Safe Use of Cable: How to Prevent Wires and Cables from Catching Fire Due to Overload of Wres

Wire and cable in the process of operation, due to the existence of resistance will be heated.The resistance of the wire is generally very small, and its heating power can be expressed by the formula Q =I^2R.Q =I^2R indicates that: for a section of wire in use (R has been basically constant), the greater the current through the wire, the greater the heating power;If the current constant, then the wire heating power is constant.The heat released during operation will be absorbed by the wire itself, thus causing the temperature of the wire to rise.

Although the wire is constantly absorbing the heat released by the current in the process of operation, its temperature will not rise unlimitlessly.Because while the wire is absorbing heat, it’s also releasing heat into the outside world, and it turns out that the temperature of the wire increases gradually when it’s energised until it stays at a certain point.At this constant point, the heat absorption and release power of the wire are the same, and the wire is in thermal equilibrium.There is a limit to the ability of wires to withstand higher temperatures, and operating above a certain maximum temperature may be hazardous.This maximum temperature also corresponds to a certain maximum current, above which the wire is overloaded.Overloading of the wire directly causes the temperature of the wire itself and the objects nearby to rise.Rising temperatures are the most direct cause of such fires.

Overload causes the insulation layer between the two wires to be damaged, causing a short circuit, burning down the equipment and causing a fire.The double wire is separated by the insulation layer between them. Overload makes the insulation layer soften and destroy, which leads to the direct contact between the two wires, causing short circuit and burning equipment.At the same time, the high temperature generated by the large current at the moment of short circuit causes the line to catch fire and fuse, and the molten beads generated fall to the combustible material to cause the fire.Overload temperature rise can also directly ignite nearby combustibles.The heat transfer of overload wire raises the temperature of combustibles nearby, and it is possible for combustibles nearby with low ignition point to ignite and cause fire.In the storage of flammable goods in the warehouse and the use of easy, combustible decoration of the building, this risk is particularly prominent.

 

The overload also puts the connections in the line under conditions of overheating, which speeds up the oxidation process.Oxidation makes the connection point produce a thin layer of oxide film, which is not easy to conduct electricity. The oxide film increases the resistance between the contact points, resulting in ignition and other phenomena, causing fire.So, how to prevent the overload of wires and cables caused by fire?

In the process of line design, we should accurately check and ratify the capacity of the site, fully consider the possibility of new capacity in the future, and choose the right type of wire.Large capacity, should choose a thicker wire.Circuit design and reasonable selection are the key steps to prevent overload.If the design selection is not correct, it will leave a congenital hidden danger that is difficult to rectify.Some small projects, places not serious design and selection.Arbitrary selection and laying of lines, this is very dangerous.New electrical appliances, electrical equipment should be fully considered the bearing capacity of the original line.If the original line does not meet the requirements, it should be re-designed and transformed.

The line should be laid by qualified electricians according to the relevant specifications.The laying conditions of the circuit directly affect the heat dissipation of the conductor.Generally speaking, the line laying should not pass through easy and combustible materials and stacking, which will lead to poor heat dissipation of the wire and heat accumulation, resulting in the possibility of igniting combustible items around, and increasing the risk of fire caused by overload;The lines laid in the ceiling of the decoration of public entertainment places should be protected by steel pipes to separate the ceiling from the line. In the case of overload and short circuit, even if there are molten beads, they will not fall off to avoid fire.

Strengthen electricity management, avoid disorderly wiring, random wiring, careful use of mobile socket.Disorderly wiring, random wiring, the use of mobile socket is actually in a certain line to add electrical equipment, increase the current and may cause overload.The jack of the mobile socket is obviously more than the fixed socket on the wall. If too much electrical equipment is used on the mobile socket, the original line must be difficult to bear.Separate lines should be set for high-power equipment and electrical appliances, and mobile sockets should not be used as wiring sources.

Accelerate the renewal and transformation of old lines to eliminate fire hazards.Old enterprises, old residential areas and other units, due to the use of a long time, many lines have been aging, more than the service life.Even if the carrying capacity of some lines is not large, the aging lines are difficult to bear such carrying capacity, and also have the risk of overload.Especially in the old residential areas, the lines have long been aging, but with the improvement of people’s living standards and the increase of household appliances, the electricity consumption is still rising year by year, which is really worse.For old lines, we should timely supervise and coordinate, and promote their rectification as soon as possible, so as to eliminate fire hazards and ensure safety.

Wire and cable common sense questions and answers

1. What are the requirements for cable piping?

Answer: (1) The distance from the top of the pipe to the ground is 0.2m in the workshop, 0.5m under the sidewalk, and 0.7m in general areas;

(2) Pipe pits should be installed at the change direction and branch, and pits should also be added when the length exceeds 30mm;

(3) The pit depth is not less than 0.8m, and the manhole diameter is not less than 0.7mm;

(4) The drain pipe should have a drainage slope of 0.5% to 1% inclined to the pit.

2. What are the requirements for the resistance of the cable conductor connection point?

Answer: The resistance of the connection point is required to be small and stable. The ratio of the resistance of the connection point to the conductor of the same length and the same cross-section should not be greater than 1 for the newly installed terminal head and intermediate head; This ratio should not be greater than 1.2.

3. What requirements should the design of cable joints and intermediate head meet?

Answer: The requirements that should be met are:

(1) High compressive strength and good conductor connection;

(2) High mechanical strength, low medium loss;

(3) Simple structure and strong sealing.

4. What is a cable fault? There are several common types?

Answer: Cable failure refers to the failure of the cable’s insulation breakdown during the preventive test or during the operation, which forces the aerial bundled cable 33-209 standard  to power out due to insulation breakdown, wire burnout, etc. Common faults include ground faults, short-circuit faults, disconnection faults, flashover faults and mixed faults.

5. How to deal with the single-phase ground fault of the cable line?

Answer: Generally speaking, the damage to the cable conductor is only partial. If it is a mechanical damage and the soil near the fault point is relatively dry, local repairs can generally be carried out and a fake connector is added, namely Without sawing the cable core, only the fault point is insulated and sealed.

6. What tests and inspections should be carried out on power cables before laying?

Answer: Before laying, check whether the type, specification and length of the cable meet the requirements and whether there is external force damage. Low-voltage cables use a 1000V megohmmeter to remotely measure the insulation resistance, and the resistance is generally not less than 10MΩ, and high-voltage cables are measured with a 2500V megger. The resistance is generally not less than 400MΩ.

7. What should be paid attention to when laying cables in the main workshop?

Answer: When laying cables in the main factory building, generally pay attention to:

(1) All control cables leading to the centralized control room should be laid overhead;

(2) 6KV cables should be laid in tunnels or pipes, and the high groundwater level can also be laid overhead or pipes;

(3) For 380V cables, tunnels, trenches or pipes should be used when the two ends of the cable are at zero meters. When one end of the equipment is on the top and the other is on the bottom, it can be partially overhead laid. When the local water level is high, it should be overhead.

8. Where are the inner and outer shielding layers of power cables? What material is used? What’s the effect?

Answer: In order to make the insulation layer and the cable conductor have better contact and eliminate the increase in the electric field strength of the conductor surface caused by the unevenness of the conductor surface, the conductor surface is generally covered with an inner shielding layer of metalized paper or semiconductor paper tape. In order to make the insulating layer and the metal sheath have better contact, generally the outer surface of the insulating layer is covered with an outer shielding layer. The material of the outer screen layer is the same as that of the inner screen layer, and sometimes copper tape or braided copper ribbon is tied outside.

9. Briefly describe the composition and performance of epoxy resin compound.

Answer: The epoxy resin compound is composed of epoxy resin added with hardener, filler, toughening agent and diluent. Has the following properties:

(1) Have sufficient mechanical strength;

(2) Excellent electrical performance;

(3) Stable electrical performance;

(4) It has sufficient adhesion to non-ferrous metals;

(5) Good corrosion resistance;

(6) When used outdoors, it is resistant to rain, light, and humidity.