After the power cable installed, in order to make it a continuous line, each section of the cable need to be connected as a whole, and a cable gland need to be used at this time. The cable gland in the middle of the power line is called the intermediate cable gland, while the cable joint at both ends of the power line is called the terminal cable gland.
Classification of cable land
According to the installation location, it can be divided into indoor and outdoor applications.
According to the material of the cable gland, they are divided into plastic and metal cable joints. Metal cable joints are further divided into porous metal cable waterproof joints, anti bending metal cable joints, double locking metal cable waterproof joints, plastic hose cable joints, metal hose cable joints, etc.
Functions of a cable land
Its main function is to make the circuit unobstructed, keep the cable sealed, and ensure the insulation level at the cable joints, ensuring its safe and reliable operation. If the sealing is poor, not only will oil leakage cause the oil immersed paper to dry up, but moisture will also invade the inside of the cable, causing a decrease in the insulation performance of the paper.
The root cause of a heating cable glandÂ
Some electrical installation workers often do not pay attention to the installation quality when laying wires: do not install insulation sleeves where they should be used; There is no junction box installed where it should be used; Even at the cable joints, it is not the twisting method used, but the illegal hook like connection method is used. This hook shaped connection method has a high contact resistance and continuously generates heat when powered on, causing nearby wooden boards to gradually dry and carbonize, ultimately leading to combustion and causing a fire.
Hot wire connections not only cause a significant amount of electrical energy loss, but also seriously affect the normal operation of electrical equipment. Mild cases include an increase in working current in the circuit, a shortened lifespan of electrical equipment, sudden interruptions in ongoing production, scientific research, medical surgery, and other activities, as well as fire and electric shock accidents, causing incalculable losses;
Tips for installing these cable glands
1. The connection of conductors requires low resistance and sufficient mechanical strength, and there should be no sharp corners at the connection. The commonly used method for connecting conductors of medium and low voltage cables is crimping, and attention should be paid to crimping:
(1) Choose a conductor connection tube with appropriate conductivity and mechanical strength;
(2) The fitting gap between the inner diameter of the pressure connecting pipe and the outer diameter of the connected wire core is taken as 0.8-1.4mm;
(3) The resistance value of the joint after crimping should not be greater than 1.2 times that of the conductor with equal cross-section, and the tensile strength of the copper conductor joint should not be less than 60N/mm2.
(4) Before crimping, apply conductive adhesive to the outer surface of the conductor and the inner surface of the connecting pipe, and use a steel wire brush to break the oxide film;
(5) Sharp corners, burrs, etc. on connecting pipes and wire conductors should be polished smooth with a file or sandpaper.
2. Internal semiconductor shielding treatment.
If the cable body has an inner shielding layer, it is necessary to restore the inner shielding layer of the pressure connecting conductor part of the joint when making the joint. A part of the inner semiconductor shielding of the cable should be left behind so that the inner shielding of the connecting head on the connecting pipe can be connected to each other, ensuring the continuity of the inner semiconductor and evenly distributing the field strength at the joint connection.
3. Processing of external semiconductor shielding.
External semiconductor shielding is a semi conductive material that acts as a uniform electric field on the insulation of cables glands. Like internal semiconductor shielding, it plays a very important role in cables and its joints. The external semiconductor port must be neat and uniform, and it is required to have a smooth transition with insulation, and to add semiconductor tape at the cable joint to connect with the external semiconductor shielding of the cable.
4. Treatment of cable reaction cone.
A reaction force cone with accurate shape and size during construction has an equal potential distribution on the entire cone surface. When making a cross-linked cable reaction cone, special cutting tools are generally used, or it can be slightly heated with a low flame and cut with a fast knife. After the basic shape is formed, it is scraped with 2mm thick glass, and finally polished with sandpaper from coarse to fine until smooth.
5. Metal shielding and grounding treatment.
The main function of metal shielding in cables and joints is to conduct short-circuit currents due to cable faults, as well as to shield electromagnetic interference from nearby communication equipment. During operation, the metal shielding is at zero potential in a well grounded state. When a cable fault occurs, it has the ability to conduct short-circuit currents in a very short period of time. The grounding wire should be reliably welded, and the metal shielding and armor tape on the cable body at both ends should be firmly welded. The grounding of the terminal head should be reliable.
6. Sealing and mechanical protection for cable gland.
The sealing and mechanical protection of the cable gland are the guarantees for ensuring the safe and reliable operation of the joint. Water and moisture should be prevented from seeping into the cable joints, and joint protection grooves or cement protection boxes should be installed at the joint positions.
Measurement methods for these cable glands
1. Temperature sensing cable type temperature measurement. Place the temperature sensing cable parallel to the cable. When the cable temperature exceeds the fixed temperature value, the sensing cable will be short circuited and an alarm signal will be sent to the control system. The disadvantages of ordinary temperature sensing cables are destructive alarms, fixed alarm temperatures, incomplete fault signals, inconvenient system installation and maintenance, and easy equipment damage; The analog linear temperature sensing cable can only use a certain local line segment as the alarm unit, so it cannot accurately locate the alarm signal generated by a certain point.
2. Thermistor type temperature measurement. Temperature values of cables can be measured using thermistors, but they are all analog outputs that require signal amplification and A/D conversion to be received. Each thermistor requires independent wiring, complex wiring, and is prone to damage and maintenance. Sensors do not have self checking function and require frequent calibration.
3. Infrared sensing temperature measurement. Infrared sensing utilizes all objects with temperatures above absolute zero to continuously emit infrared radiation energy into the surrounding space. The size and wavelength distribution of infrared radiation energy from an object are closely related to its surface temperature. Therefore, by measuring the infrared energy emitted by an object itself, its surface temperature can be accurately determined. Infrared sensing is non-contact measurement, therefore it has good safety. Its disadvantage is that it is greatly affected by the emissivity of objects, environment, and mist during temperature measurement, and has poor anti-interference ability.
4. Thermocouple temperature measurement. Thermocouples need to use dedicated compensation wires to transmit signals, and the transmission distance should not be too long, which is not suitable for the actual situation of a wide distribution of cable heads; Thermistors are usually platinum resistors that require three wire transmission and balanced bridge output. The transmission distance should not be too long, and the anti-interference ability is poor.
5. Integrated circuit temperature measurement. There are various types of integrated circuit type temperature measuring components, among which the current output type has a large internal resistance and is suitable for long-distance transmission. It is usually small in size and can be sealed with anti-corrosion, moisture-proof, and high-temperature resistant thermal conductive silicone at the measured point. The external wiring is led out by two lines to transmit data, but it is greatly affected by electromagnetic interference at the measured point.
6. Fiber optic distributed temperature monitoring. The fiber optic distributed temperature measurement system is a relatively advanced system. Temperature measurement is achieved by generating backward Raman scattering temperature effects through laser pulses transmitted through optical fibers. The latest fiber optic distributed temperature monitoring system can allow fiber optic loop length to reach 12 km and measurement accuracy to reach ± 1 ° C. The advantage of using a fiber optic distributed temperature monitoring system is that it can accurately locate the hotspots along the cable line, display and record the temperature change curve trajectory, and display the location of hotspots on the line. This system requires the ability to determine the location of hotspots during installation, but relies on pre manufactured monitoring equipment and optical/thermal data processing, which not only increases additional equipment investment, but also faces maintenance and repair issues for newly added equipment.
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