Causes of blackening of copper wire of heavy rubber sheathed flexible cable

2.1 catalytic aging of copper is an important reason for rubber hair adhesion
The experiment of the former Soviet Union Institute of AAAC Cable science proved that copper infiltrated into the insulating rubber from the contact with rubber during vulcanization, and the thickness of 1.0-2.0mm contained 0.009-0.0027% copper. As we all know, trace copper has a great damage to rubber, that is, heavy metal is the catalytic aging of rubber. During the process of insulation vulcanization, qiulanm precipitates some free sulfur to react with copper to form active copper containing groups: CH3 ■ ch2-ch-c-ch2- ■ ■ SS ■ ■ cucu, when aging, weak s-s-bond breaks, forming active copper containing base: cu-s-, which acts with rubber, and acts with oxygen, breaks down long bond molecules of rubber, making rubber soft and sticky, which is a combination of low molecular chain. The French Academy of rubber also pointed out that if there are harmful metals in rubber, such as copper, manganese and other heavy metal salts, the rubber viscosity will occur regardless of the type of promoter.

2.2 sulfur migration to the surface of insulating rubber and copper wire in rubber sheath cable
The possibility of sulfur diffusion in cable sheath rubber was confirmed by the use of radioisotope by former Soviet scientists. The diffusion coefficient of free sulfur is about 10-6cm2 / s at 130-150 ℃ in the vulcanized rubber based on natural rubber. In the continuous vulcanization factory, when vulcanizing sheath rubber, the temperature is between 185-200 ℃, and the diffusion coefficient is greater. Because of the diffusion of free sulfur in rubber sheath, the structure of the colum rubber is changed, and the polysulfide bond may be formed. These polysulfide compounds migrate through chemical decomposition and chemical combination, namely “chemical diffusion”. Due to the migration, not only the structure of the insulating rubber can be changed, the heat resistance of the rubber can be reduced, but the reaction between sulfur and copper surface will result in copper sulfide and cuprous sulfide, which leads to the blackening of copper wire. In turn, copper sulfide and cuprous sulfide accelerate the aging of rubber, and lead to the occurrence of adhesion.

3. Reasons for processing technology
3.1 reasons for processing rubber
In the insulation formula based on the combination of natural rubber and SBR, the plastic of rubber needs to be improved by plastic refining. In order to produce, some factories use internal mixer to improve plasticity by adding a small amount of chemical plasticizer, accelerator M. If the temperature of plastic refining and rubber filtration are not well controlled, high temperature above 140 ℃ appears. When the raw rubber is put on the opening mill, it passes through the drum slowly. Because the rubber is affected by hot oxygen and accelerator M, it will be found that the rubber surface seems to be coated with oil. In fact, rubber molecules are more serious in promoting the chain breaking under the promotion of chemical plasticizer, A relatively soft and sticky rubber with smaller molecular weight was produced.
Although the rubber was mixed with SBR and then mixed with insulating rubber, these small molecular weight natural rubber were evenly dispersed in the rubber. After the rubber was extruded on the copper wire for continuous vulcanization, there might be no problem at that time, but a hidden danger was buried for the rubber copper ABC Cables. That is, the local copper wire sticking phenomenon will appear in the first place for these small molecular weight natural rubber.

The process of adding vulcanizing agent and accelerator to insulating rubber is also very important. Some small factories add vulcanizing agent on the mixer, that is, pour the pot containing vulcanizate into the middle of the drum, with many in the middle, and less on both sides. When the vulcanizate into rubber, the number of turning triangle was less, which would make the vulcanizate distributed unevenly in the rubber. In this way, copper wire blackening is easy to appear in many places with vulcanizing agent when extrusion is continuous vulcanization. In the blackening place for a long time, the phenomenon of rubber adhesive copper wire will appear.
3.2 reasons for vulcanization of insulating rubber
In order to pursue the production, some enterprises only have 60 meters long continuous vulcanization tubes, 1.3mpa steam pressure, and the vulcanization speed is 120 m / min. thus, the residence time of insulating rubber in the pipe is only 30 seconds. Rubber itself is a bad conductor of heat. The surface temperature of the insulating core is more than 190 ℃, and when the temperature is transferred to the inner rubber contacting with copper wire, it is also absorbed by copper wire. When the copper wire is heated to close to the inner rubber temperature, the vulcanized rubber wire core has been discharged from the vulcanizing tube. So the temperature of the inner rubber is relatively low, about 170 ℃, and the vulcanization tube will be left only a few seconds. When it enters the cooling and winding, the insulation rubber will not be vulcanized enough.
In order to achieve sufficient vulcanization, the amount of promoter TMTD (as vulcanizing agent) is up to 3.4%. The excess of vulcanizing agent also releases more free sulfur during the curing process. Besides the crosslinking rubber molecules, there are also excess free sulfur. This is the reason why the copper wire surface is blackened.
In short, it is still difficult to solve the problem of copper wire blackening. Every process from copper wire to rubber should be taken seriously to achieve better results. The key to the problem is the choice of rubber species and the adoption of vulcanization system. The solution to this problem needs to go through the test of time.

Introduction of conductor calculation formula of wire and cable

1、 Wire and AAC Cable material consumption
The conventional calculation method of copper weight without conversion: sectional area * 8.89 = kg / km
For example: 120 * 8.89 = 1066.8kg/km
1. Quantity of Conductor:
(Kg/Km)=d^2 * 0.7854 * G * N * K1 * K2 * C /
D = diameter of copper wire, g = specific gravity of copper wire, n = number of wires, K1 = twisting rate of copper wire, K2 = twisting rate of core wire, C = number of insulated core wires
2. Insulation amount:
(Kg/Km)=(D^2 – d^2)* 0.7854 * G * C * K2
D = outer diameter of insulation d = outer diameter of conductor g = specific gravity of insulation K2 = twisting rate of core wire C = number of insulation core wires
3. Dosage of external coating:
(Kg/Km)= ( D1^2 – D^2 ) * 0.7854 * G
D1 = finished outer diameter d = upper process outer diameter g = insulation specific gravity


4. Amount of wrapping tape:
(Kg/Km)= D^2 * 0.7854 * t * G * Z
D = outer diameter of upper process t = thickness of cladding g = specific gravity of cladding z = overlap ratio (1 / 4lap = 1.25)
5. Winding amount:
(Kg/Km)= d^2 * 0.7854 * G * N * Z
D = copper wire diameter n = number of wires g = specific gravity z = twist in rate
6. Weaving amount:
(Kg/Km)= d^2 * 0.7854 * T * N * G / cos θ
θ = Atan (2 * 3.1416 * (D + D * 2)) * mesh / 25.4/t
D = diameter of braided copper wire t = number of ingots n = number of bars per ingot g = specific gravity of copper
Specific gravity of material:
89; Cu -8; 50; Ag -10; Aluminum – 2.70; Zinc-7.05; 90; Ni -8; 30; tin-7; Steel -7.80; Lead-11.40; Aluminum foil mylar-1.80; 35; Myra -1.37
PVC-1.45; LDPE-0.92; HDPE-0.96; PEF (foaming) – 0.65; FRPE-1.7; Teflon(FEP)2.2; Nylon-0.97; PP-0.97; PU-1.21
55; cotton belt -0; PP rope -0.55; Cotton yarn-0.48
2、 Calculation formula of material outside conductor
1. Sheath thickness: outer diameter before extrusion × 035 + 1 (for power cables, the nominal thickness of sheath of single core ABC Cable shall not be less than 1.4mm, and that of multi-core cable shall not be less than 1.8mm)
2. On line measurement of sheath thickness: sheath thickness = (perimeter after sheath extrusion – Perimeter before sheath extrusion) / 2 π
Or sheath thickness = (perimeter after sheath extrusion – Perimeter before sheath extrusion) × zero point one five nine two


3. Thinnest point of insulation thickness: nominal value × 90%-0.1
4. Thinnest point of single core sheath: nominal value × 85%-0.1
5. Thinnest point of multi-core sheath: nominal value × 80%-0.2
6. Steel wire armor: number=
{π ×( Outer diameter of inner sheath + diameter of steel wire) ×λ)
Weight = π × Wire diameter? ×ρ× L × Number of roots ×λ
7. Weight of insulation and sheath = π ×( Pre extrusion outer diameter + thickness) × thickness × L ×ρ
8. Weight of steel strip = {π ×( Outer diameter before wrapping + 2 × Thickness – 1) × two × thickness ×ρ× L}/(1+K)
9. Weight of tape = {π} ×( Outer diameter + number of layers before wrapping × Thickness) × Number of layers × thickness ×ρ× L}/(1±K)
Where: K is the overlap rate or gap rate, if it is overlap, it is 1-k; In case of gap, it is 1 + K
ρ Is the specific gravity of the material; L is the length of cable; λ Stranding coefficient

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.

What operation data should be collected for cable fault?

To analyze the cause of the fault, first of all, we need to investigate the field situation of the fault and check the real fault, and collect the following installation and operation data.
(1) inspection and analysis of AAAC Cable or connector and terminal fault
1. Connector or terminal failure
(1) whether the joint sleeve or terminal box is complete and whether there is any trace of water invasion;

(2) whether the casing is cracked or flashover;
(3) whether the insulating glue is filled, whether there is a gap inside, and whether the ABC Cable oil is deteriorated;
(4) whether the conductor connection is in good condition, if necessary, the contact resistance of the crimping conductor should be measured;
(5) whether there is moisture in the conductor and insulation, and whether there is damage in the bending part;

(6) the path of insulation breakdown or flashover;

(7) sealing condition of metal sheath;
(8) the condition of lead (aluminum) sheath at and near the stripped part of armor;
(9) equipotential bonding between armor and inner sheath;
(10) shielding connection of metal sheath of three core cable.
2. Cable body fault
(1) whether the conductor has barbs, sharp corners, broken and uneven strands;
(2) whether there is moisture in the conductor and insulation, whether there are wrinkles and creases on the paper tape, and whether the covering of each layer is uniform; Whether the gap overlaps repeatedly, whether the wrapping is too loose, whether the impregnation is sufficient, whether the insulating oil is deteriorated or waxy, whether the paper tape is brittle, and whether there are traces of carbonized branches on the surface, etc;
(3) whether the cross-linked insulation has impurities, pores, dendritic discharge traces, whether the insulation thickness and eccentricity meet the requirements of relevant standards, and whether the shielding layer is broken;
(4) whether there are impurities, oxides or pores in the metal sheath, whether the pressing lead (aluminum) is uniform, whether there are obvious seams or pinholes, and whether the sheath has mechanical damage, vibration deformation, cracking, tension cracking, etc;
(5) whether the outer protective layer of the metal sheath is damaged or has traces of mechanical damage;
(6) whether the overlapping distance of armor layer meets the standard, whether there is corrosion or damage by external force, and whether the outer sheath is in good condition.
(2) data collection of line operation and installation
(1) learn about cable line fault trip and relay protection action from dispatching department;
(2) the name of the line and the starting and ending terminals;
(3) the time and place of the failure;
(4) cable specification: such as voltage grade, type, conductor section, insulation type, manufacturer name, purchase date;
(5) device record: such as installation date and climate, connector or terminal design type. Type of insulating agent and heating temperature;
(6) field installation: such as cable bending radius size, terminal device height. The arrangement and grounding of three-phase single core cable, cable embedding mode, elevation, cover plate position, etc;
(7) surrounding environment: such as the ground conditions near the fault point, whether there are new excavation, piling or pipe laying projects, whether there are acid or alkali components in the soil, whether there are small stones, whether there are chemical factories in the nearby area, etc;
(8) operation conditions: such as cable line load and temperature, etc;
(9) calibration record: including test voltage, time, leakage current and insulation resistance value and historical record.
through the analysis of the real fault, combined with the above investigation materials and data, we can get more correct analysis results.

Why are mineral insulated cables so hot?

In recent years, the city has developed rapidly
Subway, station, hospital, school, high-rise housing
Expansion of public facilities construction
The number of high-rise buildings has soared
The demand for urban electricity is rising
Such a densely populated focus area
Serious consequences of fire
In recent years, the fire spread trend of such sites presents similar pattern

Analysis of experts in building and safety fire protection
There are still many AAC Cables for public facilities
No fire protection
It is difficult to play an appropriate role in the fire
The law expressly stipulates that the fire protection system needs mineral insulated cable in the code of fire protection design of building engineering to pass the fire inspection and acceptance.
Because mineral insulated ABC Cable can not only ensure the fire power supply within the fire duration (more than 180 minutes), but also will not delay the fire, no smoke and secondary disasters, thus winning valuable time for fire rescue.
It can be said that mineral insulated cable is a special cable used in the fire protection of important national facilities, large public areas, high-rise buildings and other places. Its fire resistance, durability, safety, reliability and economy are irreplaceable by traditional power cables.
1、 Structure form of mineral insulated cable
Chengtiantai mineral insulation cable is a kind of cable which is wrapped with copper sheath and separated from the conductor and sheath with magnesium oxide powder as inorganic insulation material (optional for outer sheath), and forms a close whole with metal core and sheath.
Magnesium oxide is also a kind of non combustible material, and it will not produce toxic smoke or other harmful substances when encountering high temperature. Therefore, the cable with magnesium oxide as the insulator of copper core and copper sheath has better performance and many advantages.
2、 The properties and advantages of mineral insulated cables
Chengtiantai mineral insulation cable mainly includes bbtrz (flexible mineral insulated fire-proof cable) and yttw (flexible inorganic mineral insulation cable), which has good fire resistance, high temperature resistance, high carrying capacity, waterproof, corrosion resistance, mechanical damage, radiation resistance, electromagnetic compatibility, beautiful and generous appearance. At the same time, the cable will not emit any smoke under fire conditions Halogen and toxic and harmful gases.
1. Fire protection performance
Because mineral cables are all made of inorganic materials, they will not cause fire, combustion or combustion supporting, and toxic gas will not be produced. Even if there is fire outside, the cable can still work normally. Mineral insulated cable is made of magnesium oxide as insulator, and the melting point of magnesium oxide is as high as 2800 ℃, which is difficult to burn easily, so the fire resistance of mineral insulated cable is excellent.
The test shows that the mineral insulated cable can be operated normally for 2 hours in a flame with temperature of 800 ℃ -900 ℃; The cable is still intact and continues to operate normally after 30 minutes of combustion under the flame of 1000 ℃.
2. High temperature resistant operation, high load flow
The normal operating temperature of mineral insulated cable can reach 250 ℃, and in special cases, the cable can be maintained in a short time at the temperature close to the melting point of copper sheath. It is very suitable for laying in high temperature places, such as metallurgy, boiler, glass furnace, blast furnace and other environments.
The cut-off flow of MgO powder is far higher than that of other cables. Because magnesia powder has better conductivity coefficient than plastic, the same working temperature and flow rate are larger. For lines with more than 16mm, one section can be reduced, and two sections can be reduced for places where people are not allowed to contact.

3. Waterproof, explosion-proof, high mechanical strength
The insulation layer of mineral insulated cable is processed by high density compaction process, and then protected by seamless copper sheath. In the product standard, it must be subject to three requirements of fire resistance, spray resistance and mechanical impact resistance, which can withstand severe impact and mechanical damage. Therefore, it not only prevents the water, moisture, oil and some chemical substances from being infringed, but also suitable for connecting the explosion-proof equipment and equipment in places with explosion risk.
4. Overload protection
When the line is overloaded, the plastic cable will cause insulation heating or breakdown due to over-current or overvoltage. For mineral insulated cables, as long as the heating temperature of copper is not reached, the cable will not be damaged. Even if the breakdown occurs in a short time, the high temperature of magnesium oxide at the breakdown point will not form carbides. After overload removal, the cable performance will not change and can still be used normally.
5. Long service life, more safe and reliable
Because the structure of mineral insulated cable is composed of copper core and insulating sheath, it is a closed whole, so it is difficult for the material to aging due to the oxygen action in the outside air. This makes the overall service life of the cable longer (2-3 times the life of the ordinary cable), and the copper sheath’s own characteristics make the cable better grounded, Therefore, the lightning protection and grounding measures are effectively implemented, so as to ensure the safety and reliability of the electrical line operation.
Due to the many excellent characteristics of mineral insulated cables, many electrical design codes or standards have been adopted in recent years, and also used in many engineering projects in different industries. It can be expected that the use of mineral insulated cables in China will increase greatly in the next ten years.

When multiple cables run in parallel, the load distribution will be uneven?

When multiple cables are running in parallel, load measurement shall be conducted regularly to understand the load distribution and correctly grasp the operation status of ACSR Cables.


The load distribution of multiple cables is uneven when running in parallel, and even one phase load of one cable will be close to zero. The main reason for the serious uneven load distribution is the large difference of contact resistance of terminal connection (especially outdoor copper aluminum transition contact).
Because, the corrosion battery with two different metal contacts will cause poor contact; Due to the change of load current, temperature and natural environment, contact resistance of contact will increase to different extent. In this way, the current distribution is not uniform. For example, three cables a, B and C are running in parallel. If the contact resistance of cable a contact is large, the current is small and the normal load is mostly transferred to ABC Cables B and C. This may cause overload of cable B and C. if the contact of cable B terminal is heated due to the increase of load, the contact will be burnt and the load will be transferred to cable C, resulting in overload, thus forming a vicious cycle, and even heating will be caused by serious uneven distribution of three-phase current on the cable armour. This phenomenon will certainly endanger the safe operation of cable lines, and should be paid full attention.


Note: in design, the cable model and path length used in parallel are the same in general. Even if the models are different, the current distribution problem will be considered to meet the actual operation requirements. The provisions of this article are designed to consider that different types of cables are used for laying parallel cables due to tight construction period and incomplete cable goods, which may cause overload of one cable and insufficient load of another cable to affect operation safety. Because of the different insulation types of cables, the maximum allowable operating temperature of the core is different, and the allowable carrying capacity of the cables with the same material and specification and different insulation types is also different. Therefore, if different types of cables are used in construction, the length of cables shall be the same as possible during laying, so as to avoid the impact of operation safety due to the disproportionate distribution of load.

For more information on cables, click Joy ’cable Blog

Instruments and accessories for cable path detection


Figure 6 t5000-3 transmitter and receiver
Instrument: t5000-3 color screen intelligent pipeline instrument
Signal output part: transmitter, direct connection, coupling clamp, LCC module
Direct connection: direct connection signal output line, used when finding the route of power cut cable
Coupling clamp: clamp method signal output line, used when finding live cable path
LCC module: it can isolate the AC voltage of 480v and below, and cooperate with transmitter to connect the AAC Cable with direct connection method, and find the live low voltage cable path and transmission cable path.
picture
Figure 7 transmitter accessories
Receiving signal part: receiver, stethoscope, A-frame

Figure 8 receiver accessories
Stethoscope: it can be used in cooperation with receiver to identify the ABC Cable live and find the target cable from multiple cables;
A-Frame: it can be used in combination with receiver to accurately locate the fault point of buried cable, and the use method is step voltage method.

3、 Transmitter wiring method
1. direct connection method
It is suitable for finding the route of power cut cable, with high current and strong signal. The transmitting signal is injected into the primary core through the direct connection, and the test phase core at the end of the cable is grounded manually. The primary core and the earth constitute the test circuit, as shown in Figure 9 below.

Figure 9 schematic diagram of direct connection
Tips:
1. the signal can not be transmitted between phases (short circuit at the end) and the test circuit can not be formed by phase. Because the current signal is in the opposite direction, the magnetic field signal is offset. The receiver is essentially the received loop magnetic field signal, and the signal will not be received;
2. the armored ground on both sides of the cable shall be removed, otherwise the current signal transmitted will be transmitted back from the armor, which will also offset the magnetic field signal;
3. in a word, the test circuit shall be one-way and only circuit.
If the cable circuit is connected incorrectly, such as poor grounding, phase sequence error, etc., the transmitter has no output current or low output current, and the “indicating square” is blank, as shown in Figure 10 below. If the circuit is connected correctly, the output current of transmitter can reach 25mA, and the “indicating square” turns black, as shown in Figure 11 below.

Figure 10 display of transmitter with wrong circuit connection
2. clamp method
It is suitable for finding the route of stop and live cable, with small current and weak signal. The transmission voltage signal is induced to the high voltage cable armour or low voltage cable zero line through coupling clamp. As shown in Figure 12, since both ends of the cable are armored or zero wire are grounded, there will be stable current signal naturally.
Tips:
1. the same point between direct connection method and clamp method is that the magnetic field signal is generated by unidirectional stable current circuit. The difference is that direct connection method transmits signal in cable core through direct connection, and clamp method is used to couple transmission signal in cable armour or zero line through clamp coupling;
2. clamp method is relatively weak in signal application, and direct connection method is preferred for power cut cable;
3. for high voltage single core live cable, direct connection method can be used to find the path in aluminum sheath. The use of clamp method and direct connection method should be reasonably selected in combination with the grounding mode of aluminum sheath.
3.1 380V cable test method
380V power off cable:
① The direct connection method of conductor is shown in Figure 9, and the one-phase core is selected as the test phase, and the end of the line core is grounded with corresponding phase core, and the zero line grounding at both ends is removed. The fire line and terrain are single-phase and unique circuit;
② Zero line direct connection method: remove zero line grounding at one end of cable, and keep zero line grounding at the other end. The direct output line is connected to the suspended zero line, and the zero line and terrain are single-phase and unique circuit, as shown in Figure 13 below;

Figure 13 zero wire direct connection
③ Clamp method, as shown in Figure 12, the output clamp is directly stuck in the cable body or cable zero line, and the signal circuit is formed between the zero line and the ground. Note: at this time, the zero line at both ends of the cable should be grounded.
380V live cable:
① Clamp method, the wiring is the same as above;
② LCC, live direct connection, transmitter on the user side directly outputs the signal on the live fire line through LCC module. For 380 power supply low voltage power supply system, neutral point of transformer in the station area is directly grounded. At this time, the live fire line and the ground form a loop. Note: this method can only be used in the user side wiring, and find the cable path from the power side, The site case is referred to the above official account.
3.2 10kV cable test method
10kV power off cable:
① The direct connection method of conductor is shown in Figure 9, and the one-phase core is selected as the test phase, and the end of the corresponding phase wire core is grounded, and the armored grounding at both ends is removed. The conductor and terrain are single-phase and unique circuit;
② Armor direct connection method: remove the armored grounding at one end of the cable, and keep the armored grounding at the other end. The direct output line is connected to the suspended armored lead wire, and the armor and terrain are single-phase and unique circuit, as shown in Figure 14 below;
③ Clamp method, as shown in Figure 12, shows that the output clamp is directly stuck on the cable body or cable armored ground wire, and the signal circuit is formed between the armor and the ground. The clamp connection demonstration is shown in Figure 15 below.
10kV live cable:
Clamp method, same as above.
3.3 test method of 110kV cable
110kV power off cable:
① Direct connection of conductors, as above;
② For the direct connection method of aluminum sheath, the grounding mode of aluminum sheath shall be referred to. For the single end grounded cable section of aluminum sheath, the direct connection signal of aluminum sheath coaxial cable conductor shall be directly connected at the protective grounding box; For the cross connected cable section with aluminum sheath, the conductor or shielding direct connection signal of aluminum sheath coaxial cable shall be conducted at any cross interconnection box.
110kV live cable:
① For the cable section with single end grounding of aluminum sheath, open the protective grounding box or cross connected box with no grounding end. First, use multimeter to measure the induced voltage of the aluminum sheath. If the induction voltage is less than 25V (t5000-3 fuse fuse fuse fuse fuse fuse voltage), direct connection method can be directly used for aluminum sheath, as shown in Figure 16 below;
② If the induction voltage of aluminum sheath without grounding terminal exceeds 25V, LCC module incoming voltage shield shall be used and the live direct connection method shall be used for test;
③ Clamp method: because the aluminum sheath of cross connected large section is directly grounded at both ends, although the cable is transposed, the overall aluminum sheath still forms a circuit with the ground. Therefore, clamp method is only applicable to the path finding of 110kV cable section cross connected by aluminum sheath, and clamp the clamp directly on the coaxial cable at the cable body or any grounding box, as shown in Figure 18 below.
Tips:
1. when finding the path of 110kV cable live, it is better to search by sections according to aluminum sheath, and apply signal from the ground end to find the cable path to the grounding end;
2. for the cable section with no grounding at the direct grounding end of one end of aluminum sheath, the clamp method signal is weak. At this time, the output power of transmitter shall be increased and the output frequency shall be adjusted to test. It is recommended to use direct connection method or live direct connection method for finding out the ungrounded end;
3. because 110kV single core cable will generate induction voltage on aluminum sheath, if clamp is directly stuck on cable body, inductive voltage will also be generated in clamp. At this time, if clamp jaws are completely closed, circulation will be formed on clamp coil, and clamp will be burned by heating, that is, paper sheet shall be pad for closing jaw, as shown in Figure 19 below.

Figure 19 jaw treatment of clamp method when using the body

How to distinguish the rigidity or flexibility of mineral insulated cable

The structure of mineral insulated ABC Cables with different names is convenient for us to design and select better, and there is no confusion about many naming rules.

What are (rigidity) and (flexibility)? For a long time, the domestic mineral insulated cables are divided into two types: rigidity and flexibility.
The internationally used magnesium oxide mineral insulated cable (IEC 60702-1:2002) uses copper solid conductor instead of stranded conductor or soft conductor which is used in our common common cable.
Later, the following MI cables were developed by domestic production enterprises:
(1) It meets the industry standard: yttw series of jg/t 313 metal sheathed inorganic mineral insulated cable and terminal with rated voltage of 0.6/1kv and below (the release and update time of the standard is 2011 and 2014);
(2) It meets national standards: RTT series of gb/t 34926 mica tape mineral insulated corrugated copper sheath cable and terminal with rated voltage of 0.6/1kv and below (the standard release time is 2017);
(3) There are also some mica tape mineral insulated cables that meet the enterprise standards: non solid conductor.
All the above cables are generally classified into flexible mineral insulated cables in the industry, only to distinguish them from rigid MgO mineral insulated AAC Cables.
1. What form of (MI) cable is identified by the specification
The code for fire protection design of buildings GB 50016-2014, article 10.1.10 of the article description states that:
Mineral insulation non combustible cable is composed of copper core, mineral insulation material, copper and other metal sheath. Besides good conductivity, mechanical and physical properties and fire resistance, it also has good non flammability. Under fire conditions, this cable can ensure the fire power consumption within the fire duration.
That is to say, mineral insulation cable recognized by the specification must be made up of copper core, mineral insulation material, copper and other metal sheath as the basic composition.
BTT series rigid mineral insulated cable structure form
According to this requirement, we will see whether the structural forms of various rigid cables and flexible cables meet the requirements.
2. Rigid cable with unique naming rule (BTT)
BTT series mineral insulated cable is the only mineral insulated cable recognized internationally. It is a kind of cable with seamless copper tube sheath and dense compacted magnesium oxide crystal powder in the middle as the insulation material. The conductor is a single copper rod cable.
The reason for being recognized internationally depends on the material used is the real high temperature resistant material, among which the melting point of copper conductor and copper sheath is 1083 ℃, and the melting point of MgO mineral insulation layer reaches 2800 ℃, so that the line can still operate safely under fire temperature of more than 1000 ℃.
Because all the materials used are inorganic materials, and the structure is dense and hard, the product has the electrical, mechanical and environmental withstand properties that can not be compared with the conventional VV, YJV, yjy and other (organic cables).
It has high temperature resistance, fire prevention, explosion-proof and non combustion. It can run continuously for a long time at 250 ℃ and can also maintain 180min short-time operation under the limit state of 1000 ℃.
For BTT series Mi cables, the domestic implementation standard is gb/t 13033-2007 mineral insulated cables and terminals with rated voltage of 750V and below, and has not been updated so far. It can be seen from the release time of the specification that the stable state of product standards can be said to be the most mature Mi cable in technology.
BTT series rigid cables have good technical performance advantages, but also have problems such as difficult construction, short manufacturing length and complex intermediate joint processing technology. In order to solve the above problems, a variety of flexible cables have been developed in China.
This also brings us a lot of difficulties in the selection of engineering design.
3. Flexible cable named (bbtrz)
When the mineral insulated cable was introduced to China in the early stage, the copper sheath was often cracked because of the immature technology. Therefore, magnesium hydroxide was used as the sheath instead of copper sheath.
So (T) is changed to (b), and B is mineral.
The product structure consists of copper strand, mica tape insulation, irradiation crosslinked polyethylene water barrier, magnesium hydroxide fire protection sheath and low smoke halogen-free sheath.

Bbtrz, as a series of flexible mineral insulated cable models, has been highly recognized by many design institutes, owners and users, and has been widely used in various large-scale building fire lines in China.
Although the fire resistance tests of C, W and Z of the circuit integrity of bs-6387 can be passed, the mineral insulated cable does not use metal sheath such as () copper).
With the development of national comprehensive level, the structure of non combustible cable of mineral insulation is officially defined as “composed of copper core, mineral insulation material, copper and other metal sheath” in the code for fire protection design of buildings (GB 50016-2014) promulgated and implemented in 2015.
Because the series does not contain metal sheath, it has gradually withdrawn from the stage of fire protection application, and is no longer classified as “mineral insulation non combustible cable”.
4. Flexible cable named after [bttrz, yttw, rttz]
In fact, the three named Mi flexible cables have the same structure, all of which are called flexible rolled copper sheathed mica tape mineral insulated cables.
The product structure consists of copper pipe sheath composed of multi strand copper strand, mica tape mineral insulation wrapping, alkali free glass fiber dense filling and copper strip longitudinal welding.
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Before the relevant domestic standards were not issued, the bttrz flexible cable, which was developed and named by the manufacturer, was mainly different from bttz rigid cable, which highlighted (soft), so it was added (R) on the basis of bttz naming.
Later, the domestic construction industry standard: jg/t 313 was promulgated and implemented in 2011, and bttrz named by the manufacturer was changed to yttw. However, this naming rule is not in line with the domestic cable naming method, and many people can not accept it.
Later, the national standard was promulgated and implemented by the National Standardization Commission in 2017: gb/t 34926, which named the former flexible cable rttz.
So bttrz = yttw = rttz. In addition, before the relevant standards were issued, the flexible cables were named by different manufacturers: bttw, Btte and btwtz, which can be described as various.
It is obvious that the flexible cables meet the requirements of gb50016-2014, and the fire resistance type test meets the most stringent requirements of C, W and Z of bs-6387.
5. Flexible cable named (ng-a, btly)
Ng-a and btly series cables belong to the same category. They use aluminum as the main material metal pipe extrusion instead of copper tube pull sheath, which not only simplifies the production process and improves the efficiency, but also reduces the product cost (aluminum is only 1/10 of the comprehensive cost of copper).
The product structure consists of copper strand, mica tape insulation, seamless aluminum tube sheath, oxygen insulation layer filling, isolation refractory layer and low smoke and halogen-free sheath.
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The reason why aluminum tube can replace copper pipe is not melted under high temperature flame, which is due to the expanded refractory layer extruded by the outer part of the aluminum pipe.
It is foamed and solidified under the fire, forming a thick barrier to block the direct effect of flame on aluminum pipe. The insulation stability of mica tape can be improved (the insulation resistance will decrease with the increase of temperature).
According to the structure form and type test, it also meets the requirements of grade C, W and Z of gb50016-2014 and bs-6387. The insulation performance of mica tape inside the insulation layer (Mg (OH) 2 or Al (OH) 2 mineral material) on the outer layer of aluminum sheath has been improved.
However, ng-a (btly) series cables are not flexible cables specified in jg/t 313 and gb/t 34926 standards, and only enterprise standards are supported at present.
6. Summary
The above Mi cables are simply combed as follows:
(1) Rigid cable: BTT, structure form and product performance are in accordance with international and domestic specifications.
(2) Flexible cable: RTT and yttw, the fire resistance performance of the product meets the most stringent level tests of C, W and Z of bs6387, and the structure form meets the national specifications.
(3) Flexible cable: ng-a and btly, the fire resistance performance of the product meets the most stringent level tests of C, W and Z of bs6387, and the structure form meets the national standards, but only [enterprise standard] is supported.
From the above situation, the performance of flexible cable meets the requirements of the specification, and the length of a single cable is long and the construction is relatively easy. We will surely think, “can flexible cable replace rigid cable?”

Choose the wire and cable, quickly identify the type!

The main function of wire and cable is to transmit electric energy, signal and realize electromagnetic conversion. The transmission of electric power, such as power cable, overhead line, etc., is the same as shaft ABC Cable, and the wire with electromagnetic conversion is like paint wrapped wire. If you choose a wire or cable, you need to quickly identify the type and type to select the right cable to use.

According to the number of transmission lines of each other with each other which are fixed together, the insulated wires can be divided into single core wires and multi-core wires. Multi core wires can also fix multiple single core wires in one insulating sheath. The multi-core wires in the same sheath can be as many as 24 cores. The parallel multi-core line is indicated by “B”, and the twisted multi-core wire is indicated by “s”.
Insulated wires can be divided into single wire and multi-core wire according to the stock number of each transmission line. Generally, the insulated wires above 6 square mm are all multi-core wires. The insulated wires with 6 square mm and below can be single wires and can also be multi-core wires. We call the single wire of 6 square mm and the following as hard wire, and multi-core wire is called copper wire.
Hard wire is indicated by “B” and copper wire is indicated by “R”. The common insulating materials of ACSR Cable are polyethylene and high pressure polyethylene. The “V” of PE indicates that “Y” is used for high-pressure polyethylene.

Bv copper core polyethylene insulated wire; BLV aluminum core polyethylene insulated wire; BVV copper core polyethylene insulation layer polyethylene protective cable; Blvv aluminum core polyethylene insulation layer polyethylene protective cable; BVR copper core polyethylene insulation copper wire;
RV copper core polyethylene insulation layer is installed with copper wire; RVB copper core polyethylene insulation layer flat electrode connecting wire copper wire; BVS copper core polyethylene insulation layer twisted copper wire; RVV copper core polyethylene insulation layer polyethylene wire sheath copper wire;
BYR high voltage polyethylene insulated soft cable; Byvr high voltage polyethylene insulation layer polyethylene wire sheath copper wire; Ry high voltage polyethylene insulated copper wire; RYV high voltage polyethylene insulation layer polyethylene wire sheath copper wire;
BVVB copper core polyethylene insulation layer polyethylene protective wire sleeve flat cable; BLVVB aluminum core polyethylene insulation layer polyethylene protective wire sleeve flat cable; Bv-105 copper core high temperature resistant 105 ℃ polyethylene insulated wire.

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The causes and solutions of overheating of wires and cables

When the power cable passes through a certain load current, it will be heated. With the increase of load current, the higher the cable surface temperature is. If it is not managed in time, the consequences can be imagined. For example, PVC cable is considered with core temperature of 70 ℃ as the upper limit, and the surface temperature will be 5-10 degrees lower. Therefore, the ACSR Cable surface temperature is basically safe below 60 degrees. Considering the power supply maintenance, the lower the temperature is, the better.


The causes of heating during operation of cables are as follows:
1. The resistance of ABC Cable conductor does not meet the requirements, which causes the cable to generate heat during operation.
2. Improper cable selection causes the conductor cross-section of the cable used is too small, and overload occurs during operation. After a long time of use, the heating and heat dissipation imbalance of the cable cause heating phenomenon.
3. The cable is arranged too densely, the ventilation and heat dissipation effect is not good, or the cable is too close to other heat sources, which affects the normal heat dissipation of the cable, and may also cause the heating phenomenon of the cable in operation.
4. The joint manufacturing technology is not good, and the crimping is not tight, which causes the contact resistance at the joint is too large, and the cable will generate heat.
5. The insulation performance of the cable is not good, which causes the insulation resistance is small, and the heating phenomenon will also occur in the operation.
6. The partial sheath of armored cable is damaged, which causes slow damage to the insulation performance after water inflow, which results in the gradual decrease of insulation resistance and also the heating phenomenon in the operation of the cable.
After the cable has a heating phenomenon, if the cause is not found, the cable will continue to be powered on continuously and will cause insulation thermal breakdown. The short circuit between phases of the cable trip phenomenon, serious fire may be caused.
After the cable has a heating phenomenon, if the cause is not found, the cable will continue to be powered on continuously and will cause insulation thermal breakdown. The short circuit between phases of the cable trip phenomenon, serious fire may be caused.


The reason and solution of plug power cord heating are everywhere in family life. The electric appliances can not be separated from the power cord. This small power cord may despise him.
The heat of the power cord of water heater is usually caused by the poor coordination with the socket. Meanwhile, the normal heating phenomenon should be considered. If the surface temperature of the plug is less than the ambient temperature plus 50 ℃, it is normal. If abnormal heat is abnormal, it is necessary to consider replacing the socket or checking the cooperation between the plug and the socket.
1. the connection between plug and socket is poor, and the load power is high, which causes the plug to heat up. Such as electric kettle, electric iron, etc.
2. the thread head in the new plug is loose, which is caused by rough production process and other reasons.
3. the old plug is used for a long time, and the insulation performance is reduced (or the plug is loose).
How to use plug wire safely
The main reason for the heating of plug wire is that the load of the wire is increased due to the long-term power consumption of electrical equipment. Special multi-function socket, if connected with several high-power electrical equipment at the same time, the power line of the socket must be overloaded, and in that case, it is very easy to cause the wire to burn.
Therefore, we should pay attention to the reasonable distribution of electricity in our life, and regularly check the plug wires and sockets to see if the wires are old and whether the contact between the plug and socket is bad. Sometimes because the production department process is not fine, the plug wire head will not be firm, so the plug in the plug power supply will cause poor contact and lead to plug heating. At the same time, every household should pay attention to that the plug will be old for a long time, and the insulation property will be reduced. It should be replaced in time. For plug wires of high-power appliances, we suggest that the wire core is used to increase the type, and it is better to use the pin with grounding. If it is found that the power socket or wire plug is hot, which causes the electric appliances to burn, do not act recklessly, and take care of the management.

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