Cable protection pipe with high temperature discoloration warning function

Abstract

The application discloses a cable protection pipe with high-temperature discoloration warning function, which comprises a base pipe layer, a heat conduction enhancement layer and a thermochromic early warning layer; the base pipe layer extends along an axial direction and has an average light transmittance of not less than 30% in a visible light wavelength range; the heat conduction enhancement layer is compounded on an inner wall surface of the base pipe layer and is used for enhancing the conduction efficiency of heat from a cable to the inner wall of the base pipe layer; the thermochromic early warning layer is arranged between the heat conduction enhancement layer and the base pipe layer, is composed of a thermochromic material, and has a color change when a temperature reaches a preset threshold value and is observed in appearance through the base pipe layer; the thermochromic early warning layer has a monotonously changed spatial non-uniform discoloration temperature threshold value distribution along the axial direction, so that different positions on the whole protection pipe produce color changes at different temperatures, the occurrence positions of temperature abnormalities are coded as spatial color change signals on the outer wall of the protection pipe, and thus accurate positioning of a heat source and quantification of a temperature level can be realized.

Description

Technical Field

[0001] This invention relates to the field of cable protection pipe technology, and in particular to a cable protection pipe with a high-temperature color change warning function. Background Technology

[0002] Cable protection pipes are metal protective pipes with a certain mechanical strength that are laid on the outer layer of cables to prevent them from being damaged.

[0003] Cable protection pipes are often laid in concealed environments such as cable trenches, cable trays, or shafts. Traditional monitoring methods, such as infrared thermometers, require point-by-point scanning, which is inefficient and cannot detect transient overheating or hot spots hidden inside. At the same time, cable trenches are often blind spots or areas with delayed response of active fire protection systems such as smoke detectors. By the time the detectors alarm, the fire may have already spread.

[0004] In existing technologies, thermochromic coatings or reversible thermochromic particles are typically applied to the surface of the protective conduit. The material's temperature-sensitive color change allows for visual assessment of whether the cable protection conduit is overheating. However, the thermochromic coating on the surface of the protective conduit is prone to peeling and failure due to scratches, chemical corrosion, and other reasons.

[0005] Another type of MPP cable protection conduit consists of, from the outside in, a transparent film layer, a color-changing layer, protective layer a, and protective layer b. Protective layer a contains a built-in temperature-sensing resistance wire. This type suffers from a complex overall structure and redundant functions. Not only does it require additional consideration of electrical connections during installation, but the wiring inside the conduit occupies extra space and increases the difficulty of threading. Furthermore, the resistance wire is at risk of aging and breakage over long-term operation. Since the color-changing layer is located outside the temperature-sensing resistance wire, it relies excessively on the reliability of electrical components; if the resistance wire fails, all warning functions are lost. In addition, the heat generated by the cable must be transferred sequentially through the cable sheath, the air inside the conduit, the inner wall of the protective conduit, the wall thickness of the protective conduit, the outer wall of the protective conduit, and the color-changing layer. Multiple thermal resistance interfaces exist during this transfer process, and the temperature of the outer wall of the conduit is inevitably lower than the temperature of the cable itself. When the color-changing layer on the outer wall of the conduit changes color, the inside of the cable may already be severely overheated, leading to a delayed high-temperature color-changing warning.

[0006] If the thermochromic material is integrated into the cable itself, the entire cable needs to be replaced if a problem occurs. This is not only costly, but also difficult to adapt to existing lines, making its use very limited. Summary of the Invention

[0007] This application provides a cable protection pipe with a high-temperature color change warning function. It actively designs a superposition scheme of "space-temperature encoding function" and cable temperature distribution. When observing the color change distribution, reference information independent of the temperature field can be extracted, realizing accurate location of heat source, quantification of temperature level, and even retaining thermal event traces after temperature recovery. It has high practical application value.

[0008] This application provides a cable protection pipe with a high temperature color change warning function, including a base pipe layer, a thermally conductive enhancement layer and a thermochromic warning layer; The base tube layer extends axially and is made of a polymer material with semi-transparent or light-diffuse transmission properties, with an average transmittance of not less than 30% in the visible light wavelength range, so as to allow observers to visually identify the color change of the thermochromic warning layer through the base tube layer. The thermally conductive enhancement layer is composited on the inner wall surface of the base tube layer to enhance the heat conduction efficiency from the cable to the inner wall of the base tube layer. The thermochromic warning layer is disposed between the thermally conductive enhancement layer and the base tube layer, and is made of thermochromic material. It changes color when the temperature reaches a preset threshold and can be observed through the base tube layer. The thermochromic warning layer has a monotonically varying spatially non-uniform color change temperature threshold distribution along the axial direction, which causes different locations on the entire protective tube to produce color changes at different temperatures, thereby encoding the location of the temperature anomaly as a spatial color change signal on the outer wall of the protective tube.

[0009] In one possible implementation, the thermochromic warning layer has a monotonically varying spatially non-uniform thermochromic temperature threshold distribution along the axial direction, specifically any of the following types: A. The color-changing initiation temperature of the thermochromic warning layer changes continuously along the axial direction of the protective tube as a linear function. It has a lower color-changing initiation temperature at the beginning and a higher color-changing initiation temperature at the end. The linear function form satisfies T_S(x)=T_0+k*x, where T_S(x) is the color-changing initiation temperature at the axial position x, T_0 is the color-changing initiation temperature at the beginning, and k is the gradient slope. The value of k ranges from 0.5℃ / m to 10℃ / m. B, the color change initiation temperature of the thermochromic warning layer changes continuously along the axial direction of the protective tube in an S-shaped function, with the rate of change being the largest in the middle section and gradually flattening at both ends, or the rate of change being higher at both ends and gradually flattening in the middle section. C, the color change initiation temperature of the thermochromic warning layer changes exponentially along the axial direction, and the location of the temperature rise can be quickly deduced from the color change abrupt point during fault location. D. The protective tube is divided into several sections of equal or unequal length along the axial direction. The color change threshold of the thermochromic warning layer in each section is approximately constant, and the color change threshold between adjacent sections changes in a step-like manner. The step-like manner between adjacent sections is either an arithmetic step or a geometric step. E, the color change initiation temperature of the thermochromic warning layer fluctuates periodically along the axial direction, and its average value increases monotonically along the axial direction, superimposed with periodic fluctuations of fixed amplitude, or a periodic peak-valley-peak distribution pattern. F, the thermochromic warning layer is a customized threshold distribution. The threshold distribution is set in an arbitrary function form along the axis according to the known fault risk distribution in a specific cable laying scheme. The highest resolution is concentrated in the area with the highest fault risk, which includes the cable joint, the bending radius of the smallest area, and the friction section of the conduit.

[0010] In one possible implementation, the difference between the maximum and minimum values ​​of the color change initiation temperature of the thermochromic warning layer along the axial direction of the protective tube, i.e., the total gradient amplitude, is between 15°C and 30°C.

[0011] In one possible implementation, the gradient slope k ranges from 1.5℃ / m to 4℃ / m.

[0012] In one possible implementation, the color-changing threshold of the thermochromic warning layer continuously increases from 55°C to 80°C, and the gradual parameter configuration of the thermochromic warning layer enables the location of the cable overheating source to be located by visually observing the boundary of the color-changing range, with a positioning accuracy of ≤0.5m.

[0013] In one possible implementation, the thermal conductivity of the thermally conductive reinforcement layer is ≥5W / (m·K), and the axial temperature diffusivity of the thermally conductive reinforcement layer satisfies the relationship dT / dx≥2·ΔT_diff with respect to the gradual threshold change rate, where dT / dx is the threshold change rate and ΔT_diff is the temperature uniformity deviation caused by the axial diffusion of the thermally conductive reinforcement layer, so as to ensure the spatial location coding accuracy of the heat source.

[0014] In one possible implementation, the thermally conductive enhancement layer is divided into at least three segments along the axial direction, with different thermal conductivity in each segment. The thermal conductivity of each segment matches the color change initiation temperature range of the corresponding segment in the thermochromic warning layer, and the distribution of the thermal conductivity along the axial direction is positively correlated with the axial distribution of the color change initiation temperature.

[0015] In one possible implementation, the base tube layer is one or more composite materials selected from modified polypropylene, chlorinated polyvinyl chloride, or high-density polyethylene, wherein at least the surface area exposed to the color change observation has a background color difference ΔE*_ab ≥ 15 that is colorless, light-colored, or contrasting with the color change color of the thermochromic warning layer, calculated according to the CIE 1976 color difference formula.

[0016] In one possible implementation, the thermochromic warning layer is composed of a microencapsulated reversible thermochromic material, and the color-changing threshold temperature is controlled by adjusting the parameters of the microencapsulation process. The parameter control methods of the microencapsulation process include microcapsule particle size distribution adjustment or microcapsule doping density axial gradient adjustment.

[0017] In one possible implementation, the thermochromic warning layer further includes one or more irreversible thermochromic particles located in at least one high-temperature zone. These irreversible thermochromic particles produce a permanent color change when the temperature reaches a predetermined second threshold, and are used to record the location and / or range of the highest temperature event that occurs during cable operation when it exceeds its preset threshold.

[0018] Beneficial effects: Compared with the prior art, the cable protection pipe with high temperature color change warning function provided in this application can deeply coordinate the thermal color change warning function with the physical protection of the base pipe layer and the heat conduction function of the thermally conductive enhancement layer by setting the thermal color change warning layer between the thermally conductive enhancement layer and the base pipe layer. The thermal color change warning layer can respond to the cable temperature rise earlier and more accurately, and the passive thermal conduction efficiency can be improved by ≥50%, the durability is better, and the failure risk of conventional electrical components can be eliminated. In addition, by designing the thermochromic warning layer to have a spatially non-uniform color-changing temperature threshold distribution with monotonic changes (increasing or decreasing) along the axial direction, different locations on the entire protective tube will produce color changes at different temperatures. This encodes the location of the temperature anomaly as a spatial color change signal on the outer wall of the protective tube, which can provide a "second information dimension" independent of the temperature field. The precise location accuracy of the heat source can reach the meter level or even the centimeter level (depending on the design density of the gradient), and the temperature level can be specifically quantified by comparing the color saturation of different sections.

[0019] These and other objects, features and advantages of the present invention will become fully apparent from the following detailed description. Detailed Implementation

[0020] The following description is intended to disclose the present invention and enable those skilled in the art to implement it. The preferred embodiments described below are merely examples, and other obvious variations will occur to those skilled in the art. The basic principles of the invention defined in the following description can be applied to other embodiments, modifications, improvements, equivalents, and other technical solutions that do not depart from the spirit and scope of the invention.

[0021] Those skilled in the art should understand that, in the disclosure of this specification, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientation or positional relationships, are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the above terms should not be construed as limiting the present invention.

[0022] Unless otherwise specified, all terms used herein (including technical and scientific terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. It will also be understood that terms, such as those defined in commonly used dictionaries, shall be interpreted as having a meaning consistent with their meaning in the context of the relevant art and shall not be interpreted as having an idealized or overly formal meaning unless expressly so defined herein.

[0023] It is understood that the term "a" should be understood as "at least one" or "one or more", that is, in one embodiment, the number of an element can be one, while in another embodiment, the number of the element can be multiple, and the term "a" should not be understood as a limitation on the number.

[0024] This application provides a cable protection pipe with a high-temperature color-changing warning function, comprising a base pipe layer, a thermally conductive reinforcement layer, and a thermochromic warning layer. The base pipe layer extends axially and is composed of a polymer material with semi-transparent or diffuse light transmission properties. The average transmittance of the base pipe layer in the visible light wavelength range is not less than 30% (tested according to GB / T 21300-2007 "Determination of Opacity" under finished pipe wall thickness conditions), allowing observers to visually identify the color change of the thermochromic warning layer through the base pipe layer. The measured data of 30% average transmittance comes from the verification of the embodiment, wherein the preferred transmittance range is 30%-80%.

[0025] Preferably, the base tube layer is one or more composite materials selected from modified polypropylene (MPP), chlorinated polyvinyl chloride (CPVC), or high-density polyethylene (HDPE), wherein at least the surface area exposed to the color change observation has a background color difference ΔE*_ab ≥ 15 (calculated according to the CIE 1976 color difference formula) that is colorless, light-colored, or contrasts with the color change color of the thermochromic warning layer. The base tube layer does not need to be completely transparent like glass; it only needs to have sufficient light transmittance (semi-transparent or even non-completely transparent with low light transmittance is acceptable) so that the observer can observe the color change of the thermochromic warning layer located inside it through the base tube layer. The national standard GB / T 21300-2007 "Determination of Opacity of Plastic Pipes and Fittings" provides a method for determining the opacity of plastic pipes, and Appendix A provides recommended light transmittance values ​​for opaque pipes and fittings. This, in turn, illustrates that there are different categories of transparent and opaque cable protection pipes in the current market. As long as the light transmittance is appropriate, it is acceptable.

[0026] The thermally conductive enhancement layer is composited on the inner wall surface of the base tube layer to enhance the heat conduction efficiency from the cable to the inner wall of the base tube layer. A thermochromic warning layer, composed of a thermochromic material, is disposed between the thermally conductive enhancement layer and the base tube layer. This thermochromic warning layer changes color when the temperature reaches a preset threshold, and the color is visually observed through the base tube layer.

[0027] The thermally conductive reinforcement layer can be made of graphene-modified thermally conductive composite material, which directly contacts the outer wall of the cable inside the base tube layer. It efficiently conducts the heat generated by the cable to the thermochromic warning layer, which accurately responds to the temperature change of the cable. The thickness of the thermally conductive reinforcement layer is not less than 0.1 mm to provide sufficient effective thickness to accommodate the thermally conductive filler to form a "thermal conductive path" (three-dimensional network) and ensure that the effective thermal conductivity is ≥5 W / (m·K).

[0028] Considering the differences in thermal expansion coefficients and elastic moduli between the thermally conductive reinforcement layer and the base tube layer, as well as the mismatch in processing temperatures, residual thermal stress can form. When the thickness of the thermally conductive reinforcement layer exceeds approximately 0.8 mm, the thermal stress at the interlayer interface exhibits a non-linear growth trend, significantly higher than in thin-layer structures. In long-term thermal cycling environments (cable switching causing periodic temperature changes), excessively thick thermally conductive reinforcement layers may experience peeling, cracking, or even delamination failure at the interface, severely impacting the long-term service life of the product. When the thickness further exceeds 1.0 mm, the thermal resistance increases significantly (R_th > 0.20 K·cm² / W), interlayer thermal stress accumulates to a high level, increasing the risk of interlayer peeling, while the marginal improvement in thermal conductivity is extremely low (doubling the thickness results in negligible improvement in thermal conductivity)—the engineering losses from excessive thickness outweigh the alternative means of reducing thermal resistance by increasing the content of thermally conductive fillers. Therefore, the thickness of the thermally conductive reinforcement layer is preferably in the range of 0.2mm to 0.5mm. This can further reduce the negative effects of thickness. Within this range, the thermal resistance is the lowest, the processing technology is the most mature and stable, it is not easy to peel off under long-term thermal cycling, and the color-changing coding accuracy is the highest.

[0029] The thermochromic warning layer has a spatially non-uniform color change temperature threshold distribution that varies monotonically (increasing or decreasing unidirectionally along the axial direction) along the axial direction, which causes different positions on the entire protective tube to produce color changes at different temperatures, thereby encoding the location of the temperature anomaly as a spatial color change signal on the outer wall of the protective tube.

[0030] Unlike the technical design of a uniform color change threshold for the entire pipe, the gradient information of the uniform color change threshold design comes from the temperature distribution of axial heat conduction. However, the temperature information only comes from the temperature distribution of the cable itself and there is no temperature calibration. At the same time, the generation mechanism of the gradient information is determined by the "real heat source", which is uncontrollable.

[0031] This application designs the thermochromic warning layer as a gradient color-changing threshold. The gradient information comes from the axial distribution function of the color-changing threshold in space, which is a manually designed value. The temperature information comes from the "difference frequency coding" relationship formed between the cable temperature distribution and the threshold distribution. Thus, by combining the threshold matching condition at the location with the "whether the color changes here" information, auxiliary positioning and discrimination can be performed. The generation mechanism of the gradient information is determined by the "designed preset threshold field", which is completely controllable by the designer. For example, the color-changing threshold range is designed to be 55℃ to 85℃ or 50℃ to 80℃. The gradient resolution of this preset threshold field is designed manually. After superimposing axial heat conduction, it can generate richer spatial banding information, which can then allow the two gradient information to interact.

[0032] In other words, under the unified color-changing threshold scheme across the entire cable, the spatial distribution of color change is entirely driven by the cable's own temperature distribution (the natural temperature field of axial heat conduction), without any artificially designed spatial coding information. The only information that maintenance personnel can see is that "the overheating center is at the deepest point of color change." This application's gradual color-changing threshold design adds a second information dimension independent of the temperature field: a pre-defined threshold axial distribution function. This distribution is set by the designer as needed (it can be linear, S-shaped, exponential, etc.) and is unrelated to the actual cable temperature distribution. Its purpose is to construct a "spatial-temperature quantification reference coordinate system." This second information dimension provides a "calibration benchmark" independent of the temperature field, allowing for the deduction of the known influence of the threshold distribution when extrapolating temperature from the color-changing signal, resulting in a more accurate estimate of the actual cable temperature and the location of the heat source.

[0033] For example, comparisons can be made through modeling: Assuming a uniform color-changing threshold scheme across the entire pipe, the threshold temperature of the color-changing material is T_C = 70℃, and the half-width of color change is ΔT = 10℃ (color change begins at 60℃ and is complete at 80℃). The cable exhibits a temperature distribution within the 8-12m range: T = 85℃ at the center point (10m), gradually decreasing along the axial direction according to a Gaussian function towards both ends. The color distribution under this condition is as follows: The areas with the strongest discoloration are concentrated in the temperature range above about 75℃ (the color is darker); In areas where the temperature is below 70℃, the color gradually lightens (becomes lighter or remains unchanged). The observer's information: The superheat center is in the vicinity of 10m.

[0034] In a gradient threshold scheme, the threshold temperature linearly changes from 60°C to 85°C along the axial direction. The same cable temperature distribution is applied to the protective conduit. The color distribution in this case would be: When the actual temperature of the 8-12m section reaches 75℃, the threshold T_C(x) corresponding to this section is approximately 75-82℃. After the actual temperature is superimposed on the design threshold, both reach high values, resulting in the deepest degree of discoloration. The actual temperature in the 4-8m section may be between 60-70℃. The threshold T_C(x) corresponding to this section is approximately 65-75℃. The temperature here is close to or exceeds the threshold, and the color change is from just beginning to the saturation edge of color change (especially close to 9m). Key difference: Observers not only see information about the superheat center being around 10m, but can also infer the axial distribution of temperature by measuring the "length of the discoloration zone" and the "chromatographic distribution characteristics of the discoloration zone", because the threshold axial distribution function T_C(x) is known in advance (design value).

[0035] In terms of positioning accuracy, the two schemes are as follows: In the unified color-changing threshold scheme for the entire pipeline, assume that the color-changing segment covers a length of L1 (located in a region with a temperature ≥ T_C). Maintenance personnel can only know that "the overheating location is near the center of L1"—no additional information is available, and the positioning accuracy depends on the broadening characteristics of the temperature field (determined by factors such as thermal conductivity), which cannot be controlled.

[0036] In the gradient thresholding scheme, based on the observed color-changing segment positions x1 and x2 (corresponding to the threshold design value), the actual temperature at that position can be accurately calculated as T_C(x), where T_C(x) is a known design function. The observed "color-changing segment length" can be directly mapped to "the cable segment length exceeding a certain temperature threshold," providing a controllable and designable positioning accuracy. By adjusting the distribution rate of T_C(x) (e.g., concentrating the process of T_C(x) gradually changing from 55℃ to 85℃ within a short distance to achieve high-resolution encoding), positioning accuracy can be improved to the level of several meters or even tens of centimeters without relying on the broadening region of the temperature field for estimation, resulting in a significant improvement in positioning accuracy.

[0037] Therefore, this application represents a substantial shift from a "passive response" to an "active, designable coding" approach to a unified color-changing threshold across the entire cable. In the unified color-changing threshold scheme, temperature distribution is the independent variable, and the color-changing signal is the dependent variable; a very simple mapping exists between them. However, in the gradient color-changing threshold scheme, we actively introduce a threshold distribution function T_C(x), transforming the color-changing signal into: S(x) = f(T_real(x) - T_C(x)). This is a difference frequency coding formula. Since T_C(x) is pre-designed and known by the designer, the observer can inversely solve for T_real(x) from S(x), thereby obtaining more accurate temperature information. That is, the location and intensity of the color change can be used simultaneously to infer the actual cable temperature, rather than simply indicating "whether it is overheated in a specific temperature range." In other words, the gradient color-changing threshold scheme introduces a spatially encoded optical reference pattern, which can both observe the temperature distribution and eliminate environmental interference through the reference pattern, thereby improving the accuracy of temperature inversion. Ultimately, it forms a two-dimensional active encoding system (color-changing intensity = function (temperature - threshold (x) ), and threshold (x) is the design value).

[0038] The spatial color change symbol caused by cable temperature variations can be displayed on the outer wall of the base pipe layer in one or more combinations of the following forms: a) Hue change signal: This is a one-dimensional colorimetric code where different axial segments present different final hues to indicate temperature levels. It is mainly applicable to multi-threshold graded protection tubes (e.g., 0-4 m segment: colorless → yellow; 4-8 m segment: yellow → orange; 8-12 m segment: orange → red). The difference in hue provides temperature level information, and inspection personnel can judge the severity of overheating by observing "which segment has turned into which color". b. Color saturation gradient signal: This is a one-dimensional saturation encoding where color saturation changes continuously along the axis, providing continuous temperature level quantification information. It is used to encode the degree to which the temperature exceeds a threshold. In some cases, color saturation gradient can work in conjunction with the color change boundary location—maintenance personnel can determine the overheating range by observing the length of the color change segment, and infer the center location of the heat source by observing the axial distribution of saturation within the same segment (e.g., high saturation at the center and low saturation on both sides). Color saturation gradient can also provide temperature information independently—experienced inspection personnel can estimate the actual temperature value by comparing saturation with a colorimetric card. c. Color change boundary position signal, which is a one-dimensional spatial coordinate code, provides one-dimensional analog quantity information, namely continuous position coordinates. The information content is greater than the binary hue change. The starting position and / or ending position of the color change of the thermochromic warning layer in the axial direction indicate the axial positioning information of the cable overheating. d. The color-changing section length signal is a one-dimensional spatial range code used to indicate the influence range of cable overheating, that is, the axial length from the start of color change to the end of color change. It can be used to determine whether overheating has spread to adjacent medium and low threshold sections, thereby providing a basis for judging the overheating trend. e. Dynamic time-series signal, which is a four-dimensional spatial + time-coded signal, is based on the difference in spatial color change signals observed at at least two different time points. It utilizes the changes in the position of the color change boundary and the length of the segment during multiple inspections to provide information on the overheating evolution trend.

[0039] It should be noted that the color change signal can be observed directly with the naked eye on the outer surface of the base tube layer (through any combination of a / b / c / d / e as shown above), or it can be enhanced under the excitation of a handheld light source or ultraviolet lamp. In addition, the color change signal can also be automatically identified and analyzed by digital image acquisition and image processing algorithms.

[0040] In some embodiments, the thermochromic warning layer has a monotonically varying spatially non-uniform color change temperature threshold distribution along the axial direction, specifically any one of the following types: A. The color-changing initiation temperature of the thermochromic warning layer changes continuously along the axial direction of the protective pipe using a linear function, exhibiting a lower color-changing initiation temperature at the beginning and a higher color-changing initiation temperature at the end. The linear function form satisfies T_S(x) = T_0 + k*x, where T_S(x) is the color-changing initiation temperature at axial position x, T_0 is the color-changing initiation temperature at the beginning, and k is the gradient slope, ranging from 0.5℃ / m to 10℃ / m. This method is suitable for long-distance straight-line cable trenches, such as tunnels or direct burial in conduits. The overheat source positioning accuracy is consistent with the inspection direction. The lower limit of k is based on the lower limit requirement of the positioning resolution by the naked eye in actual inspection observation, while the upper limit of k, set at 10℃ / m, is based on the upper limit of the continuous feeding flow control accuracy in the compensation extrusion process. The numerical range is highly coupled with physical feasibility. Taking a 12m long protective pipe with a linearly distributed axial gradient threshold as an example, the test data are as follows: Five detection points were taken along the pipe length, and the measured discoloration initiation temperatures were: point a (0m) 55±1℃, point b (3m) 61±1℃, point c (6m) 67±1℃, point d (9m) 73±1℃, and point e (12m) 80±1℃. The linear fitting R² = 0.997, indicating excellent gradient uniformity. For scenarios where there may be multiple joints in the cable laying path, such as when the cable joint is located in the middle of a pipe segment, a mirror-symmetric linear gradient can be designed, i.e., low at both ends and high in the middle. In this case, the threshold distribution can be designed as a mirror-symmetric distribution of "55℃ at one end → 80℃ in the middle → 55℃ at the other end". The formula is expressed piecewise as: T_S(x)=T_0+k·x (x≤L / 2), T_S(x)=T_0+k·(Lx) (x≥L / 2). The highest threshold area in the middle section corresponds to the location of the cable joint, ensuring higher-precision temperature monitoring of the node most likely to fail. B. The color change initiation temperature of the thermochromic warning layer changes continuously along the axial direction of the protective pipe in an S-shaped function. The rate of change is the largest in the middle section and gradually flattens at both ends. That is, the middle point of the S-shaped curve is the position with the fastest gradual change rate. By selecting parameters, the threshold change is slow in the areas at both ends of the protective pipe (inlet and outlet) and fast in the middle section. This form can concentrate the "weighted resolution" of temperature information encoding in the middle pipe section. It is suitable for scenarios where different sections have different requirements for temperature resolution. For example, known fault-prone areas are concentrated in the middle section (such as the pipe section between two cable joints), or sections in tunnels with blind spots in inspection have higher physical requirements for temperature resolution. Taking a 12m pipe as an example, the test data shows that in the 0-2m section, T_S gradually changes from 55℃ to 58℃ (slope approximately 1.5℃ / m); in the 2-10m section, T_S rapidly changes from 58℃ to 77℃ (average slope approximately 2.4℃ / m); and in the 10-12m section, T_S gradually changes from 77℃ to 80℃ (slope approximately 1.5℃ / m). Within the middle 8m section, T_S changes by 19℃, achieving a temperature resolution of approximately 0.42℃ / cm.

[0041] In the continuous variation of the S-shaped function, another form is characterized by a higher rate of change at both ends and a gradual flattening in the middle. This means the initial and final regions are more sensitive to temperature changes, while the middle section is relatively less sensitive. This form is suitable for scenarios requiring high early warning sensitivity at both ends, such as easily inspected areas like utility tunnel entrances and exits. The hyperbolic tangent function (tanh) or its linear combinations can be used for function fitting. When temperature anomalies occur near entrances and exits, the color change signal can be detected early. C. The color-changing initiation temperature of the thermochromic warning layer changes exponentially along the axial direction. During fault location, the location of the temperature surge can be quickly deduced from the color change abrupt change point. A relatively common exponential function can be used. Since the threshold distribution is exponential, the location of the temperature surge can be quickly deduced from the color change abrupt change point during fault location. This is mainly applicable when a single fault source occurs on the side with a large gradual change rate, the temperature gradient rapidly crosses the threshold distribution band, the color-changing boundary is clear, and it can be quickly identified during inspection. Test data: Taking a 12m pipe as an example, T_S at each section: 55℃ at 0m; 55℃ at 3m. +25×(1-0.7³)=55+25×0.657≈71℃; at 6m, 55+25×(1-0.7^6)=55+25×0.882≈77℃; at 9m, 55+25×(1-0.7^9)=55+25×0.960≈79℃; at 12m, 55+25×(1-0.7^12)=55+25×0.986≈79.7℃ (close to 80℃, actual temperature may fluctuate slightly due to process precision and material selection. Here, A=25 and B=ln(0.7)≈-0.3567 are used, satisfying the decreasing exponential law). The temperature resolution is significantly higher in the first 0-6m segment than in the latter 6-12m segment. High-precision positioning can be achieved when overheating occurs in the 0-6m segment. D. The protective tube is divided into several sections of equal or unequal length along the axial direction. The color change threshold of the thermochromic warning layer within each section is approximately constant, and the color change threshold between adjacent sections exhibits a step-like jump. The step-like jump between adjacent sections is in the form of an arithmetic progression or a geometric progression. This form essentially approximates a continuous gradient function in an interval manner. Compared to the continuous gradient scheme, the main advantage of the step gradient is its simpler manufacturing process. It only requires dividing the extrusion process into several stages, with a constant material supply in each stage, eliminating the need for real-time continuous adjustment of the proportions. The larger the number of steps N in the step gradient, the better the approximation of the continuous gradient, but the equipment complexity also increases accordingly. The equilibrium point is usually between N=3-8 segments. A simple example of an arithmetic progression is four segments: 65℃ / 70℃ / 75℃ / 80℃. A geometric progression can be designed as 65℃ / 70℃ / 78℃ / 90℃, etc., with denser temperature sampling points set in the high-temperature segments to improve the accuracy of the high-temperature overheat warning. E. The color-changing initiation temperature of the thermochromic warning layer fluctuates periodically along the axial direction, with its average value monotonically increasing along the axial direction. This is superimposed with periodic fluctuations of fixed amplitude, or a periodic peak-valley-peak distribution. This pattern is mainly used to identify the periodic structural characteristics of the cable (such as repetitive joint positions). For example, when there is a joint every 2m in the cable, a threshold fluctuation with a period length of 2m can be set to make each joint position correspond to a higher temperature resolution, achieving precise temperature monitoring of each joint position. F, the thermochromic warning layer is a customized threshold distribution. The threshold distribution is set in an arbitrary function form along the axial direction according to the known fault risk distribution in a specific cable laying scheme. The highest resolution is concentrated in the area with the highest fault risk, which includes cable joints, the area with the smallest bending radius, and the friction section through the conduit. This is mainly to achieve fixed-point high-precision monitoring. In addition, this customized threshold distribution can also compensate for changes in ambient temperature along the laying path. For example, a lower threshold is set in the section with high ambient temperature and a higher threshold is set in the section with low ambient temperature, so that the warning activation condition of the entire line is normalized to "equal temperature difference between cable and environment", realizing a more intelligent environmental adaptive warning.

[0042] In some embodiments, the difference between the maximum and minimum values ​​of the color-changing initiation temperature along the axial direction of the thermally induced color-changing warning layer, i.e., the total gradient amplitude, is between 15°C and 30°C. If the total amplitude is too small, the temperature encoding range of the entire protection tube is limited (e.g., from 60°C to 72°C), which cannot cover heating events beyond 72°C (e.g., 85°C, 95°C). If the entire protection tube changes color prematurely, the high-temperature warning information will be completely lost. On the other hand, if the total amplitude is too large, the threshold change gradient between adjacent positions is too gentle, and the spatial resolution of the temperature resolution is correspondingly reduced (the temperature increment per unit distance is too small), making it difficult for maintenance personnel to determine the precise overheating location from a slight color change of tens of centimeters. Furthermore, if the minimum value of the color-changing initiation temperature is set too low (e.g., 35°C), high ambient temperatures in summer may trigger false alarms, thus seriously interfering with maintenance judgment.

[0043] Therefore, a design with a total gradient amplitude between 15℃ and 30℃ can cover the normal operating temperature range and the general overheating temperature range of the cable (such as a gradient from 55℃ to 85℃), while ensuring that the gradient rate is clearly distinguishable and that environmental interference is minimal. Tests show that the color change initiation offset distance that inspectors can observe within each meter of length is approximately 0.4m, which is sufficient to distinguish the approximate location of the overheating area; the gradient amplitude ΔT of each interval is approximately 2.1℃, and the temperature resolution matches the actual inspection requirements. In short, the lower limit of 15℃ ensures coverage of the ΔT offset between the normal operating temperature range of the cable and the critical overheating condition, avoiding false signals of premature color change of the entire protective tube; the upper limit of 30℃ ensures that the temperature positioning resolution reaches an acceptable engineering level.

[0044] In some embodiments, the gradient slope (the change in color-changing initiation temperature per unit length) k is preferably in the range of 1.5℃ / m to 4℃ / m. If k is too large, it means that the threshold change rate is too fast, and the threshold difference between two adjacent visually distinguishable positions within a short distance (e.g., 2-5 cm) will exceed 5-10℃. In this case, when hot spots occur, the color-changing signal may only appear near 1-2 discrete positions, failing to form a gradient color-changing band of a certain length. At the same time, an excessively large k will also increase the difficulty of the fabrication process. If the value of k is too small, it means that the temperature resolution is too low, which is not conducive to positioning accuracy; at the same time, if the total length is fixed, k can only be guaranteed by increasing the total gradient width ΔT_total, which can easily lead to the problems mentioned above regarding the total gradient width.

[0045] In some embodiments, the color-changing threshold of the thermochromic warning layer continuously increases from 55°C to 80°C. The gradual parameter configuration of the thermochromic warning layer allows the location of the cable overheat source to be determined by visually observing the boundary of the color-changing range, with a positioning accuracy of ≤0.5m. Assuming a total pipe length of 12m, the test fixture is artificially heated to 70°C and placed 4.2m from the starting end. Observing the starting position of the color change (temperature deviation occurs at approximately 4.2m), the center point of the color-changing span can be used to determine the center location of the overheating point. Optical positioning measurements show that the deviation between the actual observed positioning and the true position is ≤0.4m, meeting the accuracy requirements.

[0046] It is worth noting that the thermally conductive reinforcement layer and the thermochromic warning layer are not simply "physically stacked" or "aggregated together," but rather a highly coupled functional system. The axial TS distribution of the thermochromic warning layer relies on the uniformity of heat conduction ensured by the thermally conductive reinforcement layer, thereby establishing a spatial mapping from the cable body temperature to the color-changing signal. The thermally conductive reinforcement layer alone cannot achieve temperature information encoding; while the gradient color-changing layer alone cannot guarantee the accuracy of the temperature mapping.

[0047] Furthermore, it is important to note that the axial temperature diffusion effect of the thermally conductive reinforcement layer itself must be considered when designing the gradient threshold distribution. Axial temperature diffusion can "smooth out" the axial temperature gradient, leading to the loss of the expected temperature difference information. Therefore, the thermal conductivity of the thermally conductive reinforcement layer is ≥5 W / (m·K). Simultaneously, the axial temperature diffusivity of the thermally conductive reinforcement layer and the gradient threshold change rate satisfy the relationship dT / dx ≥ 2·ΔT_diff (under standard laboratory testing conditions), where dT / dx is the threshold change rate and ΔT_diff is the temperature equalization deviation caused by the axial diffusion of the thermally conductive reinforcement layer, ensuring the accuracy of the spatial location encoding of the heat source. The thermally conductive reinforcement layer is typically a polymer-based thermally conductive composite material, which not only enhances radial heat conduction but also possesses good axial heat conduction capabilities. By optimizing the axial gradient threshold distribution, the resolution of spatial encoding and the axial diffusion characteristics of the thermally conductive reinforcement layer can be mutually compensated or balanced, achieving the following design: 1. The rate of change of the gradient threshold distribution is not less than the axial temperature drop rate caused by the axial diffusion of the thermally conductive reinforcement layer, so as to ensure that even if the thermally conductive reinforcement layer smooths out some temperature differences, the change of the threshold is still greater than the axial temperature difference, and the color change signal can reflect a clear trend of axial temperature change. 2. An anisotropic thermal conductivity enhancement layer design is adopted (in-plane thermal conductivity > radial thermal conductivity) to control the degree of axial diffusion within a controllable range (≤1℃ / m).

[0048] The high thermal conductivity of the thermally conductive reinforcement layer reduces the temperature difference between the cable body temperature and the outer wall temperature of the protective tube (ΔT≤2℃), which is the physical premise for the gradient color-changing layer threshold to accurately reflect the cable temperature.

[0049] In some embodiments, the thermally conductive enhancement layer is divided into at least three segments along the axial direction. The thermal conductivity of each segment is different, and the thermal conductivity of each segment matches the color change initiation temperature range of the corresponding segment in the thermochromic warning layer (that is, the position and number of segments of the thermally conductive enhancement layer correspond one-to-one with the position and number of segments of the thermochromic warning layer). At the same time, the distribution of the thermal conductivity along the axial direction is positively correlated with the axial distribution of the color change initiation temperature. For example, in the low threshold range (55℃-65℃), the thermal conductivity of the thermal enhancement layer is set to 5-8 W / (m·K); in the medium threshold range (66℃-75℃), the thermal conductivity of the thermal enhancement layer increases to 10-15 W / (m·K); and in the high threshold range (76℃-85℃), the thermal conductivity of the thermal enhancement layer further increases to ≥20 W / (m·K). This segmented thermal enhancement layer and segmented gradual threshold design are matched with each other, which can ensure that the mapping relationship between cable temperature and pipe wall temperature remains consistent throughout the entire warning temperature range, and the constraint that the temperature difference is always ≤2℃ can be met in each segment.

[0050] In some embodiments: The protective tube is 12 m long. The base tube layer is made of CPVC material, and the thermally conductive reinforcement layer is made of CPVC material modified by graphene / boron nitride composite filler. The thermal conductivity is 6.8 W / (m·K) and the thickness is 0.3 mm. It is prepared by anisotropic directional arrangement process, so that the radial thermal conductivity reaches 4.8 W / (m·K) and the in-plane thermal conductivity is 12.5 W / (m·K).

[0051] The thermochromic warning layer is formed in a single step using a three-layer co-extrusion process and is positioned between the thermally conductive reinforcement layer and the base tube layer. A linearly gradient threshold distribution is adopted along the axial direction, with ΔT_total = 25℃ (58℃ → 83℃) and a gradient slope k = 2.1℃ / m. During fabrication, two independent metering feeders are used to supply 58℃ and 83℃ threshold microcapsules respectively. The ratio changes linearly from 100%:0% to 0%:100% during the 12m extrusion process, with a metering deviation of ±2% and a ratio change rate of 8.3% / m, consistent with gradient doping process parameters. The thermal conductivity of the thermally conductive reinforcement layer is 6.8 W / (m·K), and the temperature difference between the inside and outside of the tube ΔT ≤ 1.8℃. The axial change rate of the threshold, dT / dx = 2.1℃ / m, and the temperature equilibrium deviation caused by axial temperature diffusion of the thermally conductive reinforcement layer is 0.8℃ / m, satisfying the design criterion of dT / dx ≥ 2·ΔT_diff.

[0052] Testing revealed that the cable overheating center positioning accuracy was ≤0.35 m, the temperature difference was ≤1.8℃, and the temperature difference under extreme conditions was ≤2℃. The comparative scheme with a uniform threshold across the entire pipe lacked precise positioning capability, while the linear gradient + low-threshold coding structure significantly improved coding mapping accuracy after addressing the temperature difference parameters, demonstrating the significant contribution of the coupling effect.

[0053] In some embodiments, the base tube layer is one or more composite materials selected from modified polypropylene, chlorinated polyvinyl chloride, or high-density polyethylene, wherein at least the surface area exposed to the color change observation has a background color difference ΔE*_ab ≥ 15, calculated according to the CIE 1976 color difference formula. This avoids the problem of the color change signal of the thermochromic warning layer being "submerged" or distorted due to the base tube layer itself absorbing specific wavelengths of visible light. Commonly available MPP cable protection pipes are usually milky white or gray with low light transmittance; CPVC power pipes are uniformly orange-red. The orange-red background color absorbs the blue-violet light band, and if the color change signal of the thermochromic material falls in the blue-violet region, its recognizability will be significantly affected.

[0054] In addition, considering that the recognizability of the color-changing signal may be affected in local light sources, backlighting, or dim environments, a small amount of reflective microbeads or fluorescence enhancement materials can be incorporated into the thermochromic warning layer so that the color-changing signal can still be observed under low illumination conditions.

[0055] Gradient doping is the core manufacturing method for achieving a continuously gradient threshold distribution along the axis. A continuous co-extrusion process is used to continuously control the chemical composition parameters of the thermochromic microcapsules, thereby imparting a gradient threshold property along the axis to the protective tube during the tube extrusion process. In some embodiments, the thermochromic warning layer is composed of a microencapsulated reversible thermochromic material, and the color-changing threshold temperature is controlled by adjusting the parameters of the microencapsulation process. The parameter control methods for the microencapsulation process include microcapsule particle size distribution control or axially gradient control of microcapsule doping density.

[0056] Among these, the control of microcapsule particle size distribution is crucial, as the particle size directly affects the response speed and threshold stability of the color-changing material. Smaller microcapsules have a larger specific surface area, faster heat transfer, and a faster color-changing response, although the color-changing initiation temperature may be slightly lower; larger microcapsules have the opposite effect. A particle size gradient can be established by gradually adjusting the emulsification speed or wall material concentration in different sections during the microcapsule synthesis stage, and then supplying microcapsules of different particle sizes to different extrusion sections. Standardized microcapsules can be applied in sections according to particle size; for example, microcapsules with d50=3μm can be used in the initial extrusion section, microcapsules with d50=10μm can be used in the final section, and the particle size can be continuously varied in the intermediate section, using the particle size gradient to extend the range of variation. Specifically, the axial gradient control of microcapsule doping density is crucial. The doping density (volume fraction) of microcapsules in the thermochromic warning layer affects the color saturation and visual clarity of the color-changing signal. High-doping-density regions produce more vivid color-changing signals, suitable for long-distance observation; low-doping-density regions can save on the amount of microcapsule material used. Ideally, the axial gradient of doping density should be designed in conjunction with the threshold gradient. For example, using a higher doping density in the high-temperature range can directly compensate for the decrease in color contrast caused by the wider ΔTSH in the high-temperature range, thereby further improving the accuracy of the warning.

[0057] ΔTSH refers to the "Temperature Span of Halochromic Transition," which is the temperature difference between the onset temperature of color change (usually the temperature at which the color begins to change noticeably to the point where the color change is complete (the temperature at which the color change reaches saturation and no longer changes significantly with increasing temperature).

[0058] Thermochromic materials are divided into reversible thermochromic materials and irreversible thermochromic materials.

[0059] The thermochromic warning layer is composed of microencapsulated reversible thermochromic materials, or more specifically, reversible thermochromic microcapsules. These microcapsules consist of a core material and a wall material. A typical core material formulation is a three-component system: a leuco dye (such as crystal violet lactone CVL), a color developer (such as bisphenol A), and a solvent (phase change medium). During the color change process, the leuco dye undergoes a reversible ring-opening (colored) and ring-closing (colorless) transformation of the lactone ring under the action of the color developer. This reaction is triggered by the solvent near the characteristic phase change temperature. The color change temperature of the reversible thermochromic microcapsules can be controlled within a wide temperature range (30℃~120℃) by changing the solvent type and ratio, and it is highly compatible with spatial coding designs such as linear gradients / S-shaped gradients.

[0060] When the cable recovers from an overheating event, the color-changing layer returns to its original state, enabling the protective tube to provide repeated warnings and maintain accurate mapping of the gradient threshold. Furthermore, for the threshold temperature gradient configuration, by adjusting the core material solvent composition and ratio of the reversible thermochromic microcapsules in different sections (i.e., changing the proportion and type of leuco dye, developer, and solvent), the section can initiate color change when the temperature rises to a preset threshold and naturally recover after the temperature decreases, thus meeting the repeated warning requirements of the entire protective tube during long-term service.

[0061] Irreversible thermochromic materials undergo a chemical color change upon heating and do not revert to their original color upon cooling. One method to achieve this is through "thermosensitive release color-developing microcapsules": the microcapsule shell ruptures at a preset color-developing temperature, releasing a pre-encapsulated colored indicator liquid to achieve irreversible color development. Another method involves an aryl alkyne and free radical initiator system, where a polymerization reaction is initiated by heating to produce a color change. In other words, irreversible methods become ineffective after the first trigger and are unsuitable for long-term, repetitive warning systems.

[0062] In some embodiments, the thermochromic warning layer further includes one or more irreversible thermochromic particles located in at least one high-temperature zone. These irreversible thermochromic particles undergo a permanent color change when the temperature reaches a predetermined second threshold, recording the location and range of the highest temperature event exceeding the preset threshold during cable operation. In this way, the irreversible thermochromic particles in the warning layer can permanently solidify evidence of a one-time extreme high-temperature event at a specific spatial location in the protective tube, providing traceable documentary evidence of high temperature for inspection personnel, indicating the location of the most severe overheating. Therefore, the irreversible thermochromic particles not only have a specific spatial arrangement but also a system integration that works synergistically with reversible thermochromic materials. Even when the reversible material returns to its original color after the temperature decreases, the permanent color change signal of the irreversible particles remains, thus providing "historical information" between two inspections. This allows the "permanent color change" of the irreversible particles and the "real-time gradual change" of the reversible microcapsules to form a mutually corroborating dual evidence chain.

[0063] In the structural design of irreversible thermochromic particles, an axially positioned embedding scheme can be adopted, where the entire length of the protective tube still uses reversible gradient threshold microcapsules as the main warning layer (e.g., a linear gradient from 65℃ to 80℃). Furthermore, in specific sections at different axial positions, irreversible thermochromic particles are embedded, or several circular or strip-shaped irreversible particle doping zones (which can be called high-temperature recording embedding zones) are mixed in. The irreversible particles in each zone have the same trigger temperature (e.g., 95℃), used to permanently mark the location of severe overheating events. Even after the cable cools down, the irreversible color mark remains on that zone of the protective tube; maintenance personnel can easily locate the severe overheating point during subsequent inspections. When the total length of the protective tube is 12m, 95℃ irreversible thermochromic particles can also be embedded only in the 8-12m high-temperature section (the area with a reversible gradient threshold of 75℃-80℃). The embedding method is as follows: embedding sites (microgrooves) are reserved on the inner surface of the base tube, and a pre-made irreversible microcapsule / matrix mixture (turns blue at 95℃) is embedded, and then extruded and compounded synchronously with the reversible gradient layer. The base tube layer can be made of a high melting point shell material (such as urea-formaldehyde resin Tg≈130±3℃) to ensure that the irreversible particles do not trigger the warning activation (temperatures below 90℃) in this section, and only show color when the cable reaches or exceeds 95℃, thus preserving evidence of the "highest temperature" at which this section was overheated.

[0064] It should be noted that those skilled in the art should distinguish between two different gradual color change phenomena: First, in existing unified threshold schemes for the entire pipe, the synchronous color gradual change response across the entire pipe is caused by the inherent color-changing temperature range of the thermochromic material (typically 2–8°C), which is essentially a one-dimensional time-temperature mapping. Second, in the gradual color-changing threshold scheme of this invention, the spatially inconsistent distribution of local color changes is caused by the continuous axial variation of the color-changing initiation temperature, which is essentially a two-dimensional space-temperature mapping. The former encodes information as "temperature level," while the latter encodes information as "temperature level + overheating location." The two have fundamentally different practical engineering applications.

[0065] Those skilled in the art should understand that the embodiments of the present invention described above are merely examples and do not limit the invention. The advantages of the present invention have been fully and effectively realized. The functional and structural principles of the present invention have been shown and explained in the embodiments; any variations or modifications can be made to the implementation of the present invention without departing from these principles.

Claims

1. A cable protection pipe with a high-temperature color-changing warning function, characterized in that, It includes a base tube layer, a thermally conductive enhancement layer, and a thermochromic warning layer; The base tube layer extends axially and is made of a polymer material with semi-transparent or light-diffuse transmission properties, with an average transmittance of not less than 30% in the visible light wavelength range, so as to allow observers to visually identify the color change of the thermochromic warning layer through the base tube layer. The thermally conductive enhancement layer is composited on the inner wall surface of the base tube layer to enhance the heat conduction efficiency from the cable to the inner wall of the base tube layer. The thermochromic warning layer is disposed between the thermally conductive enhancement layer and the base tube layer, and is made of thermochromic material. It changes color when the temperature reaches a preset threshold and can be observed through the base tube layer. The thermochromic warning layer has a monotonically varying spatially non-uniform color change temperature threshold distribution along the axial direction, which causes different locations on the entire protective tube to produce color changes at different temperatures, thereby encoding the location of the temperature anomaly as a spatial color change signal on the outer wall of the protective tube.

2. The cable protection pipe with high-temperature color-changing warning function as described in claim 1, characterized in that, The thermochromic warning layer described herein has a monotonically varying spatially non-uniform thermochromic temperature threshold distribution along the axial direction, specifically any one of the following types: A. The color-changing initiation temperature of the thermochromic warning layer changes continuously along the axial direction of the protective tube as a linear function. It has a lower color-changing initiation temperature at the beginning and a higher color-changing initiation temperature at the end. The linear function form satisfies T_S(x)=T_0+k*x, where T_S(x) is the color-changing initiation temperature at the axial position x, T_0 is the color-changing initiation temperature at the beginning, and k is the gradient slope. The value of k ranges from 0.5℃ / m to 10℃ / m. B, the color change initiation temperature of the thermochromic warning layer changes continuously along the axial direction of the protective tube in an S-shaped function, with the rate of change being the largest in the middle section and gradually flattening at both ends, or the rate of change being higher at both ends and gradually flattening in the middle section. C, the color change initiation temperature of the thermochromic warning layer changes exponentially along the axial direction, and the location of the temperature rise can be quickly deduced from the color change abrupt point during fault location. D. The protective tube is divided into several sections of equal or unequal length along the axial direction. The color change threshold of the thermochromic warning layer in each section is approximately constant, and the color change threshold between adjacent sections changes in a step-like manner. The step-like manner between adjacent sections is either an arithmetic step or a geometric step. E, the color change initiation temperature of the thermochromic warning layer fluctuates periodically along the axial direction, and its average value increases monotonically along the axial direction, superimposed with periodic fluctuations of fixed amplitude, or a periodic peak-valley-peak distribution pattern. F, the thermochromic warning layer is a customized threshold distribution. The threshold distribution is set in an arbitrary function form along the axis according to the known fault risk distribution in a specific cable laying scheme. The highest resolution is concentrated in the area with the highest fault risk, which includes the cable joint, the bending radius of the smallest area, and the friction section of the conduit.

3. The cable protection pipe with high-temperature color-changing warning function as described in claim 2, characterized in that, The difference between the maximum and minimum values ​​of the color change initiation temperature of the thermochromic warning layer along the axial direction of the protective tube, i.e. the total gradient amplitude, is between 15℃ and 30℃.

4. The cable protection pipe with high-temperature color-changing warning function as described in claim 3, characterized in that, The gradient slope k ranges from 1.5℃ / m to 4℃ / m.

5. The cable protection pipe with high-temperature color-changing warning function as described in claim 2, characterized in that, The color-changing threshold of the thermochromic warning layer increases continuously from 55°C to 80°C. The gradual parameter configuration of the thermochromic warning layer allows the location of the cable overheating source to be located by visually observing the boundary of the color-changing range, with a positioning accuracy of ≤0.5m.

6. The cable protection pipe with high-temperature color-changing warning function as described in claim 1, characterized in that, The thermal conductivity of the thermally conductive reinforcement layer is ≥5W / (m·K). The axial temperature diffusivity of the thermally conductive reinforcement layer and the gradual threshold change rate satisfy the relationship dT / dx≥2·ΔT_diff, where dT / dx is the threshold change rate and ΔT_diff is the temperature uniformity deviation caused by the axial diffusion of the thermally conductive reinforcement layer, so as to ensure the spatial location coding accuracy of the heat source.

7. The cable protection pipe with high-temperature color-changing warning function as described in claim 6, characterized in that, The thermally conductive enhancement layer is divided into at least three sections along the axial direction. The thermal conductivity of each section is different, and the thermal conductivity of each section matches the color change initiation temperature range of the corresponding section in the thermochromic warning layer. The distribution of the thermal conductivity along the axial direction is positively correlated with the axial distribution of the color change initiation temperature.

8. The cable protection pipe with high-temperature color-changing warning function as described in claim 1, characterized in that, The base tube layer is one or more composite materials selected from modified polypropylene, chlorinated polyvinyl chloride, or high-density polyethylene, wherein at least the surface area exposed to the color change observation has a background color difference ΔE*_ab≥15 that is colorless, light-colored, or contrasts with the color change color of the thermochromic warning layer, calculated according to the CIE 1976 color difference formula.

9. The cable protection pipe with high-temperature color-changing warning function as described in claim 1, characterized in that, The thermochromic warning layer is composed of microencapsulated reversible thermochromic material. The color change threshold temperature is controlled by adjusting the parameters of the microencapsulation process. The parameter control methods of the microencapsulation process include microcapsule particle size distribution adjustment or microcapsule doping density axial gradient adjustment.

10. The cable protection pipe with high-temperature color-changing warning function as described in claim 9, characterized in that, The thermochromic warning layer also includes one or more irreversible thermochromic particles located in at least one high-temperature zone. The irreversible thermochromic particles produce a permanent color change when the temperature reaches a predetermined second threshold, which is used to record the location and / or range when the highest temperature event of the cable during operation exceeds its preset threshold.