All kinds of electrical equipment in our production, scientific research and life are designed according to certain requirements and connected by wires. There are inevitably a large number of electrical joints and wire joints in the lines.
Electrical equipment internal electrical joints, before the factory will generally pass a strict inspection test, before the operation of the test, there are generally no problems.
And the external wiring connector of equipment, usually can only be found through manual inspection and inspection, it is the weak link of the line.
The author has personally seen and heard that a copper and aluminum terminal at the power main switch on the second floor of an office building of a research institute and the special terminal for air conditioning on the third floor of a dormitory building often get hot, black and emit a bad smell of coke for a long time, and is determined to study the heating of the wire connection.
In the process of electricity production and life, the accidents such as wire break caused by overheating of equipment wire joint account for a part of the overall electrical accidents, which bring great threat to the safety supply and power supply, so we must not despise them.
Investigate its reason, it is on duty personnel job responsibility is not strong on the one hand, patrol is not in place, on the other hand it is a lot of workers lack the attention to wiring joint, the skill that sends hot spot to check and measure.
Therefore, it is very important to attach importance to the engineering installation and connection of wire joints and learn how to find, prevent and deal with joint heating.
1. The phenomenon of wire joint heating and its hazards
The phenomenon of wire joint heating is generally manifested as peculiar smell around the wire joint, which can be smelled. This is because the wire joint heating makes its insulation layer produce smell.
Secondly, the wire joints smoke, red, outsourcing insulation layer blackened, angry, or even broken line.
Hot wire connection not only can cause large amounts of electric energy loss, but also seriously affect the normal work of electric equipment, while the working current of the circuit increases, shorten the life electric equipment, heavy then suddenly interrupted the ongoing production, scientific research, medical surgery, and other activities, will lead to fire and electric shock accidents, etc., caused incalculable loss.
2. The cause of wire connection heating
Some electrical installation and construction personnel often do not pay attention to the installation quality when laying the wire: do not install the casing where the insulation casing should be used;
There are no junction boxes where they should be used;
Even in the junction of the wire is not the use of splicing method, but the use of illegal hook connection method.
The contact resistance of this kind of hook connection method is very big, constant heating when electrification, can make nearby plank gradually dry, carbonization, produce combustion finally, cause fire.
Comprehensive analysis, the author may have the following several reasons: equipment installation, repair joint is not tight, when an electric current passes through may be fever, redness, smoking, even in a blaze, bolt, wires and switches, electrical appliances through the lines column connection, due to careless when construction, do not add the gasket on the terminal, gong cap is not tight, it will also make contact resistance increases, such as a hotel, power distribution box is for this reason, the conductor heating melt, bead falls below carton, cause a fire.
Long running joint will loose, some joint during installation quality is good, but due to the reason of heat bilges cold shrink, or long-term by vibration, can make the loose connection, such as long running copper aluminum joints, joint interface is no tin or silver plated not hanging the, due to the contact surface have serious galvanic corrosion generated oxide film, make joint resistance increases, very easy to become hair hot;
At ordinary times, the contact with small current flows through the system, and the current increases suddenly due to the sudden change of the system, resulting in the contact heating;
Short circuit fault occurs in the system, overcurrent causes insufficient capacity or defective contacts, instant fire and so on.
It appears that the contact heating is mainly caused by the increased contact resistance.
According to Joule-Lenz's law:
Q = 1.0032 I2Rt (coke)
If we assume that a contact resistance of 0.0005 ohms flows through a current of 300 amps, its calorimetry for 1 second is:
Q1 = 1.0032 * 3002 * 0.0005 * 1 = 45.14 (coke)
It can be seen that the contact in the outdoor air, the temperature is not high, long-term operation is no problem.
However, if the contact resistance of this contact rises to 0.5 ohm and the current flowing through is 300 amps, its calorimetry for 1 second will be:
Q2 = 1.0032 * 3002 * 0.5 * 1 = 45140 (coke)
That's a thousand times Q1!
And the voltage drop at both ends of the line contact reaches:
U = IR = 300 * 0.5 = 150 (v)
Therefore, the contact is bound to generate serious heat and may produce two situations: one is the contact melting, because there is no tension, may automatically adhesion, welding death, temporarily relieve the degree of heat.
The second is the tension, the contact melting, wire breaking accident or false state.
From the above analysis, it can be seen that contact heating is caused by excessive contact resistance, resulting in a large voltage drop. The load current is still very important for contact heating, and its heating degree is proportional to the size of contact resistance and the square of current.
Therefore, when severe heat is found at the contact point, the power distribution station or service department should be contacted first to reduce the load of the line at the point, and then the treatment plan should be studied.
This is one of the most effective ways to reduce fever in an emergency.
3. Check the methods and techniques of contact heating
The operating temperature of electrical equipment joints shall not be greater than 70℃.
When the temperature of the joint reaches or exceeds 70℃, it must be treated.
There are many ways to check hot spots. The following are several commonly used methods to check hot spots.
3.1 method.
The easiest way to detect the smell of burning near the wire ends is to smell through the nose.
If the burnt smell is confirmed to come from the wire connector, the connector temperature must be above 70℃.
3.2 Candle test method.
Tie the candle to the end of the insulation tie rod to touch the contact, such as slow melting, the temperature is about 55℃;
If it melts and flows quickly, the temperature is about 70℃ above;
Such as fast melting and smoke, the temperature of about 200℃ above.
3.3 Observation method.
(1) If the insulation tape outside the wire connector is yellow and black, the joint temperature must be above 70℃.
(2) Contact observation method in rainy days: the contact is dry and the temperature is above 50℃.
Such as rain immediately vaporized, the temperature of about 100℃ above;
Like a "SHH" sound, large raindrops form a rolling shape, the temperature of about 200℃ above.
Check contact heating in rainy days, easy to find, high efficiency.
(3) Snow melting method: Snow melting at the contact point, the temperature is about 0℃ above;
If the contact is dry, the temperature is about 50℃ above.
(4) Observe the hot gas flow method: When the temperature difference between the heating element and the air reaches about 20℃, the tiny airflow can be seen.
When the temperature of the joint reaches 100℃, the "hot gas flow" is very obvious.
If the joint temperature reaches above 200℃, the "hot gas flow" is very easy to be seen.
If the joint is made up of several contacts, the thermal stream can also tell which contacts are hot.
To clearly see the thermals, you have to look in the background behind them.
The "background" to be chosen is black, gray or other dark, linear, mesh, etc.
Wires and equipment can be used as background.
When checking a contact, the inspector should constantly change the standing position, so that the shape and Angle of the contact are consistent and parallel, and then slowly make the top of the contact close to the "background", leaving only a small gap for shaking.
If there is shaking, it indicates that the contact is hot;
If the shaking is big, the fever is serious.
3.4 Use a thermometer.
Infrared thermometer is an advanced instrument, which is convenient to use and has a good effect of regular contact detection.
4. Prevention and treatment
Heat treatment and prevention of wire connections on the original reliable wiring.
The so-called prevention of wire joint heating, in essence, is to correct construction, standard wiring.
Good joint, also prevent the joint heating, wire joint heating problem will not appear, also do not need to deal with.
Therefore, wire joints should be strong, tight, beautiful shape, no overlap, bending, crack, and concave-convex phenomenon;
The mechanical strength of the joint shall not be less than 80% of the mechanical strength of the wire;
The insulation strength of the joint shall not be lower than that of the conductor.
In the course of installation and maintenance, the number of wire joints should be reduced as far as possible, and excessive wire joints should not be used.
Particular attention should be paid to the joints of mobile and outdoor lines by stipulating that no more than one joint is allowed between two adjacent wire stalks.
When the aluminum wire connection below 10mm2 is pressed, the oxide film on the surface of the aluminum wire, oil dirt and dirt on the inner wall of the connecting pipe should be removed and coated with vaseline zinc paste;
Apply solder to the joints when using various welding methods;
Use the end connection when the heavy pressure.
When clamping method is used for external engineering line connection, the oxide layer on the surface of the wire and the contact surface of the connecting pipe should be cleaned with a fine steel wire brush first, and then coated with a layer of vaseline zinc paste or power compound grease. Each pit should be finished at one time without any break, and the joint and exposed parts should also be coated with a layer of power compound grease.
For hand-cut connection ends, when connecting single strand wires (core wires over 2.5mm2), the total length of the core wire winding should be greater than 25~35mm and the core wire winding should be checked back. Finally, insulation treatment should be carried out, such as installing insulation jacket and protective seal.
FIG. 2 Schematic Diagram of single-strand connection Winding of multi-strand aluminum core wire intersecting with multi-strand core wire, not only ensuring the total length of single-strand connection, but also the left and right ends of the wire intersect first, and then they are wound clockwise to imitate single-strand winding.
Generally, 4mm2 aluminum core wire is connected by screw cap or safety cap.
Aluminum core wire above 6mm2 shall be connected by aluminum casing or gas welding.
The outer diameter, length and dimension deviation of aluminum sleeve tube for extrusion shall conform to the relevant regulations.
The connection of different metal wire should have reliable transition metal fittings, etc.
When there are copper wires, aluminum wires and aluminum alloy wires connected to each other in the circuit, copper and aluminum transition joints (commonly known as wire nose) should be used, and coated with conductive paste to increase the conductivity, especially when multiple aluminum core wires are connected with equipment and electrical appliances, copper and aluminum transition terminals should be used for compression.
If there is no cu-Al transition terminal, aluminum terminal can be temporarily replaced, but the contact with equipment, electrical appliances should be lined with a layer of tin foil, to reduce the electrochemical corrosion effect, and the compression screw must be with a spring pad.
It is not allowed to compress the coil of aluminum core wire itself.
When the multi-strand aluminum core wire is connected with the multi-strand copper core wire, the copper core wire can be rinsed with tin and then pressed with the aluminum sleeve.
A single strand of 2.5mm2 aluminum core wire is connected with 2.5mm2 copper core wire, or can be connected with a screw cap, of course, copper core wire is best rinsed tin.
Single strands of 2.5 was aluminum or copper conductor with soft copper wire connection, can be more strands of copper cord after rinse tin coil on the aluminum conductor, single strands of aluminium conductor from the many strands of soft wire of copper core even thick copper conductor section size large difference of the joint for coarse copper conductor cross section size of large difference of joints, hexagonal hydraulic connection pad should be adopted in the method, and unfavorable use clip-on connection.
For example: 16mm pick?
With a 30 mm pick?
Copper core connection.
Following this, the branching joint can also be hydraulically connected.
New overhead insulated distribution lines with rated voltage up to and including 1KV are not allowed to be wound connected, but should be connected with wire clips and connectors.
When the line is LGJ-400/65 or less of the wire, 100T hydraulic press for the pressing work;
And lgJ-500/45 and less than the wire, with 200T level hydraulic press for compression is appropriate.
The upper and lower dies shall be aligned with the hydraulic connection, the pipe shall be flat, and the pipe orifice shall coincide with the mark.
After the pipe is pressed, it should be filed away and polished.
After the pipe should not be obvious distortion and bending phenomenon, otherwise it should be straightened.
For overhead lines that cannot be clamped or hydraulically connected, the bursting pressure method can be used, but the lines must be highly clean.
Knowing the correct wiring method, it is not difficult to deal with the hot fault of the joint.
In short, after finding the cause of the heat, if it is confirmed that it is a simple wiring problem rather than line overload, etc., the joint can be decisively removed and then rewired according to the correct corresponding wiring method mentioned in this paper.
Conclusion 5.
(1) The fault of heating of electric wire joints is common, so people must pay great attention to it. In the teaching material of electrician, the content of heating of electric wire joints should be recorded into special chapters to introduce it.
(2) The heating of the wire contact is mainly caused by the poor contact of the wire connection, which increases the contact resistance;
(3) The joint heating can be prevented. The key to the treatment and prevention of the joint heating lies in the correct wiring.