Pithy formula and interpretation of common cable selection

In our daily sales, we may encounter customers asking about ACSR Cable selection in the process of maintenance and repair. As a cable salesman, choosing the right cable for customers is the top priority of this work.
picture
First, the actual power of the load should be considered, at the same time, the maximum current to be borne by the cable should be calculated, and then the appropriate cable should be selected. The following is the relevant contents sorted out by the editor in AAAC Cable treasure. I hope I can provide some help for my colleagues.

[doggerel]: five out of ten, two out of one hundred, fifty-three out of four, and 75% off for buried casing.
[explanation]: according to the total current required by the insulated conductor, when the total current is less than 10a, the cross-sectional area of the conductor per square millimeter can pass 5A current, if it is more than 100A, it can only pass 2A current per square millimeter, 4A current per square millimeter between 10 ~ 50A and 3a current per square millimeter between 50 ~ 100A. After this calculation, In case of buried or casing laying, the current value that can be passed shall be multiplied by 0.75.
The calculation method given above obtains the allowable current per square millimeter. The selection of the actual specific conductor section should be: first determine the current required by the electrical equipment (including rated current and appropriate margin), then compare the current range in the doggerel according to the required current value, and then the conductor section that should be selected can be obtained by “allowable current per square millimeter” belonging to the range at the required current value.


For example, the rated current of an electrical equipment is 20A. Considering a certain margin, the required current is determined to be 22a. Then, find out that the current range of 22a current in the above doggerel belongs to 10 ~ 50a, and the available current per square millimeter is 4a. Finally, divide 22a by 4a to obtain the conductor section s that should be selected as:
S = 22 / 4 = 5.5 (mm2). According to this value, the conductor with corresponding section can be selected. If the calculated sectional area is not in the conductor section specification series, a slightly larger conductor section can be selected.

Maintenance contents of cable channel

1. Replace the damaged well cover, cover plate and protection plate, and complete the missing well cover, cover plate and protection plate.
2. Maintain the working shaft stop.
3. Clean up the water and debris in the passage.
4 stairs (ladder) for tunnel maintenance personnel entering and leaving the shaft

5. Maintain the ventilation, lighting, drainage and low voltage power supply system in the tunnel.
6. Maintain fire prevention and isolation facilities and fire fighting facilities in cable trench and tunnel.
7 prune and cut branches and vines with insufficient safety distance around AAAC Cable terminal tower (pole) and T-joint platform.
8 repair the grounding down lead with loose connection, poor grounding, corrosion and other defects.
9 replace the missing, faded and damaged stakes, warning signs and signs, and timely correct the inclined stakes, warning signs and signs.
10. Carry out anti-corrosion treatment for corroded cable support, and replace or repair the missing, damaged and severely corroded support parts.
11. Bailey frame, I-beam and other facilities can be used to protect the cable trench in operation, and the suspension and support protection shall be well done. When the suspension protection is carried out, the cable trench body or row pipe shall be protected as a whole, and direct suspension of bare cables is prohibited.


12. If the cable passage in the green belt or sidewalk is changed to slow lane or fast lane, it should be relocated. Before the relocation, the relevant parties should be required to take reinforcement measures according to the load-bearing road standard to protect the working shaft, drainage pipe and cable trench.
10.2.13 when there are excavators, cranes and other large machinery passing through the non load bearing AAC Cable channel, the relevant parties shall be required to take protective measures such as setting steel plates above, and the protective measures shall prevent noise from disturbing the residents.
10.2.14 if the elevation of the working shaft or trench body is inconsistent with the surrounding due to the change of the environment of the cable passage, the elevation of the working shaft or trench body shall be adjusted by prefabricated shaft or cast-in-place method.

What are the cable maintenance items?

1. Manhole and drainage pipe
(1) water samples were taken for chemical analysis.
(2) remove the water in the well and remove the sludge.
(3) paint the iron parts such as cable bracket and hook.

(4) check the well cover and ventilation, and check the well body for settlement and cracks.
(5) dredge the spare pipe hole.

(6) check the condition of ACSR Cables and joints in the manhole to see if there is any oil leakage, whether the insulation gasket on the bracket is in good condition and whether the grounding is good.
(7) check the circuit name plate.
(8) check whether there is electric erosion, and measure the potential and current distribution of cables in the manhole.
2. Cable trench and tunnel
(1) check whether the door lock is normal and whether the access is smooth.
(2) check whether there is water seepage and ponding in the tunnel, remove the ponding and repair the leakage.

(3) check whether the ABC Cable bracket falls.
(4) check whether the insulation gasket between the metal sheath of the cable and connector and the bracket is intact, and whether there is any damage on the bracket; Whether the bracket has fallen off.
(5) check whether the fireproof tape, coating, blocking material and fireproof box are in good condition, whether the fireproof equipment and ventilation equipment are perfect and normal, and record the room temperature.

(6) check whether the grounding condition is good, and measure the grounding resistance if necessary.

(7) clean cable trench and tunnel.
(8) check the cable and cable joint for oil leakage.
(9) check cable tunnel lighting.

Learn cable knowledge, start here

1、 Basic introduction of wire and cable
Wire and cable: usually twisted by several wires or groups of wires [at least two in each group], similar to rope. Each group of wires is insulated from each other and twisted around the center of a wire. The whole wire is covered with a highly insulating covering layer. It is mainly used for transmission, distribution of electric energy or transmission of electric signals.
Wires and ACSR Cables are mainly composed of the following four parts
1. Conductive core: made of high conductivity material (copper or aluminum). According to the requirements of laying and using conditions for cable flexibility, each wire core may be made of a single wire or multiple wires.
2. Insulating layer: the insulating material used for cables shall have high insulation resistance. The common insulation materials used in cables are oil impregnated paper, PVC, PE, XLPE, rubber, etc.


3. Sealed sheath: protect insulated wire core from mechanical, moisture, moisture, chemicals, light and other damage. For the insulation susceptible to moisture, lead or aluminum extrusion sealing sheath is generally used.
4. Protective coating: used to protect the sealing sheath from mechanical damage. Generally, galvanized steel strip, steel wire or copper strip, copper wire, etc. are used as armor to wrap around the sheath (called armored cable), and the armor layer plays the role of electric field shielding and preventing external electromagnetic interference at the same time. In order to avoid the corrosion of steel strip and steel wire by the surrounding medium, they are usually coated with asphalt or wrapped with impregnated jute layer or extruded with polyethylene or PVC sleeve.
2、 Wire and cable specifications
Wire and cable specification is the meaning of the expression of the number of cores and section size of wire and cable. The complete naming of wires and AAC Cables is usually more complex, so people sometimes use a simple name (usually the name of a category) combined with model specifications to replace the complete name. For example, “low voltage cable” represents all plastic insulated power cables of 0.6/1kv class. It can be said that as long as the standard models and specifications of wires and cables are written, specific products can be identified.
3、 Wire and cable application classification
(1) Classified by insulating materials, such as oil impregnated paper insulated cable, PVC cable, XLPE cable, etc.
(2) According to the use classification, it is divided into power cable, communication cable and control cable. They are used in power system, information transmission system, mechanical equipment and instrument system.
1. Power system
The wire and cable products used in power system mainly include overhead bare wire, bus bar (bus bar), power cable (plastic cable, oil paper power cable (basically replaced by plastic power cable), rubber sheathed cable, overhead insulated cable), branch cable (replacing part of bus bar), electromagnetic wire and electric equipment wire and cable for power equipment.


2. Information transmission system
The wires and cables used in information transmission system mainly include local telephone cable, television cable, electronic cable, radio frequency cable, optical fiber cable, data cable, electromagnetic wire, power communication or other composite cables.
3. Mechanical equipment and instrument system
Except for overhead bare wires, almost all other products have applications, but mainly power cables, electromagnetic wires, data cables, instrument cables, etc.
(3) According to product classification, it can be divided into five categories
1. Bare wire and bare conductor products
The main features of this kind of products are: pure conductor metal, no insulation and sheath layer, such as ACSR, Cu al bus bar, electric locomotive line, etc; The processing technology is mainly pressure processing, such as melting, calendering, drawing, stranding / tight stranding, etc; Products are mainly used in suburbs, rural areas, user main line, switch cabinet, etc.
2. Power cable
The main features of this kind of products are: extruding (winding) the insulating layer outside the conductor, such as overhead insulated cable, or twisting several cores (corresponding to the phase line, zero line and ground wire of power system), such as overhead insulated cable with more than two cores, or adding sheath layer, such as plastic / rubber sheathed wire and cable. The main process technologies include drawing, stranding, insulation extrusion (wrapping), cabling, armor, sheath extrusion, etc. different process combinations of various products have certain differences.
The products are mainly used in the transmission of strong electric energy in power generation, distribution, transmission, transformation and power supply lines, with large current (tens to thousands of a) and high voltage (220 V to 500 kV and above).
3. Wires and cables for electrical equipment
The main features of these products are: a wide range of varieties and specifications, a wide range of applications, the use of voltage in 1kV and below more, in the face of special occasions continue to derive new products, such as fire-resistant cable, flame retardant cable, low smoke halogen-free / low smoke halogen-free cable, termite proof, mouse proof cable, oil / cold / temperature / wear-resistant cable, medical / agricultural / mining cable, thin-walled wire, etc.
4. Communication cable and optical fiber
From simple telephone and telegraph cables in the past to thousands of pairs of telephone cables, coaxial cables, optical cables, data cables, and even combined communication cables. This kind of product structure size is usually small and uniform, high manufacturing accuracy requirements.
5. Electromagnetic wire (winding wire)
It is mainly used for various motors, instruments, etc.
4、 What’s the difference between wire and cable?
In fact, there is no strict boundary between “wire” and “cable”. Generally, the products with few cores, small diameter and simple structure are called wires, the ones without insulation are called bare wires, and the others are called cables; The conductor with larger cross-sectional area (more than 6 square mm) is called large wire, the smaller one (less than or equal to 6 square mm) is called small wire, and the insulated wire is also called cloth wire. But with the expansion of the scope of use, many varieties of “cable in the cable”, “cable in the cable”. So there is no need to make a strict distinction. In daily habits, people call household cloth wires wires and power cables for short.
The wire is composed of one or several soft wires with light and soft protective layer; The cable is composed of one or several insulated wires, which are wrapped with a tough outer layer made of metal or rubber. Cables and wires are generally composed of core wire, insulation sheath and protective sheath.

Types of cable faults

(1) Mechanical damage accounts for a large proportion of AAC Cable accidents. There are three main causes of mechanical damage.
(1) it is directly damaged by external force. If the cable is damaged by accident in the process of urban construction, transportation, or excavation, piling, lifting and handling of underground pipeline engineering.
(2) damage during cable installation: for example, the cable is damaged due to excessive traction, or the insulation or shielding layer is damaged due to too small cable bending.
(3) damage caused by other factors, such as expansion of insulating glue in joint or terminal and cracking of shell or nearby cable sheath; Without proper cable expansion arc, the cable sheath installed on the pipe orifice or bracket is damaged due to thermal mechanical force; Excessive tension caused by land subsidence, breaking the joint or conductor; The breakage of terminal porcelain bushing; Due to the vibration of large mechanical parts or running vehicles nearby, the lead bag is damaged.
(2) damp insulation
(1) because of the joint box or terminal box structure is not sealed or poor installation, resulting in water.
(2) poor cable manufacturing: there are small holes or cracks in the metal sheath of the ACSR Cable.
(3) the metal sheath is pierced or corroded by foreign objects.
in the fault repair work, the damp insulation must be removed. At the same time, check whether there is moisture in the conductor and remove it.


Insulation aging
cable insulation has been operating under the action of electricity and heat for a long time, and its physical properties change, which leads to the decrease of insulation strength or the increase of dielectric loss, and finally causes insulation breakdown, which is called insulation aging. If the operation time exceeds the designed service life of the cable, the aging of the insulation is called normal aging, which belongs to improper operation. If the similar situation occurs in a short period of time, it is regarded as premature aging of the insulation.
Overvoltage (IV)
it is rare to damage cable equipment due to lightning stroke or other impulse overvoltage. Generally, 3 ~ 4 times of instantaneous overvoltage will not affect the cable with good insulation. However, there are still some cases that the cable line is broken down when it is struck by lightning, such as the insulation of the vertical part of the terminal is dry, which may be broken down when it is struck by lightning.
Overheating
there are many factors for cable overheating. Cable overload operation (over cable ampacity or under abnormal operation mode) will cause cable overheating; In areas with dense cables, poor ventilation of cable trench and tunnel, cables passing through dry ducts, etc., insulation damage may be accelerated due to overheating of cables. The vertical part of oil paper cable will lose insulating oil quickly due to overheating, resulting in dry insulation and even coking of insulating paper. Overload can also accelerate the recrystallization of lead clad grains, resulting in lead clad fatigue damage. In the oil paper cable line, when the shell of the joint filled with a large amount of asphalt insulating glue is welded with copper sheet, due to the large thermal expansion coefficient of the asphalt insulating glue itself, it is difficult to avoid the overload. The expansion of asphalt insulating glue will cause the copper shell to crack, or lead sheath of a section of cable at the lead sealing joint to crack.
Due to the fire or the influence of other adjacent cable faults and other external overheating damage, most of them can be determined from the burning phenomenon of the protective layer outside the cable, which is easier to identify.
(VI) material defects
this is a congenital defect of the cable. There are mainly mechanical damage of lead sheath, impurities in lead sheath, and joints in pressing lead. These are not easy to see outside the armor, and are often found in the process of accident repair. The end of the plastic cable is not sealed. After a large amount of moisture enters into the conductor, the insulation will gradually form “water branch” under the action of voltage, leading to insulation breakdown.

it belongs to congenital defects of cable accessories, such as sand holes in cast iron, insufficient mechanical strength of porcelain parts, rough and loose processing of assembly parts, non-conforming specification of waterproof rubber ring, rubber aging, etc. If the installation unit has a sound acceptance system, it can basically find these disadvantages and plug the loopholes.
(7) poor design
such as the joint and terminal waterproof design is not careful, the selection of materials is improper, the process requirements are not strict, the mechanical strength is poor and so on. For new equipment, new materials and new technology, trial operation should be arranged first, and then gradually popularized according to the maturity of operation experience, so that it is easy to correct even if problems occur.

How to prevent cable corona discharge?

Corona discharge occurs in the air gap of indoor heat shrinkable, cold shrinkable or nylon AAC Cable terminal due to the action of electric field at the junction of semiconductor shielding layer and insulation layer of three core or single core cable.
When the terminal position of the cable installed in the indoor switch cabinet or metering cabinet of the distribution station is low, the ponding in the cable trench or the poor ventilation of the distribution station, and the indoor air humidity is high, the corona discharge phenomenon will also occur.
In order to prevent the corona discharge of indoor terminal, the filling material of trident of three core cable terminal must be filled. In addition to the filling material, the heat shrinkable or cold shrinkable terminal should also pay attention to the elimination of gas in the pipe during the heat shrinkable or cold shrinkable process without leaving air gap. Special attention should be paid at the junction of the two media.
The experimental results show that after the heat shrinkable terminal is moved, the discharge amount of partial discharge test will increase, and the re heating shrinkage partial discharge will decrease. Therefore, if corona discharge occurs at the heat shrinkable terminal, it will disappear after reheating and shrinking to eliminate the air gap.

If the cable trench in the station is flooded and the ventilation is poor, measures such as drainage, improving ventilation and keeping the station environment clean shall be taken( The above text is excerpted from “technical questions and answers for power cable installation and operation”)
reference:
[1] Shi Chuanqing, chief editor. Question and answer of power ABC Cable installation and operation technology. Beijing: China Electric Power Press, 2007
Extended reading:
Corona discharge, the most common form of gas discharge, refers to the partial self-sustaining discharge of gas medium in non-uniform electric field. In the vicinity of the tip electrode with a small radius of curvature, the local electric field intensity exceeds the ionization field intensity of the gas, which makes the gas ionize and excite, resulting in corona discharge. When corona occurs, light can be seen around the electrode, accompanied by a hissing sound. Corona discharge can be a relatively stable discharge form, or it can be an early development stage in the process of gap breakdown in uneven electric field.

Methods to prevent cable corrosion

(1) The cable manufacturer is required to use the protective layer outside the metal sheath of the AAC Cable when manufacturing the cable, and the aluminum sheathed cable must have polyethylene or PVC outer sheath.
2) When selecting the cable route, it is necessary to make full investigation, collect the soil data near the area where the line passes, and carry out chemical analysis, so as to judge the degree of soil and groundwater erosion. If necessary, measures should be taken, such as changing the route of the line, partially replacing the bad soil, or adding special protective measures, such as putting the cable through the corrosion-resistant pipe.


Prevent external damage and theft of facilities
Design and infrastructure stage
The setting of cable route, auxiliary equipment and facilities (ground grounding box, entrance and exit, ventilation Pavilion, etc.) shall be approved by the planning department. It should be avoided that the cable passage is adjacent to the thermal pipeline, flammable and explosive pipeline (oil and gas) and corrosive medium pipeline.
Note: it is a new clause. It is proposed that the route, auxiliary equipment and facilities should be approved by the planning department, and the requirements of avoiding the cable passage adjacent to thermal pipelines, flammable and explosive pipelines (oil and gas) and pipelines with corrosive media should be avoided.
It is difficult to understand the corrosion degree of the cable on the running cable line. In the area where the corrosion of the cable has been found or the leakage of some chemicals piled on the ACSR Cable line, the soil should be dug to check the cable, and the nearby soil should be chemically analyzed to determine the damage degree
2. Methods to prevent electrolytic corrosion
(1) strengthen the insulation between the metal sheath of the cable and the huge metal objects nearby.
(2) installation of drainage or forced drainage, polarity drainage equipment, cathode station, etc.
(3) for areas with serious electrolytic corrosion, shielding pipes should be installed.
stray current is related to the distribution of electrical track, the maintenance of joints and the layout of other underground metal pipelines. In order to protect the underground pipelines and communication cables, the departments of underground oil and gas pipelines and local communication cables sometimes adopt cathodic protection. Therefore, the cable operation Department must systematically measure the potential difference between the lead sheath and the earth and other pipelines, the stray current density of the lead sheath and the current density flowing into the earth, so as to accumulate data. When we know which area is in danger of corrosion, we should take preventive measures in time, and work together with relevant units to try to eliminate the source of stray current and do a good job of corrosion prevention.


the operation experience shows that when the current density from the cable metal sheath reaches the average value of one day and night, the metal sheath will be corroded. This paper introduces a method of measuring stray current density auxiliary electrode method,
the auxiliary electrode is made of a section of cable similar to the tested cable, and its length should be such that the contact surface between the electrode and the earth is not less than 500. Peel off the outer protective layer of the electrode surface, wipe off the armored surface, weld the connecting wire, and pour asphalt or other insulating materials on the solder joint insulation and both ends of the electrode.
In addition to cable corrosion, there are also corrosion phenomena such as metal bracket, metal hoop and grounding wire in cable channel.

How to make a perfect cable identification?

The most commonly used label for ACSR Cable identification is covered with protective film. This kind of label is sticky and has a layer of transparent protective film outside the printing part, which can protect the label printing font from abrasion. In addition, single cable / jumper can also use non coated label, flag label and heat shrinkable sleeve label. Common material types include vinyl, polyester and polyfluoroethylene, as shown in the figure.

For bundled cables, it is recommended to use identification plates for identification. This kind of sign can be printed by printer, fixed with nylon tie or felt belt and AAC Cable bundle, and can be placed horizontally or vertically. The sign itself should have good tear resistance and conform to ROHS standards.

The most commonly used label for cable identification is covered with protective film. This kind of label is sticky and has a layer of transparent protective film outside the printing part, which can protect the label printing font from abrasion. In addition, single cable / jumper can also use non coated label, flag label and heat shrinkable sleeve label, as shown in the figure.
5.3 label of wiring panel / outlet panel
The wiring panel identification is mainly plane identification, which requires that the material can withstand the test of the environment, meet the environmental requirements of RoHS, maintain good image quality in various solvents, and can be pasted to various surfaces including low surface energy plastics. The label should be printed and not filled in by hand. It should be clear and easy to read. All labels shall be clear and complete and meet the requirements of the environment, as shown in the figure

Factors affecting insulation resistance of wires and cables

Small insulation resistance is often encountered in AAC Cable production, which is affected by many factors. In fact, there are four main factors that have a great influence on the coefficient of insulation resistance.

1、 The influence of temperature
With the increase of temperature, the insulation resistance coefficient decreases. This is due to the increase of thermal motion and the increase of ion production and migration. Under the action of voltage, the conduction current formed by ion motion increases and the insulation resistance decreases.
The theory and practice show that the insulation resistance coefficient decreases exponentially with the increase of temperature, and the conductivity increases exponentially with the increase of temperature.
2、 Influence of electric field intensity
When the electric field strength is relatively low, the mobility of ions increases in proportion with the increase of electric field strength. The ionic current and electric field strength follow Ohm’s law. When the electric field intensity is relatively high, with the increase of electric field intensity, the mobility of ions gradually changes from linear relationship to exponential relationship. When the electric field is close to breakdown, a large number of electrons will migrate and the insulation resistance coefficient will be greatly reduced.
The withstand voltage test voltage of various wire and ACSR Cable products specified in the standard is in the stage of ion mobility increasing in proportion to the electric field strength, so the influence of electric field strength on the insulation resistance coefficient cannot be reflected. The effect of electric field on the insulation resistance coefficient is obviously reflected when the sample is subjected to breakdown test.


3、 Influence of humidity
Due to the large conductivity of water, the size of water molecules is much smaller than that of polymer molecules. Under the action of heat, the polymer macromolecules and the constituent chain move relatively, so that water molecules can easily penetrate into the polymer, increase the conductive ions in the polymer, and reduce the insulation resistance.
The standard specifies the immersion test of various wires and cables. For example, the rubber specimen is immersed in water for 24 hours before the insulation resistance is measured. The purpose is to meet the influence of moisture and water on electrical performance during use.
Insulation resistance is one of the main electrical properties of insulating materials, and it is also an important index of wire and cable products or materials. Generally, the insulation resistance should not be lower than a certain value. If the insulation resistance value is too low, the leakage current along the wire and cable line will inevitably increase, resulting in the waste of electric energy. At the same time, electric energy will be turned into heat energy to prepare for thermal breakdown and increase the possibility of thermal breakdown.
4、 Influence of material purity
The impurity mixed into the material increases the conductive particles in the material and reduces the insulation resistance. Therefore, the insulation resistance of a certain rubber plastic material will reflect the purity of the material and verify whether it meets the standard.
In the process of wire and cable production, the process does not strictly comply with the operating procedures, mixed impurities and materials Blister due to moisture, insulation core deviation or outer diameter size is less than the standard, insulation delamination or crack, insulation scratch, etc., It will reduce the insulation resistance of the product.
Therefore, in order to check the insulation resistance, it is necessary to check whether there are any problems in the process operation. During the use of wires and cables, measuring the change of insulation resistance can also check the insulation damage and prevent accidents.

For more information on cables, click Joy ’cable Blog

Is the thicker the cable insulation, the better?

Many friends may think that the thicker the AAC Cable, the better the insulation performance, the more durable the cable, and the better the quality. In fact, this idea is taken for granted, and there are some misunderstandings.
This is because the insulation performance of cables depends on the technical indicators. The state has strict technical standards for cables. Compared with the cable thickness specified in the national standards, it is unnecessary to say that if the cable thickness is thinner, even if it is thicker, it is also unqualified as long as it exceeds the specified value.

When the thickness exceeds the standard, the performance of the cable will be affected, such as the service life will be greatly shortened. Because when the cable is laid and put into use, it will be in the state of continuous power supply, and the heat will be generated in the process of power supply. If the thickness exceeds the standard, although the insulation looks better, the result is that the heat is difficult to dissipate( Learn more cable technology, please click here, a large number of dry goods waiting for you) with the accumulation of time, more and more heat accumulation, of course, will affect the normal service life of the cable.


When the situation is serious, it will even lead to short circuit, power failure, causing greater damage.
The so-called “too much is not as good as it can be”. Those who meet the national standards are the best.

For more information on cables, click Joy ’cable Blog