Features of rigid mineral insulated cables
1. Main advantages of rigid mineral insulated cable
1) complete fire protection
The rigid mineral insulated cables themselves are not burned at all and will not cause fire.
Even in the case of flame-burning, as long as the flame temperature is lower than the melting point temperature of copper, the flame will be removed without replacement and can continue to be used.
There is no poisonous smoke or gas in the case of a flame barbecue.
2) the overload protection capability is strong
When the line is overloaded, the cable will not be damaged as long as the heat fails to reach the melting point temperature of copper.
Even if the breakdown is instantaneous, the breakdown of the magnesium oxide crystal will not form the carbide, and the cable performance will not change after the overload has been eliminated, and it can continue to be used normally.
3) high working temperature
Due to the insulating layer of magnesium oxide crystal melting point temperature is much higher than the melting point of copper, thus cable highest working temperature is 250 ℃, in the short term can be close to the melting point of copper under the temperature 1083 ℃ to continue running.
4) good anti-corrosion and explosion-proof properties
Because adopts the seamless copper sheath, mineral insulated cable with waterproof, moisture, oil and chemical substance abuse, the performance of copper tube is quite mechanical strength it has good explosion proof performance.
5) long service life
The rigid mineral insulated cable is composed of inorganic materials, so there is no insulation aging, and the service life can be more than three times that of ordinary cable.
6) it is more flexible
Rigid mineral insulated cable can with ordinary cable laying the bridge rack, also can be used in a special seat Ming fu, more flexibility than ordinary cable laying, also saves the investment of cable tray, can reduce overall project cost.
2. Main disadvantages of rigid mineral insulated cables
1) high investment cost
Due to the rigid mineral insulated cable outer sheath is made of seamless copper tube, the overall copper content than ordinary cable, at the same time, rigid mineral insulated cable construction technology of equipment requirements higher than the common cable, mineral insulated cable price rigidity and about 30% higher than a normal cable.
2) the receiver is vulnerable
The insulation layer is made up of mineral magnesia, which can react chemically with moisture in the air and produce a conductive magnesium hydroxide.
Cable head, in the construction of the cable end strip bare conductor, the insulation resistance of the cable in more than 10 M Ω commonly, but if not completed in 1 hour cable head production, insulation resistance can fall below 10 M Ω, can appear even fall below 0.5 M Ω situation, such as do not pay attention to cut the outer, and failed to timely find and seal moistureproof processing, insulating value will soon fall and will gradually drop to zero, this will cause the cable cannot be used.
3) construction is difficult
Rigid mineral insulated cable hardness is higher, compared with common cable weight about 2 times of general cable, not easy to achieve parallel neat when laying the look and feel of the effect, and long lines, connectors, more difficult to find fault point, so the construction is difficult, in and out of the box office and bridge in bending forming difficulties.
4) the construction workload is larger
Rigid mineral insulated cable and all specifications of more than 35 was both for single core cable, such as 1 70 was cable, cable only 3 x 70 + 2 x 35 five conductors in the same outer sheath, and BTTZ to achieve the same specifications must be from 3 root 70 was add 1 35 was single core cable split.
The delivery length of single-core cables is short, such as 240mm2 of cable delivery length of 69 meters, and if the length of laying distance increases the production of a large number of intermediate joints, the construction workload is multiplied.
5) the voltage resistance level is only 750V;
6) the weight is large and weighs about twice as much as the general cable.
7) the production length is limited, and the copper tube drawing technology has limited the single continuous length.
(the length of the line is long, and the magnesium oxide is very easy to react with moisture in the air, which is prone to moisture absorption and difficulties in locating the fault.)
8) the production quality of intermediate and terminal heads needs to be improved.
9) hard and hard steel, it is not easy to achieve parallel and neat observation effect when laying, difficult to bend molding in and out of the wiring box and bridge frame.
10) magnesium oxide powder ACTS as an insulating layer. It can produce thick and thin, and the process can be worn with a slight differential. The rated voltage is up to 450/750v.
3. Problems in construction
1) the cable insulation resistance can not meet the requirements;
2) during the test run, the short-circuit tripping accident (the number of joints, the cable is subjected to the decrease of the bending of the bending extrusion, etc.);
3) as long as the rework, it will cause the laying of the cable lost its original trim and beauty.
(this paper introduces the main construction of rigid mineral insulated cables with BTTZ cable as an example.)