is located centrally of the closure portion 7, and bears against an external surface of the closure element of the cap. An O-ring gasket 32 surrounds the valve and is compressed between an internal surface of the closure element of the cap and the annular rim 21 of the valve.

As regards the functioning of the valve 11, Figure 2 shows the valve in a closed condition for preventing water from penetrating into the cable end cap 5. When the pressure of the gas released by the insulation 3 of the cable has reached a value sufficient to generate an internal thrust exceeding the sum of the elastic bias force of the closure element 27 and the adhesive force attributable to the viscous layer 31, the closure element 27 abruptly detaches from the member 24 to put the valve in an open condition as shown in Figure 3. Preferably the valve is designed in such a way that movement to the open condition occurs when the pressure of the gas reaches 10 mbar.

After some time, the pressure of the gas decreases and the closure element 27 abruptly reattaches itself to the cover 24 to put the valve back into its closed condition.

In a modification of the illustrated embodiment, the valve is mounted on the closure element of cap 5 by means of a housing, in which the valve is accommodated, this housing being fastened to the cap.

In an alternative embodiment the valve may be a pneumatic valve.

Instead of the valve 11 fitted in the port 8 of the cap, a semi-permeable member may extend across the port. This member, or diaphragm, may advantageously comprise a layer of microporous polytetrafluoroethylene having pores of a diameter less than or equal to 0.5mm. Like the above-described valve, the semi-permeable member ensures sealing relative to extraneous liquids, at least under a pressure equal to 1 bar, but is gas permeable.

Claims (9)

CLAIMS:

1. A cap for protecting an end of an insulated electrical cable, comprising a tubular portion fittable over the end of the cable and closed at one end by a closure portion which is provided with means adapted to allow gas contained in the cable to escape from the cap and to prevent extraneous liquid from penetrating into the cap.

2. A cap according to claim 1, wherein said means comprises a valve including a gas outlet port, a closure element for closing said port and means adapted to apply the closure element to the gas outlet port against a pressure exerted by the said gas and to allow outlet of gas through said port when the pressure exerted by the gas reaches a predetermined value.

3. A cap according to claim 2, wherein said predetermined value is 10 mbar.

4. A cap according to claim 2 or 3, wherein the closure element is elastic and the means for applying the latter to the gas outlet port comprises a protrusion bearing only on a central region of the closure element facing the said port.

5. A cap according to claim 4, wherein the valve comprises a chamber which is delimited by a peripheral wall and two end walls arranged opposite each other, and in which is located said closure element, said gas outlet port being provided in one of the said end walls, the said protrusion extending from the other said end wall for applying the element to said gas outlet port and a gas duct being provided in said other end wall.

6. A cap according to claim 4 or 5, having a viscous layer placed between the closure element and a surface delimiting the said gas outlet port.

7. A cap according to claim 1, wherein the said means comprise a semi-permeable member which is gaspermeable and liquid-tight and which extends across a port provided in said closure portion of the cap.

8. A cap for protecting an end of an insulated cable substantially as hereinbefore described with reference to the accompanying drawings.

9. An insulated electrical cable having at least at one end, a cap according to any one of the preceding claims.