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What is the function of inner and outer shielding of cable medium voltage and high voltage cables?

The so-called "shielding" on the cable structure is essentially a measure to improve the electric field distribution. The cable conductor is made up of multiple wires twisted, and it is easy to form an air gap between it and the insulating layer. The surface of the conductor is not smooth, which will cause the electric field to concentrate. Add a layer of shielding layer of semi-conductive material on the surface of the conductor, which has the same potential as the shielded conductor and is in good contact with the insulating layer, so as to avoid partial discharge between the conductor and the insulating layer. This layer of shielding is the inner shielding layer.   Also, there may be gaps at the contact between the insulating surface and the sheath, which is a factor that causes partial discharge. Therefore, a layer of semi-conductive material shielding layer is added on the surface of the insulating layer, which has good contact with the shielded insulating layer and is in contact with the metal sheath. Sheath equipotential, so as to avoid partial discharge between the insulation layer and the sheath, this layer of shielding is the outer shielding layer. For extruded insulated cables without metal sheath, in addition to the semi-conductive shielding layer, a metal shielding layer wrapped with copper tape or copper wire should be added. The function of this metal shielding layer is to pass capacitive current during normal operation; When the system is short-circuited, it acts as a channel for short-circuit current and also plays a role in shielding the electric field. It can be seen that if the outer semiconductor layer and copper shielding in the cable do not exist, the possibility of insulation breakdown between the core and the core of the three-core cable is very high.

2022

11/15

Types and properties of common flame-retardant wires and cables

1. Ordinary flame-retardant wires and cables The insulating and sheathing materials of ordinary flame-retardant wires and cables are generally polymer materials containing halogen (or modified by adding halogen containing flame retardant). Polyvinyl chloride (PVC) is commonly used. Ordinary PVC resin has the characteristics of high electrical insulation, chemical resistance, abrasion resistance, aging resistance and low price. However, when PVC burns, it will release hydrogen chloride, carbon monoxide, carbon dioxide, various aromatic hydrocarbons, chlorinated compounds and other toxic and harmful gases. 2. Low smoke halogen-free flame retardant wire and cable The insulation and sheath materials of low smoke and halogen-free flame-retardant wires and cables are mostly thermoplastic elastomer materials, which do not contain halogens such as fluorine, chlorine, bromine and iodine. Mercury, chromium, cadmium, lead and other heavy metal elements that pollute the environment are excluded from the production process. Polyamide is often used as the material. The flame-retardant mechanism of this cable is that of interrupt switching. Interrupted exchange flame retardant refers to taking away part of the heat generated during the combustion of flame retardant materials, so that the materials can not maintain the thermal decomposition temperature, and therefore can not continue to produce combustible gas, so the combustion is self extinguished.   Zhenglan Cable Technology Co., Ltd manufacture flame retardant wire, low smoke halogen-free flame retardanet wire, fire resistant wire, flame retardant and fire resistant wire. Welcome your inquiry.  

2022

11/05

Why Single core Armored Cable Should Use Non magnetic Materials?

When a single core cable is started or stopped by alternating current or direct current, a corresponding alternating magnetic field will be generated around the cable, and an induced current will be generated in the metal material. This current flows inside the metal material, much like a vortex of water, so it is called eddy current, or eddy current for short. The thermal effect of eddy current can heat the metal materials in a short time. If the heat cannot be emitted in time, it will cause the cable insulation layer to melt or accelerate the aging, and the insulation performance will be weakened, thus causing cable breakdown.   The armored steel strip has good magnetic conductivity, which can be magnetized under a small magnetic field and generate eddy current. Non magnetic materials such as stainless steel tape, copper tape, aluminum tape or non-magnetic metal wire shall be used for armouring of single core cables.   In addition, it is better to lay single core cables in finished form during construction. Single core cables should avoid magnetic materials as far as possible, such as iron pipes, iron clamps, etc., which can form closed loop magnetic materials.   In addition, for a three core cable, the alternating magnetic fields generated by the three core phase relationship cancel each other, so that the sum of the induced current vectors is 0, and no magnetic force lines pass through the armor. Therefore, steel tape armouring can be used.

2022

09/19

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