Environment-friendly anti-corrosion overhead insulated cable

Abstract

The application discloses an environment-friendly anti-corrosion overhead insulated cable and particularly relates to the technical field of electric wires and cables, which comprises a conductor, a conductor shielding layer, an insulation layer, an insulation shielding layer, a metal shielding layer, a water-blocking layer, an inner protective layer and an outer protective layer. The application is provided with a clamping section and a buffer section. The clamping section is high in rigidity and is suitable for the rigid clamping requirement of fittings. The buffer section dissipates vibration energy through expansion and contraction deformation, greatly reduces the alternating stress in the clamping area, inhibits fretting wear and micro-crack initiation, and increases the pressure in the filling cavity by using the compression deformation of the buffer section when vibration is triggered. When the micro-crack penetrates the outer wall of the filling cavity, the corrosion inhibitor is automatically released, seeps along the crack, forms a film to block the corrosion of corrosive media, and the release amount is self-adaptively matched with the vibration intensity, thereby avoiding invalid loss. Meanwhile, the buffer section reduces the alternating stress transmitted to the filling cavity area, can inhibit the rapid expansion of cracks, delays the abrasion failure of the wire clamp, reduces the operation and maintenance frequency, and improves the power supply reliability.

Description

Technical Field

[0001] This invention relates to the field of wire and cable technology, and more specifically, to an environmentally friendly and corrosion-resistant overhead insulated cable. Background Technology

[0002] Overhead insulated cables (or simply overhead cables) are overhead conductors equipped with insulation layers and protective sheaths. They are a type of specialized cable manufactured using a process similar to that of cross-linked cables, representing a new type of power transmission method between overhead conductors and underground cables. Overhead cables are mostly single-core structures, and can be classified into different types according to their structure, such as hard aluminum wire structure, hard-drawn copper wire structure, aluminum alloy wire structure, and steel core support structure.

[0003] Chinese patent application number CN202510824881.7 discloses a weather-resistant insulated overhead cable. The inner sheath layer of this invention uses neoprene rubber as the main raw material. Neoprene rubber has excellent weather resistance, ozone resistance, chemical corrosion resistance and flame retardancy. Carbon black can enhance the wear resistance and tear resistance of the inner sheath layer, while improving its weather resistance. Anti-aging agents can effectively slow down the aging rate of the inner sheath layer. Plasticizers improve the flexibility and processing performance of neoprene rubber. The inner sheath layer wraps around the shielding layer, which provides further protection for the inner insulation layer and the shielding layer. Its good weather resistance can resist the erosion of the internal structure by the external environment. It works synergistically with the outer insulation layer to enhance the overall weather resistance of the cable.

[0004] While the aforementioned inventions have improved the ability of overhead cables to resist external climate erosion and delay sheath aging, outdoor overhead insulated cables, in addition to being subjected to static environmental erosion such as sun and rain, are also subject to high-frequency vibrations generated by light winds over a long period of time. This causes repeated fretting wear at the clamp fixing points. This wear thins the cable surface, induces micro-cracks, and accumulates powdery abrasive debris. As the wear continues to deepen, the outer protective structure is gradually destroyed, providing a channel for corrosive media to penetrate, thereby aggravating electrochemical corrosion and stress concentration at the contact points. This not only significantly reduces insulation performance but also seriously threatens power supply reliability, forcing an increase in the frequency of operation and maintenance, and driving up the total life cycle cost.

[0005] This invention provides an environmentally friendly and corrosion-resistant overhead insulated cable, which aims to solve the problem of fretting wear at the cable clamp caused by wind vibration, which leads to sheath cracking and abrasion damage, ultimately resulting in deterioration of insulation performance and a surge in operation and maintenance costs. Summary of the Invention

[0006] The purpose of this invention is to provide an environmentally friendly and corrosion-resistant overhead insulated cable to solve the problem mentioned in the background art of fretting wear at the cable clamp caused by wind vibration, which leads to sheath cracking and abrasion damage, ultimately resulting in deterioration of insulation performance and a surge in maintenance costs.

[0007] To achieve the above objectives, the present invention provides the following technical solution: an environmentally friendly and corrosion-resistant overhead insulated cable, comprising, from the inside out, a conductor, a conductor shielding layer, an insulation layer, an insulation shielding layer, a metal shielding layer, a water-blocking layer, an inner sheath, and an outer sheath; The outer protective layer is provided with a clamping section, a buffer section and a regular section along the axial direction. The clamping section is used to clamp with hardware. The clamping section is provided with a filling cavity for storing and releasing corrosion inhibitor. The buffer section is used to dissipate vibration energy and regulate the pressure in the filling cavity when elastic deformation occurs. When a microcrack is generated on the surface of the clamping section and connects to the filling cavity, the corrosion inhibitor is driven to be released outward.

[0008] Preferably, the clamping section is further provided with a squeezing groove, the squeezing groove is connected to the filling cavity through a connecting groove, and a first piston is slidably connected in the squeezing groove. The squeezing groove is connected to the internal space of the buffer section through a venting groove.

[0009] Preferably, when the buffer section undergoes axial compression deformation, its internal volume decreases, and the pressurized gas enters the extrusion groove through the venting groove, pushing the first piston to slide towards the filling cavity, thereby increasing the pressure inside the filling cavity.

[0010] Preferably, the outer periphery of the clamping segment is fixedly connected with two mounting rings, which are respectively disposed at both ends of the clamping segment along the axial direction.

[0011] Preferably, the mounting ring has a drive groove, a second piston is slidably connected in the drive groove, and an elastic element for driving the second piston to reset is also provided in the drive groove.

[0012] Preferably, a support member is fixedly connected to one end of the conventional section near the buffer section, and the support member is connected to the second piston member by a pull rope.

[0013] Preferably, a flow guide groove is provided on the outer periphery of the clamping section, one end of the flow guide groove is connected to the driving groove, and the other end passes through the mounting ring on the other side.

[0014] Preferably, when the buffer section undergoes axial tensile deformation, the pull rope pulls the second piston to slide and causes the elastic element to store energy; When the buffer section undergoes axial compression deformation, the elastic element resets and drives the second piston to slide in the opposite direction, so as to blow the gas in the drive groove out through the guide groove.

[0015] Preferably, the corrosion inhibitor in the filling cavity seeps outward when a microcrack is generated on the surface of the clamping section that extends into the filling cavity.

[0016] Preferably, the two ends of the buffer section are respectively connected to the clamping section and the conventional section, and the clamping section, the buffer section and the conventional section together constitute the axially continuous whole of the outer protective layer.

[0017] The technical effects and advantages of this invention are as follows: 1. This invention, through the design of clamping sections and buffer sections, etc., uses the clamping sections to adapt to the rigid clamping requirements of hardware with high rigidity, and the buffer sections to dissipate vibration energy through expansion and contraction deformation, significantly reducing the alternating stress in the clamping area, suppressing fretting wear and microcrack initiation. When vibration is triggered, the compression deformation of the buffer section increases the pressure inside the filling cavity. When the microcrack penetrates the outer wall of the filling cavity, the corrosion inhibitor is automatically released, seeping along the crack to form a film and blocking the erosion of the corrosive medium. The release amount is adaptively matched with the vibration intensity to avoid ineffective losses. At the same time, the buffer section reduces the alternating stress transmitted to the filling cavity area, which can suppress the rapid propagation of cracks, thereby delaying the wear failure at the clamp, reducing the frequency of operation and maintenance, and improving the reliability of power supply. 2. This invention, through the design of structures such as drive grooves and guide grooves, utilizes the reciprocating expansion and contraction deformation of the buffer section to drive the reciprocating sliding of the second piston component within the drive groove. Combined with the energy storage and reset of the elastic component, the gas within the drive groove is squeezed during the compression phase of the buffer section and continuously blown through the guide groove to the interface area between the hardware end and the cable outer sheath. On the one hand, this can weaken the adhesion of salt spray, powder abrasive, and corrosion deposits at the interface edge, preventing them from being repeatedly squeezed into the contact interface and forming an abrasive medium. On the other hand, it can actively remove trace amounts of moisture that have seeped into the interface, reducing local humidity and preventing the formation of a viscous dirt layer that is both conductive and abrasive, thus extending the cable's service life and reducing maintenance intervention. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the overall structure of the present invention.

[0019] Figure 2 This is a partial enlarged view of the overall structure of the present invention.

[0020] Figure 3 This is a radial sectional view of the overall structure of the present invention.

[0021] Figure 4 This is an axial cross-sectional view of the outer protective layer structure of the present invention.

[0022] Figure 5 For the present invention Figure 4 Enlarged view of the structure of part A.

[0023] The attached figures are labeled as follows: 1. Conductor; 11. Conductor shielding layer; 12. Insulation layer; 13. Insulation shielding layer; 14. Metal shielding layer; 15. Water-blocking layer; 16. Inner protective layer; 17. Outer protective layer; 2. Clamping section; 21. Buffer section; 22. Regular section; 23. Filling cavity; 24. Clearance groove; 25. Extrusion groove; 26. First piston component; 27. Ventilation groove; 28. Connecting groove; 3. Mounting ring; 31. Drive groove; 32. Second piston component; 33. Elastic component; 34. Support component; 35. Pull rope; 36. Guide groove. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example 1

[0025] refer to Figures 1 to 5 An environmentally friendly and corrosion-resistant overhead insulated cable according to an embodiment of the present invention includes, from the inside out, a conductor 1, a conductor shielding layer 11, an insulation layer 12, an insulation shielding layer 13, a metal shielding layer 14, a water-blocking layer 15, an inner sheath 16, and an outer sheath 17.

[0026] refer to Figure 1 and Figure 2 The outer sheath 17 is provided with a clamping section 2 along the cable axis. Both ends of the clamping section 2 are connected to a buffer section 21. Both ends of the buffer section 21 are connected to a regular section 22. The clamping section 2, the buffer section 21 and the regular section 22 together constitute the axial continuous whole of the outer sheath 17.

[0027] refer to Figure 3 and Figure 4 The clamping section 2 is made of rigid polymer material, which has a higher hardness than the buffer section 21 and the conventional section 22. It is used to contact and clamp the inner wall of the fitting. The clamping section 2 has multiple sets of filling cavities 23 along the cable axis. Each set of filling cavities 23 is circumferentially equidistant, and each filling cavity 23 is encapsulated with a corrosion inhibitor. When the outer sheath 17 cracks due to wear, the corrosion inhibitor is released. The released corrosion inhibitor penetrates upward along the micro-cracks under capillary action to the crack opening and inner wall, forming a passivation protective film to neutralize or block the electrochemical action of the penetrating corrosive medium.

[0028] refer to Figure 4 and Figure 5The buffer section 21 is made of an elastomer material and is designed as a corrugated tube structure, which gives the buffer section 21 the ability to stretch and deform elastically. The inner circumference of the buffer section 21 is provided with a relief groove 24 to provide deformation space for the buffer section 21. Both ends of the clamping section 2 are provided with a set of extrusion grooves 25. The number of extrusion grooves 25 and the position of each set of filling cavities 23 are the same and correspond to each other. The first piston 26 is slidably connected inside each extrusion groove 25.

[0029] refer to Figure 5 Each extrusion groove 25 is provided with a ventilation groove 27 between it and the corresponding clearance groove 24, and each extrusion groove 25 is provided with a connecting groove 28 between it and the corresponding filling cavity 23. Every two adjacent filling cavities 23 are interconnected.

[0030] During cable installation, the cable is clamped in clamping section 2 by fittings. The inner wall of the fittings is tightly attached to the outer surface of clamping section 2 to form a fixed constraint. After the overhead line is put into operation, under the action of a continuous light wind, the cable body generates high-frequency, low-amplitude vibrations, which propagate along the cable axis to the location of clamping section 2.

[0031] Since the clamping section 2 is rigidly fixed by the fittings, the vibration wave cannot be effectively dissipated inside the clamping section 2. Instead, it is transmitted to the buffer sections 21 at both ends of the clamping section 2. The buffer section 21 is a corrugated tube structure with a greater amount of expansion and contraction than the conventional section 22. When the vibration wave enters the buffer section 21, the crests and troughs of the corrugated tube structure can undergo stretching and compression deformation, converting the vibration mechanical energy into elastic deformation energy and gradually dissipating it, thereby significantly reducing the amplitude of the alternating stress transmitted to the end of the clamping section 2.

[0032] During the bending deformation of the buffer section 21, compression deformation occurs. When the compression deformation of the buffer section 21 causes the volume of the relief groove 24 to be compressed, the pressurized gas in the relief groove 24 enters the corresponding extrusion groove 25 inside the clamping section 2 through the venting groove 27. This pushes the first piston 26 to slide along the extrusion groove 25 towards the clamping section 2. The sliding of the first piston 26 reduces the effective volume inside the extrusion groove 25, thereby squeezing the corrosion inhibitor liquid in the filling cavity 23 and causing the pressure inside the filling cavity 23 to rise synchronously. Under normal conditions, the filling cavity 23 is a closed cavity with its outer cavity wall intact. Although the corrosion inhibitor inside is under high pressure, it is effectively sealed due to the physical isolation of the cavity wall and cannot be released to the outside.

[0033] When microcracks develop on the outer surface of the clamping section 2 due to long-term fretting wear, and the microcracks continue to expand to penetrate the outer wall of the filling cavity 23, the increased pressure inside the filling cavity 23 drives the corrosion inhibitor encapsulated therein to seep outward along the microcrack channels.

[0034] After seeping out, the corrosion inhibitor continues to climb along the inner wall of the microcrack under capillary action until it reaches the opening of the microcrack and the surface of the outer sheath 17. The corrosion inhibitor is rapidly adsorbed on the inner wall of the crack and near the opening to form a dense passivation protective film. The passivation film physically isolates the exposed outer sheath 17 material inside the crack from external corrosive media such as salt spray, moisture and acid rain. At the same time, the chemical adsorption of the corrosion inhibitor molecules inhibits the cathodic and anodic reactions of electrochemical corrosion, thereby blocking the path of corrosive media to penetrate deep into the cable along the crack channel.

[0035] When the vibration weakens or the bending deformation of the buffer section 21 recovers, the volume of the clearance groove 24 gradually increases and forms a negative pressure. The gas in the extrusion groove 25 is drawn back through the ventilation groove 27, which drives the first piston 26 to slide outward and reset along the extrusion groove 25. The pressure in the filling cavity 23 decreases accordingly, and the seepage rate of the corrosion inhibitor slows down until it stops. During this process, when the vibration intensity increases and the wear intensifies, the amount of corrosion inhibitor released increases. When the vibration weakens or stops, the amount of corrosion inhibitor released automatically decreases to avoid ineffective consumption of the corrosion inhibitor.

[0036] It should be noted that, since the corrugated tubular structure of the buffer section 21 absorbs most of the vibration energy outside the hardware clamping boundary, the alternating stress at the connection interface between the end of the clamping section 2 and the buffer section 21 is significantly reduced, effectively maintaining the integrity of the outer surface protective layer of the clamping section 2 in the end region, slowing down the propagation of microcracks from the edge of the clamping section 2 to the inside, and further ensuring the long-term sealing effectiveness of the corrosion inhibitor filling cavity 23 structure.

[0037] In summary, through the design of structures such as clamping section 2 and buffer section 21, clamping section 2 adapts to the rigid clamping requirements of hardware with high rigidity, and buffer section 21 dissipates vibration energy through expansion and contraction deformation, significantly reducing alternating stress in the clamping area, suppressing fretting wear and microcrack initiation. When vibration is triggered, the compressive deformation of buffer section 21 increases the pressure inside the filling cavity 23, and automatically releases corrosion inhibitor when microcracks penetrate the outer wall of filling cavity 23. The inhibitor seeps along the crack to form a film to block the erosion of corrosive media. The release amount is adaptively matched with the vibration intensity to avoid ineffective losses. At the same time, buffer section 21 reduces the alternating stress transmitted to the filling cavity 23 area, which can suppress the rapid propagation of cracks, thereby delaying the wear failure at the clamp, reducing the frequency of operation and maintenance, and improving the reliability of power supply. Example 2

[0038] In actual operation, the powder and shavings produced by fretting wear and the salt spray and moisture that seep in from the outside are easily retained and mixed, forming a viscous dirt layer that is both abrasive and conductive. This not only aggravates the mechanical wear of the surface of the clamping section 2, but also provides a conductive path for electrochemical corrosion, weakening the protective effect of the corrosion inhibitor. At the same time, the long-term retention of the corrosive medium will accelerate the propagation of microcracks into the filling cavity 23 area, destroying the sealing performance of the corrosion inhibitor. Therefore, this embodiment improves the device described in the above embodiment.

[0039] refer to Figures 2 to 5 The outer circumferential walls at both ends of the clamping section 2 are fixedly connected with mounting rings 3. The interior of each mounting ring 3 is provided with an annular drive groove 31. The interior of each drive groove 31 is slidably connected with a second piston 32. The interior of each drive groove 31 is fitted with an elastic element 33. One end of each elastic element 33 is connected to the end of the corresponding second piston 32 away from the buffer section 21, and the other end is connected to the inner wall of the drive groove 31.

[0040] refer to Figure 4 and Figure 5 The end of the regular section 22 near the buffer section 21 is fixedly connected to a plurality of circumferentially distributed support members 34. Each support member 34 is fixedly connected to a pull rope 35 on the side near the clamping section 2. The other end of each pull rope 35 extends into the corresponding drive groove 31 and is fixedly connected to the second piston member 32.

[0041] refer to Figure 2 , Figure 4 and Figure 5 Two sets of guide grooves 36 are provided on the outer circumference of the clamping section 2. One end of each guide groove 36 is connected to the interior of the corresponding drive groove 31, and the other end passes through another mounting ring 3.

[0042] During cable operation, the fittings are clamped in the clamping section 2 and positioned between the two mounting rings 3. When the overhead line is subjected to continuous light wind and generates high-frequency vibration, the axial vibration wave of the cable body is transmitted along the outer sheath 17 to the buffer sections 21 on both sides of the clamping section 2. The corrugated structure of the buffer section 21 undergoes alternating stretching and compression deformation, which drives the support member 34 fixed at the end of the conventional section 22 to move synchronously.

[0043] When the buffer section 21 bends and undergoes tensile deformation, the end of the conventional section 22 drives the support member 34 to move away from the clamping section 2. The support member 34, through the pull rope 35 fixedly connected to it, pulls the second piston member 32 located in the drive groove 31 inside the mounting ring 3, causing the second piston member 32 to slide along the drive groove 31 away from the clamping section 2. During this process, the second piston member 32 stretches the elastic member 33 sleeved in the drive groove 31, causing the elastic member 33 to store energy. At the same time, the volume of the drive groove 31 on the side of the second piston member 32 near the clamping section 2 increases and stores gas.

[0044] When the buffer section 21 bends and recovers, the end of the regular section 22 drives the support member 34 to move closer to the clamping section 2, and the pull rope 35 loosens accordingly, releasing the tension on the second piston member 32. At this time, the elastic member 33, which is in a compressed state, releases its elastic force, pushing the second piston member 32 to slide and reset along the drive groove 31 closer to the clamping section 2, and squeezing the gas in the drive groove 31 near the clamping section 2, generating a blowing effect. This blows away the salt spray droplets, moisture, corrosive deposits, and powdery abrasive shavings that are attached to or accumulated inside the guide groove 36 and at the junction of the hardware end and the outer wall of the clamping section 2, preventing impurities from continuously penetrating into the contact interface edge between the hardware and the cable.

[0045] As the vibration continues, the buffer section 21 alternately stretches and recovers, thereby purifying the airflow. On the one hand, this disrupts the adhesion of impurities and prevents corrosive media from accumulating on the surface of the clamping section 2 for a long time. On the other hand, it actively removes trace amounts of corrosive moisture that have penetrated into the interface between the clamping section 2 and the hardware, reducing the local humidity at the contact interface and preventing the formation of a viscous dirt layer with conductive and abrasive properties. This ensures the long-term stability of the contact state between the outer wall of the clamping section 2 and the hardware.

[0046] In summary, by setting up structures such as the drive groove 31 and the guide groove 36, the reciprocating expansion and contraction deformation of the buffer section 21 drives the reciprocating sliding of the second piston 32 in the drive groove 31. With the energy storage and reset of the elastic element 33, the gas in the drive groove 31 is squeezed during the compression stage of the buffer section 21 and continuously blown into the interface area between the hardware end and the outer sheath 17 of the cable through the guide groove 36. On the one hand, this can weaken the adhesion of salt spray, powder abrasive and corrosion deposits at the interface edge, and prevent them from being repeatedly squeezed into the contact interface and forming an abrasive medium. On the other hand, it can actively remove the trace amount of moisture that has penetrated into the interface, reduce local humidity, prevent the formation of a viscous dirt layer that is both conductive and abrasive, extend the service life of the cable, and reduce maintenance intervention.

[0047] In conclusion, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An environmentally friendly and corrosion-resistant overhead insulated cable, characterized in that, It includes, from the inside out, a conductor (1), a conductor shielding layer (11), an insulation layer (12), an insulation shielding layer (13), a metal shielding layer (14), a water-blocking layer (15), an inner sheath (16), and an outer sheath (17). The outer protective layer (17) is provided with a clamping section (2), a buffer section (21) and a regular section (22) along the axial direction. The clamping section (2) is used to clamp with hardware. The clamping section (2) is provided with a filling cavity (23) for storing and releasing corrosion inhibitor. The buffer section (21) is used to dissipate vibration energy when elastic deformation occurs and to adjust the pressure in the filling cavity (23). When a microcrack is generated on the surface of the clamping section (2) that connects to the filling cavity (23), the corrosion inhibitor is driven to be released outward.

2. The environmentally friendly and corrosion-resistant overhead insulated cable according to claim 1, characterized in that, The clamping section (2) is also provided with a squeezing groove (25). The squeezing groove (25) is connected to the filling cavity (23) through a connecting groove (28). A first piston (26) is slidably connected in the squeezing groove (25). The squeezing groove (25) is connected to the internal space of the buffer section (21) through a venting groove (27).

3. The environmentally friendly and corrosion-resistant overhead insulated cable according to claim 2, characterized in that, When the buffer section (21) undergoes axial compression deformation, its internal volume decreases, and the pressurized gas enters the extrusion groove (25) through the ventilation groove (27), pushing the first piston (26) to slide towards the filling cavity (23) to increase the pressure inside the filling cavity (23).

4. The environmentally friendly and corrosion-resistant overhead insulated cable according to claim 1, characterized in that, Two mounting rings (3) are fixedly connected to the outer periphery of the clamping section (2), and the two mounting rings (3) are respectively located at both ends of the clamping section (2) along the axial direction.

5. The environmentally friendly and corrosion-resistant overhead insulated cable according to claim 4, characterized in that, The mounting ring (3) has a drive groove (31) inside, and a second piston (32) is slidably connected in the drive groove (31). The drive groove (31) also has an elastic element (33) for driving the second piston (32) to reset.

6. The environmentally friendly and corrosion-resistant overhead insulated cable according to claim 5, characterized in that, The conventional section (22) is fixedly connected to a support member (34) at one end near the buffer section (21), and the support member (34) is connected to the second piston member (32) by a pull rope (35).

7. The environmentally friendly and corrosion-resistant overhead insulated cable according to claim 6, characterized in that, The clamping section (2) has a flow guide groove (36) on its outer periphery. One end of the flow guide groove (36) is connected to the drive groove (31), and the other end passes through the mounting ring (3) on the other side.

8. The environmentally friendly and corrosion-resistant overhead insulated cable according to claim 7, characterized in that, When the buffer section (21) undergoes axial tensile deformation, the pull rope (35) pulls the second piston (32) to slide and causes the elastic element (33) to store energy; When the buffer section (21) undergoes axial compression deformation, the elastic element (33) resets and drives the second piston element (32) to slide in the opposite direction, so as to blow the gas in the drive groove (31) out through the guide groove (36).

9. The environmentally friendly and corrosion-resistant overhead insulated cable according to claim 1, characterized in that, The corrosion inhibitor in the filling cavity (23) seeps outward when a microcrack is generated on the surface of the clamping section (2) that extends into the filling cavity (23).

10. The environmentally friendly and corrosion-resistant overhead insulated cable according to claim 1, characterized in that, The two ends of the buffer section (21) are respectively connected to the clamping section (2) and the conventional section (22). The clamping section (2), the buffer section (21) and the conventional section (22) together constitute the axial continuous whole of the outer protective layer (17).

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