What are the basic causes of wire and cable failure?

Cable failures may seem like there are many causes, but there are actually only four basic causes that cause cable failure in the field (open or underground) when a cable is being dragged. They are: mechanical damage; The bending radius exceeds the recommended value; current overload; Excessive tension.

Single-causes, or a combination of these, can lead to serious downtime. Recognizing the symptoms of a problem can help cable users identify the problem and work to address it. When one of the four basic triggers appears, either the cable becomes immediately unusable, or a series of problems occur one after the other, indicating that the cable has failed.

1. Mechanical damage.

Direct manifestation: The outer sheath of the cable is often torn or fractured, and there are burrs or friction marks on the surface of the sheath without cracking.

Indirect manifestations: There are no signs of damage on the outside of the sheath, but the wire insulation inside the cable is partially or completely broken. If it is not completely failed, it will lead to leakage, tripping and shutdown may cause analysis

Sharp rocks, mine landslides, sharp and overloaded edges of winch cable reels.

Corrective Action: Cable operators, helpers, and other operators need to understand the properties of the soft copper stranded wire and rubber material inside the cable. There is still a long way to go in identifying product performance and limitations and reducing mechanical damage.

2. The bending radius exceeds the recommended value.

When the cable is bent and its bend radius is much smaller than the manufacturer's recommended bend radius, the internal components of the cable are susceptible to organic damage. When dragging the cable, you should avoid problems such as twisting, knotting, knotting, knotting, etc. Otherwise, the cable knot loops will become tighter and tighter, and the final bend radius will only be twice the diameter of the cable instead of 12 to 16 times the normal one.

If the bend radius is less than the recommended value for the cable, damage to the conductor and insulation can quickly occur. Small-diameter steel cables can be used to cut their sheath and/or squeeze the core until the insulation is damaged. Possible causes:

Wrong dragging method Corrective action for wiring technology: Correct bend radius training for the operator. The use of larger diameter steel cables and slings can reduce operational damage.

3. How to avoid screwing:

The knotting is initially caused by the knot ring, most often on the reel from which it is fixed"Pull"It occurs at the time of the cable and is not easy to unwind. Screwing can lead to permanence"Weaknesses"。Do not pull the loop, but slowly.

Keep the cable straight open from the winding loop"Scroll"Reels or coils of thread.

4. Current overload.

Performance: The cable insulation can withstand a temperature rating of 90 °C, but the sheath is not. Sheath synthesis is to form the highest mechanical strength, which is its basic function.

If the conductor operates in 90 °C free air and the current does not exceed the rated current, the cable service life can be achieved.

The following six are important causes of wire and cable obstruction:

Clause. 1. Environment and temperature. The external environment and heat source of the cable will also lead to excessive cable temperature, insulation breakdown, and even anger.

Clause. Second, the insulation is damp. This is also a common occurrence and usually occurs at the point where the cable is buried or in the pipe. For example, if the cable discussion is not made up properly and the discussion is made under the requirements of wet weather, the discussion will be flooded or mixed with water vapor.

Over time, water branches are formed under the action of the electric field, which gradually infringes on the absolute flange strength of the cable and causes obstacles.

Clause. 3. Damage by external forces. According to incomplete statistics, external damage can be described as the No. 1 "killer" of wires and cables.

Due to the acceleration of China's urbanization rate, almost every will be doing a larger range of reform projects, in the construction process of the city differential level damage to the cable, some will directly block the cable completely, and will leave the cable "sequelae, so that the protection of the cable is too low, and the rate of progress of the cable obstruction."

Clause. Fourth, perpetual overload and trouble. When people are overloaded, things will fall, and cables are no exception. Due to the thermal effect of the current, the flow of the load through the cable must cause the conductor to heat up.

At the same time, the skin effect of the electric charge.

As well as the eddy current consumption of steel armor and the consumption of insulating medium, additional heat will also be generated, so that the temperature of the cable will increase. During constant overload operation, excessive temperature will accelerate the aging of the insulation and even cause the insulation to break down. Especially in the scorching summer, the temperature rise of the cable often leads to the breakdown of the thin insulation of the cable, so in the summer, the cable is also very obstructed.

Clause. Fifth, the cable discussion obstruction. Cable discussion is the thinnest key point of the cable course, and it often occurs when the cable discussion is hindered by the direct failure of the staff (poor construction).

In the process of making the cable, if the construction staff discusses the original network such as insufficient crimping and insufficient heating, the insulation of the cable head will be very low, which will cause chaos.

Clause. 6. Chemical corruption. The cable is directly buried in the area with the effect of liquid rolling acid and alkali, which will cause the armor, lead skin or outer sheath of the wire and cable to be corrupted, and the cover layer will be subjected to chemical corrosion or electrolytic corrosion for a long time, resulting in the failure of the cover layer and low insulation, which will also lead to the obstruction of the cable - the cable corrosion environment of the unit is commensurate and severe.

1. External damage. For example, the construction of unqualified cable laying and installation is easy to cause mechanical damage, and it is easy to damage the underground cable in civil construction.

Sometimes if the damage is not serious, it may lead to a complete breakdown failure of the damaged part for several months or even years, and sometimes it may cause serious damage and a short circuit fault may occur, which directly affects the safe production of electrical units.

Second, the insulation is damp. For example: wire and cable joints are made unqualified and in humid climates, joints can make water or water vapor, under the action of the electric field for a long time r stratum water tree mixing, dielectric strength gradually caused damage to the failure of the cable.

3. Chemical corrosion. In areas of acid-alkali interaction, due to long-term exposure to chemical or galvanic corrosion by armored cables, wires or corrosive protective layers, the outer protective layer is often caused, resulting in poor insulation of the protective layer, and can also lead to cable failure.

Fourth, long-term overload operation. Overload operation, due to the thermal effect of the load current through the wire and cable will inevitably lead to heating in the conductor, at the same time, the skin effect and the eddy current loss of the charge, the dielectric loss of the steel armor can also generate additional heat, thus making the cable temperature.

5. Cable joint failure. Wire and cable glands are the weakest link of the cable that leads to frequent cable gland failures and direct negligence. In the manufacturing method of cable joints, if the crimping is not tight, the heating is not sufficient, which leads to the reduction of the insulation of the cable head, resulting in accidents.

There are six major faults:

1. Low-resistance fault, the cable insulation material is damaged, and the grounding fault occurs. The insulation resistance RF is less than 10Z0 (Z0 is the wave impedance of the cable, generally between 10 and 40). In general, the probability of low-resistance faults in low-voltage power cables and control cables is higher in the field.

Second, the open circuit fault, the continuity of the metal part of the cable is damaged, the wire is broken, and the insulation material at the fault point is also damaged to varying degrees. The insulation resistance RF is infinity ( ) when measured by a megohmmeter in the field, but during the DC withstand voltage test, there will be electrical breakdown; Check the conduction of the core wire, there are breakpoints. The site is generally in the form of a one-phase or two-phase disconnection and grounding.

3. High-resistance fault, cable insulation material is damaged, and grounding fault occurs. The insulation resistance RF is greater than 10z0 with a megohmmeter in the field, and electrical breakdown will occur during the DC high-voltage pulse test. High-resistance faults are cable faults with a high probability of high-voltage power cables (6kV or 10kV power cables), which can reach more than 80% of the total faults.

Fourth, the flashover fault, the insulation material of the cable has been damaged, and the flashover fault occurs. The insulation resistance RF is infinity ( ) measured by a megohmmeter in the field, but in the DC withstand voltage or high voltage pulse test, flashover electrical breakdown will occur. Flashover faults are difficult to detect, especially when a flashover fault occurs during a preventive test of a newly laid cable.

The DC flashover method is generally used for detection in the field.

5. Breakdown fault, in actual work, the cable insulation damage event triggered by the preventive test is customarily called cable breakdown. This kind of fault occurs under the DC experimental voltage, and its insulation damage is electrical breakdown, and the grounding point is generally intact lead or copper skin, and there is no obvious deformation on the outside (except for mechanical trauma). Most of the cable breakdown faults are simple grounding faults, and their grounding faults are high, and there is no carbonization point in the anatomical fault point and insulating material, but the aging structure of carbon holes and water branches can be found through the instrument.

6. Operation fault, which refers to the cable fault determined by eliminating the possibility of other power component failures in the operation of the power system of the factory, the cable leads of the cable feeder line, the motor and the transformer, the voltage fluctuation of the high-voltage secondary circuit or the discovery of grounding signal (grounding tripping of power components with grounding protection). This type of failure is characterized by ambiguity. The extreme form of cable operation failure is cable discharge (such as a phase short circuit caused by two points of grounding); Another part of the operation fault in the stop point inspection, due to the withstand voltage can not be turned on, the cable breakdown fault (such as cable aging, insulation defects, etc.).

Due to the wide laying area and long time of the cable, in a variety of complex environments, its insulation layer is easy to age or be corroded, and it is also easy to be affected by external forces, so power workers need to constantly sum up experience in the actual work, so that after the cable fault occurs, the cause and failure point of the fault can be quickly judged to ensure the normal operation of the power system.

Common cable faults are as follows:

1) Flashover failure.

The cable is well insulated at low voltage and will not be faulty. However, as long as the voltage value rises to a certain range, or a certain voltage continues to rise after a period of time, it will instantly break through the insulator and cause flashover fault.

2) One-phase core wire disconnection or multi-phase wire disconnection.

In the continuous test of cable conductors, the insulation resistance of each conductor of the cable is consistent with the relevant regulations, but if it is found that one or more phases cannot be continuous during the inspection, then it means that the one-phase core wire is broken or the multi-phase is broken.

3) One or two cores of the three-core cable are grounded.

The discontinuity of one or two conductors of a three-core cable is measured with an insulating shake meter, and then a telemetry of the insulation resistance of one or two cores to the ground is carried out. If there is a much lower than normal insulation resistance between the cores, this insulation resistance value higher than 1000 ohms is called a high resistance ground fault; Otherwise, it is a low-resistance ground fault. Both of these faults are referred to as disconnection and ground faults.

4) Three-phase core short circuit.

The size of the grounding resistance during the short circuit is the basis for judging the short-circuit fault of the three-phase core of the cable. There are two types of short-circuit faults: low-impedance short-circuit faults and high-impedance short-circuit faults.

When the three-phase core is short-circuited, a ground resistance of less than 1000 ohms is a low-resistance short-circuit fault, and vice versa.

When the cable line fails, the overvoltage and grounding current may damage the outer sheath of the cable, overvoltage protection device, etc., so the grounding system should be carefully checked during the line inspection. The common detection method is GD-4136L multi-pulse cable fault test system, and the low-voltage pulse method is used to detect the fault. If necessary, the withstand voltage test should also be carried out to avoid secondary failures after the cable belt defects are put into operation.

The reasons for the failure are as follows:

1. Insulation aging and deterioration: In the process of power supply, the cable insulation will be subjected to the current to bring about heating, chemical and other effects, and the insulating medium will undergo chemical changes such as deformation and softening, so that the insulation level of the medium will decrease. Problems can arise with cables for a long time.

2. Overload work: The overload of the cable makes the temperature too high, which will cause poor heat dissipation and accelerate the damage of the cable.

3. Man-made damage: mechanical construction, artificial excavation and other external forces cause cable deformation, which will lead to excessive bending, damage to the inner insulation or lead to air gap inside the insulation.

4. Cladding corrosion: under the electrolysis or other chemical action, the lead pack of the cable is corroded, due to the different nature and degree of corrosion, the lead pack has red, yellow, orange and light yellow compounds or sponge-like pores.

5. Natural factors: the super voltage of lightning weather directly breaks down the cable, and the stress on the cable exceeds the allowable value, causing damage.

There are usually several reasons for oil leaks:

1. Valve oil leakage, valve oil leakage is generally because the valve is blocked or damaged, and the solution is to replace the valve.

2. The joint is loose, and the solution is to tighten the joint or replace the joint.

3. That is, you have problems with the pump and hydraulic oil pipe.

The first thing to do is to find out what the problem is, and then decide what to do next based on the problem. If it is a cable fault problem, it is necessary to find the fault point through the corresponding equipment, and then carry out the corresponding repair work.

If the tripping phenomenon occurs, it is strictly forbidden to close the second time without eliminating the cause of the fault; Prevent inrush current from burning out cables and distribution facilities;

After the failure occurred, hang the obvious strict closing sign and send people to the scene to be optimistic;

Look for the cause of the fault in turn.