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The article explains the working principles and application ranges of differential temperature measurement cables, constant temperature measurement cables, linear continuous heat-seeking thermocouple measurement cables, networked temperature sensors, and fiber optic temperature sensors, and provides recommendations for online temperature monitoring in cable trenches of power systems. Due to the ease of installation for power cables and their ability to meet the aesthetic requirements of municipal construction, the use of cables has been increasing rapidly in recent years amid the development of power infrastructure. However, the increasing number of cables also places significant pressure on daily operation and maintenance tasks; even the slightest negligence in maintenance can easily lead to fire accidents. Once a cable trench catches fire, it not only disrupts power supply but may also trigger a chain reaction, resulting in significant losses. 1. Causes of fires in cable trenches. There are many reasons for cable combustion; these may include defects in the quality or design of the cables themselves, as well as other issues such as improper construction or maintenance. ①The design is unreasonable. The cable does not have sufficient overload capacity. ②Improper construction. During installation, no sand or soft soil was laid at the bottom of the cable trench, nor were any covers placed there; moreover, the bending radius of the cables was too small ； During installation, the cables were not laid in layers as required, the insulation distance was insufficient, the laying density was too high, and mechanical damage to the cables occurred due to the installation process. ③Material aging. Due to differences in the cable’s material and manufacturing process, there are variations in its lifespan; especially in cable trenches with harsh conditions, the aging process accelerates. ④Short-circuit accident. Under the impact of short-circuit current, the insulation performance of the cable deteriorates significantly, making short circuits more likely to occur. ⑤The cable connector is overheating. Issues such as loose crimping of cable terminals, asymmetrical geometric positions of the three-phase crimping points, metal burrs remaining after crimping, and excessive grounding resistance can all cause heating at the cable connections. ⑥Other reasons. Accidents such as water infiltration in cable trenches or damage caused by rats can harm the cables, creating a risk of fire. 2. Traditional preventive measures: At present, the power industry still primarily relies on planned maintenance for the management of cable trenches, generally using regular inspections to assess the operating condition of these trenches. This not only results in significant economic waste, but potential fire hazards are also difficult to detect in a timely manner; such a management approach has become a weakness in the management of cable trenches. For newly constructed cable trenches, the following measures can be taken to prevent fire accidents: ① Install fire-resistant sections. Fire-resistant sections can be composed of fire-retardant coatings and fire barriers, or they can be made of lightweight, enclosed fire-resistant enclosures. ②Set up a firewall. The wall is constructed using ordinary red bricks; drainage holes are provided above the drainage ditch at the bottom, and fire-retardant coating is applied to both sides of the wall to create a complete fire barrier. Fire barriers made of slag wool should be installed at the points where cable trenches enter buildings or at intersections of cable trenches. ③Apply fireproof sealing. A mixture of asbestos, mud, yellow sand, and lime is used as a sealing filler to seal cable shafts and various openings, thereby achieving fire prevention purposes. ④Fire monitoring. Install automatic fire alarm systems; these alarms should be able to emit warning signals in a timely manner, so that fire-fighting measures can be taken promptly to prevent the spread of the fire, extinguish it at an early stage, and avoid or minimize fire-related losses. For cable trenches that are already in use, fire accidents can be prevented from the following aspects: ① Scientific and rational scheduling to avoid overload operation as much as possible ； Carry out preventive testing tasks, and regularly conduct voltage withstanding tests on cables ； Timely eliminate weak links and potential accident hazards during operation. For cables with longer transmission distances, the testing period should be appropriately extended and the voltage withstanding standard reduced. ②Check the grounding status of cables and connectors. Check whether the grounding of the cables and connectors is proper, and pay attention to analyzing changes in the grounding resistance. ③Conduct temperature testing on critical areas. Monitor the temperature of cable splices, and analyze their operating conditions based on temperature changes. ④Fire monitoring. Install automatic fire alarm systems to detect fires promptly, extinguish them in a timely manner, and reduce losses. 3. Application of online temperature monitoring technology for cable trenches in power systems. The rated current-carrying capacity of a cable is determined by the maximum allowable operating temperature of the cable insulation, that is, by the maximum allowable temperature on the outer surface of the cable core. When a cable is in operation, the surface temperature of its conductors is closely related not only to the magnitude of the load current, but also to the losses within the cable’s insulation, the cable’s heat transfer properties, and the ambient temperature. For cables that are already in operation, the cable structure as well as the phase and spatial relationships with other cables, and the voltage and frequency of the load have all been determined; the total heat generation of the cable is related only to the temperature of the conductors. Therefore, by using the cable surface temperature values provided by the temperature measurement system, along with the cable’s heat dissipation coefficient and the environmental conditions in which it is installed, it is possible to establish a relationship between the sheath surface temperature and the cable core temperature or its current-carrying capacity. This allows for the inverse calculation of the cable core temperature, which can then be compared with the allowable temperature. Meanwhile, the actual load rate of the cable circuit can be determined, enabling early detection of any potential issues with the cable. This facilitates non-contact, online diagnosis of the cable core temperature, thereby improving the overall management level of power grid operations. The vast majority of fire accidents in cable trenches are caused by overheating of cables and cable terminals. The process from overheating to the occurrence of a fire is relatively slow, as sufficient heat needs to accumulate first; therefore, it is necessary to conduct online monitoring of the temperature in cable trenches and at cable terminals. This allows such fire accidents to be prevented at an early stage. There are various methods for online cable temperature monitoring; below, the simple-to-implement and effective methods will be mainly introduced. ①Three types of temperature measurement cables. Temperature measurement cables are power cables made from materials similar to those used in ordinary cables, making them convenient to install. The temperature measurement cables currently in use can be divided into differential temperature cables, constant temperature cables, and flexible thermocouple line detector (FTLD) cables. Differential temperature cables use special thermosensitive materials to estimate the induced temperature based on the cable’s resistance, with calibration performed in advance. Changes in ambient temperature during use have a significant impact on the measurement accuracy of the cable, but the associated cost is relatively low. A constant-temperature cable consists of two elastic steel wires, each coated with a thermosensitive material, which are then twisted together to form the cable. When the temperature rises to the set value, the insulation resistance of the thermosensitive insulating material changes abruptly, resulting in a near-short circuit, which in turn triggers an alarm signal; it functions as a temperature switch. Although the above two types of cables have temperature measurement capabilities and are relatively easy to install, their temperature measurement is not accurate enough; moreover, they perform temperature measurement at only single points, making them suitable for measuring temperatures in short-distance cable circuits. The FTLD thermosensitive thermocouple is an achievement in the development of temperature sensor technology; it utilizes the thermoelectric effect to continuously and automatically generate a microvoltage signal corresponding to the temperature of the highest temperature point within its range of length. Due to its unique intrinsically safe properties and excellent accuracy in detecting temperature changes, it was initially adopted by the United States as a cutting-edge technology device and installed in the compartments of aircraft carriers, destroyers, and military aircraft. Currently, in China, there are three main sources from which thermocouples used for fire detection are obtained: they are imported directly from the United States; domestically, only Yunnan Changhui Instrument Manufacturing Co., Ltd. and Shenzhen Diantong Technology Co., Ltd. produce low-temperature thermocouples for detecting fires in power cables. The combustion process of a substance can generally be divided into an early stage, a smoldering stage, a flame exothermic stage, and a decay stage. In the early stages, due to the preheating and vaporization that occur as the material starts to burn, flammable gases and invisible aerosol particles are primarily produced; there is no visible smoke or flame, and very little heat is generated. At this stage, the fire is confined to a limited area within the potentially problematic zone, and early detection and warning of the fire should begin at this point. Traditional fire detectors are unable to detect temperature in real time, whereas FTLD thermosensitive thermocouples utilize the thermoelectric effect of K-type thermocouples to continuously and automatically generate monitoring signals. Its advantage is that, at temperatures ranging from -40°C to 180°C, it allows users to determine not only the magnitude and rate of temperature changes but also their location. Providing accurate temperature feedback in a timely manner before a fire occurs further enhances the reliability of the fire detection and alarm system. FTLD thermoelectric sensors capable of detecting heat can continuously and in real time monitor the highest temperature in a monitored area, overcoming the limitation of traditional \"point-type\" temperature sensors, which can only measure the temperature at a single point; therefore, such thermoelectric sensors represent an ideal choice for temperature measurement cables. ②Network temperature measurement: Due to the limited space and long distances in cable trenches, it is difficult to install conventional temperature sensors, making their use challenging. With the advancement of technology, digital and networked sensors have effectively overcome the shortcomings of traditional temperature sensors. For example, the DS18B20 digital and networked temperature sensor developed by DALLAS in the United States employs a unique approach to successfully resolve the conflict between digitization, networking, and cost, thereby making it possible to create monitoring systems that are easy to use, economical, and reliable. The DS18B20 utilizes intelligent fieldbus technology; only digital signals are transmitted within the entire system, and both the sensors and data acquisition modules can be connected to the network. This makes the system more reliable and effective at detecting potential overheating and fire hazards resulting from the aging of cables and their connections. It has a strong ability to predict fires caused by cable overheating at an early stage, providing a solid guarantee for the safe operation of on-site equipment. Due to its simple construction and easy networking, this solution has been successfully applied in many sites. Thermocouples with heat-sensing capability, when equipped with temperature transmitters, convert temperature signals into 4-20mA outputs, making it easier to integrate them into various control systems and the Internet of Things. They are not subject to technical barriers or price constraints, and their advantages in terms of versatility, reliability, and cost-effectiveness are even more evident. ③Fiber optic temperature measurement: The so-called distributed temperature sensing refers to sensors that operate in a distributed manner, as opposed to traditional point-type temperature sensors. DTS can enable a single sensor to detect linear areas or even planar areas. DTS technology is based on the principle that certain special light waves, as they propagate through optical fibers, carry temperature information from various points along their path; it is implemented by combining techniques such as weak light signal detection and ultra-high-speed signal monitoring. By using optical fibers as sensors, DTS technology features high precision, strong stability, and a long service life, making it one of the trends in the field of temperature sensing. DTS can continuously obtain temperature information along the detection optical cable, with no monitoring blind spots. Moreover, the fiber itself is made of quartz material, which provides complete electrical insulation ； At the same time, the signals from fiber optic sensors are transmitted via optical fibers, which ensures their intrinsic safety and enables them to function properly even in harsh electromagnetic environments. DTS can measure distances over long ranges, making it suitable for remote monitoring; it enables remote surveillance over distances of up to ten kilometers without the need for repeaters ； DTS also boasts advantages such as high sensitivity, high measurement accuracy, and a long service life, making it suitable for accurate temperature monitoring in long-distance cable trenches. More importantly, DTS can not only monitor abnormal hot spots on the cable but also monitor its current-carrying capacity. This enables DTS to effectively prevent fire accidents and also provides a reference for analyzing the operating condition of cables. Thermometry cables are simple to install and have low costs, but they require an external power source; therefore, they are suitable only for applications with short measurement distances, few measurement points, and low precision requirements. Network-based temperature measurement is more complex than cable-based temperature measurement, but it offers higher precision, greater flexibility in network configuration, stronger resistance to interference from digital signals, and lower costs. However, it still requires an external power source; therefore, it is only suitable for use in cable trenches over short distances with few monitoring points. The installation of fiber optic temperature sensors is generally not difficult; they offer high precision, a long lifespan, reliable signal transmission, and do not require an external power source. However, the initial investment is relatively high. Therefore, they are suitable for long-distance, intensive temperature monitoring. For cable trenches in critical power supply lines, it is recommended to use DTS fiber optic temperature sensors along with thermocouples for heat detection. The fire hazards in the cable trenches of power systems must be given great attention. With a scientific approach, whether it is newly built cable trenches or those already in use, real-time temperature monitoring can be achieved through advanced online temperature sensing methods; it is even possible to monitor the current-carrying capacity of the cables. This not only helps to prevent fires but also improves the level of grid operation management, advancing us further toward smart grids. Source: Changhui Instruments http://yunrun.com.cn/