The service life of cables is an inevitable problem in the use of cables. The popularity of electricity has been very high, the production and manufacturing of all walks of life are inseparable from electricity, electricity is more inseparable from cable.
Then the service life of the cable directly affects the safety of production. The service life of the cable is determined by the oxidation induction period of the sheath material. General cable design use 20 years, the actual life is much longer than this value.
Leading to cable insulation reduction of the culprit factors
External force damage
According to the operation analysis in recent years, especially in today’s rapid economic development, a considerable number of cable faults are caused by mechanical damage.
Insulation of be affected with damp be affected with damp
This is also very common, and usually occurs at the joint of the cable buried directly or in the pipe. For example: cable joint production is not qualified and in humid climate conditions do joint, will make the joint water or mixed with water vapor, a long time under the action of the electric field to form water branches, gradually damage the insulation strength of the cable and cause failure.
The cable is directly buried in the area with acid and alkali effect, which often causes the armored, lead or outer sheath of the cable to be corroded. The protective layer is subjected to chemical corrosion or electrolytic corrosion for a long time, resulting in the failure of the protective layer and the reduction of insulation, which will also lead to cable failure. Chemical: the unit of cable corrosion is quite serious.
Long-term over load operation
Overload operation, due to the thermal effect of the current, the load current through the cable will inevitably lead to conductor heating, at the same time the skin effect of charge and the eddy current loss of steel armor, insulation medium loss will also produce additional heat, so that the cable temperature increases. Long-term overload operation, excessive temperature will accelerate the aging of insulation, and insulation breakdown. Especially in the hot summer, the temperature rise of the cable often leads to the weak insulation of the cable is the first breakdown, so in the summer, the cable fault is particularly many.
Cable joint failure
Cable joint is the weakest part in cable lines, and cable joint faults caused by personnel’s direct fault (poor construction) often occur. In the process of making cable joints, if the original network is not tightly pressed or heated adequately, the insulation of cable heads will be reduced, which will cause accidents.
Environment and Temperature
The external environment and heat source of the cable may also cause the cable temperature to be too high, insulation breakdown, or even explosion and fire.
Normal aging of the cable body or natural disasters and other reasons.
Five causes of cable fire and explosion
Insulation damage causes short circuit fault
The protective lead skin of power cable is damaged during laying or the cable insulation is damaged by mechanical damage in operation, which causes the insulation breakdown between cable phases or lead skin, and the generated arc causes the insulation material and the cable outer protective layer material to burn and catch fire.
Cables are overloaded for a long time
For a long time of overload operation, the operating temperature of the cable insulation material exceeds the maximum allowable temperature of normal heating, so that the cable insulation aging and drying up. This insulation aging and drying phenomenon usually occurs on the whole cable. Due to the aging and drying of cable insulation, the insulation material loses or reduces the insulation performance and mechanical properties, which is prone to breakdown and ignition combustion, and even multiple combustion fires occur simultaneously along the entire length of the cable.
Insulation breakdown of intermediate joint box
The middle joint of the cable joint box is not tightly pressed, welded or the joint material is not properly selected, and the joint oxidizes, heats and flows glue in operation; When making the cable middle joint, the quality of the insulating agent perfused in the middle joint box does not meet the requirements. When perfused with the insulating agent, there are pores in the box memory and the cable box is poorly sealed, damaged and leaked into the moisture. The above factors can cause the insulation breakdown, form a short circuit, and make the cable explode and catch fire.
Cable end burning
The surface of cable head is contaminated by moisture, the porcelain sleeve of cable head is broken and the distance between the leading wire and the leading wire is too small, which leads to flashover fire, and the insulation of cable head surface and the insulation of leading wire are burned.
External fire and heat sources cause cable fire
Such as oil system fire spread, oil circuit breaker explosion fire spread, boiler pulverization system or coal transmission system coal spontaneous combustion, high temperature steam pipeline baking, acid and alkali chemical corrosion, welding sparks and other kinder, can make the cable fire.
Top 10 tips to prevent cable fire and explosion
Ensure that the construction quality, especially the production quality of the cable head must strictly meet the requirements.
Test cables regularly and deal with problems immediately.
Install fire alarm device to detect fire immediately and avoid cable fire.
The cable trench should be kept dry to prevent the cable from being damp, resulting in insulation decline and short circuit.
Enhance cable operation monitoring to avoid over-load operation.
Occasionally clean the dust accumulated on the cable to prevent the dust from spontaneous combustion caused by cable fire.
Enhance the regular check and maintenance of cable circuit switch and protection to ensure its reliable operation.
The cables should be laid at a sufficient distance from the heat pipe. The control cables should be more than 0.5 meters. Power cable ≥l meters; Control cables and power cables should be slotted, layered and arranged separately, and overlapping between layers is impossible; For parts that do not conform to the demarcation, flame retardant and heat insulation measures should be taken.
Equip with necessary fire fighting equipment and facilities. Overhead cable fire can be put out with commonly used fire extinguishing equipment, but automatic or remote control fire extinguishing devices, such as 1301 fire extinguishing device and water spray fire extinguishing device, should be installed at cable intersections, shafts, channels and tunnels.
Take fire prevention and flame retardant measures. Cable fire retardant measures are:
(a) Coating the surface of the cable with fire retardant paint.
(b) Wrap the fire-proof parts of the cable with fire-proof straps.
(c) The cable through the wall, the bottom of the plate, the hole in the shaft with refractory materials to close, avoid the cable fire, high temperature smoke diffusion and spread caused by the expansion of the fire surface.
(d) Heat-resistant fire insulation boards, segmented partitions and fire doors shall be set between cable layers to prevent combustion between cable layers and expand the fire.
(e) Wrap the cable with an adiabatic fire-resistant material. When a fire occurs around the cable, the wrapped cable shall be covered with an adiabatic fire-resistant material and fire
Isolation protects against burning. If the cable itself catches fire, due to the lack of oxygen in the dressing body, the fire can be extinguished to avoid the fire spreading to the dressing body.
Improper use of wire and cable preservation methods used in decoration buildings will lead to the reduction of the service life of wire and cable. We should take the correct storage method of wire and cable: in order to improve the user’s awareness of the service life of wire and cable, Xiaobian popularization in the usual storage and laying process, need to pay attention to the following matters:
When using wire and cable laying, should pay attention to:
1.a lot of wires and cables on the outer wall are directly laid on the outer wall. At this time, we should pay attention not to artificial damage on the one hand, but also pay attention to sunscreen, because ultraviolet irradiation will accelerate the aging of cables, so as to shorten the service life
2.the pipe will generally use plastic or metal pipe to protect the cable, at this time we need to pay attention to not damage the pipe, in addition to the thermal conductivity of metal pipe should also pay attention to, so as not to cause disasters. If the pipe location is underground, remind us to also need to control the drying degree of the pipe.
3.there are a lot of LAN cables under the eaves are under the eaves, at this time do not pay attention to not exposure and the environmental temperature is not high.
4.Suspended or overhead cables will be used in the laying process of many communication cables such as RS485 cables. At this time, we generally need to consider the sagging and pressure of cables, and sometimes we need to take bundling to assist.
5.If it is laid directly in the underground cable trench, it is generally good to check the dryness and humidity in the trench regularly.
When storing wires and cables, the following points should be paid attention to:
1.When storing the wire, contact with acid, alkali and mineral oil is strictly prohibited, and these corrosive substances should be isolated and stored;
2.The warehouse where the wires are stored shall not have harmful gases that destroy insulation and corrode metals;
3.as far as possible to avoid the storage of cables in the open air in a bare way, the wire tray is not allowed to lay flat;
4.the wire in custody, should be rolled regularly (3 months in summer, other seasons can be extended as appropriate). When rolling, the side of the disk will be rolled up to the bottom, so as to avoid moisture and rot. When storing, always pay attention to whether the wire head is intact;
5.wire storage period to the product delivery period is limited, generally should not exceed one and a half years, the longest not more than two years;
6.It is strictly prohibited to drop the cable or cable tray from a height during transportation, especially at a low temperature (generally about 5℃ and below). Throwing or dropping the cable will likely lead to insulation and jacket cracking;
7.When hoisting packages, it is forbidden to lift several plates at the same time. In vehicles, ships and other means of transport, the cable tray should be fixed with appropriate methods to prevent collision or tipping over, to prevent mechanical damage to the cable.