The role and advantages of steel tape armor in armored cables

The purpose of armored cables is to improve the mechanical strength of the cable sheath and increase the ability of the cable to resist rat bites, insects, and light mechanical rolling.

However, iron strips with strong magnetic permeability are usually used for steel tape armoring, so there is an implicit condition: the sum of the currents passing through all cable cores in the armored layer at any moment should be zero. If the sum of the currents is not zero, a magnetic field will be generated around the cable. The changing magnetic field will lead to the eddy current phenomenon in the magnetically conductive armored steel tape layer, that is, a circular current is formed in the steel tape, and the steel tape heats up, which in turn melts the cable insulation layer, and finally leads to the breakdown of the cable core to the steel tape, that is, the cable to the steel tape. Ground breakdown (armored steel tape is usually grounded).

The strength of the magnetic field is related to the magnitude of the current that generates the magnetic field. The larger the current, the stronger the magnetic field. Therefore, the cable breakdown caused by ordinary steel tape armor has nothing to do with the length of the cable, but the time required for breakdown is related to the magnitude of the current. If the current is small, the breakdown time will be long, and if the current is large, the breakdown time will be short.

On the other hand, eddy currents dissipate energy, causing severe voltage drops in the cables and the load may not start. The magnitude of the voltage drop is related to the magnitude of the current passing through the cable and the length of the cable. The greater the current, the longer the cable, and the greater the voltage drop.

When the single-core cable is armored, it has been determined from the structure that the sum of the current passing through the cable core in the armored layer is not zero, so the single-core cable cannot be armored with magnetic materials such as ordinary steel tape.

In the use of cables, the following situations should also be paid attention to:

1.When cables and wires are laid through iron pipes, the phase and neutral cables of the same line should be passed through the same iron pipe instead of passing through different iron pipes.

2.In the lines connected by armored cables, all phase lines and neutral lines should be in the same cable.

3.When the cables are installed in parallel, please use steel tape armored cables. Although the production process of the cables is completely consistent, the best cables cannot guarantee that the corresponding cores are strictly consistent when used in parallel, and it is impossible to ensure that they are connected to the same phase line during installation. Or the connection points of the cable cores on the neutral line are strictly consistent, as well as the aging of the line after long-term use, loose joints, etc., cannot guarantee that the current sum of the phase line and the neutral line passing through the same cable is zero, let alone a multi-core The insulating cores of the armored cable are connected in parallel on the same phase, which is actually equivalent to a single-core armored cable at this time.

In some cases, single-core armored cables must be used. At this time, it can be armored with copper tape, stainless steel tape or aluminum tape (the aluminum alloy tape for armor contains iron and should not be used), or other non-magnetic materials for armor. At this time, just like using non-ferrous utensils to boil water on the induction cooker, the water will not heat up. Armoring with these materials results in increased manufacturing costs for the cable. Among them, the armoring of copper tape and stainless steel tape will lead to a large increase in the cost of the cable, while the increase of aluminum tape will be relatively small.