​Composition of High-Voltage Cable Shielding Layer

The benefits of using shielding layers for high-voltage wire harnesses and high-voltage connectors.
The shielding layer of high-voltage cables is divided into shielding wire braiding layer and aluminum foil layer. The conventional shielding layer structures are:
① Shield wire braid only
② Shielding wire braid (close to the inner insulation layer) + aluminum foil layer (close to the outer insulation layer)
③ Three states: aluminum foil layer (close to the inner insulation layer) + shielding wire braiding layer (close to the outer insulation layer).

Shielded high-voltage connectors and wiring harnesses

Different forms of shielding layers of high-voltage cables

Of course, some high-voltage conductors use braided mesh, aluminum tubes, or a combination of the two to directly cover the outer layer of the cable to provide EMC protection for the conductors.
Figure – Different forms of shielding layer composition

(1) Shielding wire braid
The essence of the shielding wire braid is a wire with a metal braid shell, which functions as a low-frequency shield. It is mainly woven from 0.2mm or 0.15mm tinned copper wire, and its weaving density must reach more than 90%.
Shielding wire diameter, braiding angle, number of wires per spindle and braiding machine tension are several important parameters for braiding shielding wire.
Conventional shielding wire is available in two specifications: 0.2mm and 0.15mm. The thicker the wire diameter, the better the shielding effect.
OEMs and high-voltage wire manufacturers generally define the weaving angle of the shielding layer within the range of 50°~60°, and the processing efficiency is highest in this range.
The number of shielding wires per spindle is determined by each conductor manufacturer. The greater the number of shielding wires per spindle, the larger the braiding pitch, and the relative tension will be correspondingly smaller.

(2) Aluminum foil layer
Aluminum foil generally uses aluminum-plastic composite tape, which is mainly composed of aluminum, high-temperature coking glue and PET material with a temperature resistance of 80°C. Its function is high frequency shielding.
The coating force of the aluminum foil wrapped around the inner insulation layer of the high-voltage conductor is preset by the manufacturing machine, and its specific size varies according to the conductor supplier.
The aluminum foil layer of most high-voltage conductors is located outside the braided layer, and a small number of high-voltage conductors have the aluminum foil layer located inside the braided layer. Either way, the aluminum foil layer should be in contact with and conductive to the braided layer.
The shielding layer needs to be grounded to guide external interference signals into the earth, thereby preventing interference signals from entering the inner core.
It should be noted that the shielding layer is not allowed to be grounded at multiple points because there will be potential differences at different grounding points. If the shielding layer is grounded at multiple points, a current will be formed in the shielding layer, a current will be induced on the wire, and interference will be induced on the signal line. Not only does it fail to shield, it actually causes interference.
When high-voltage conductors leave the factory, both the aluminum foil and the braided shielding layer are in an unbroken state (that is, they are completely wrapped in the insulation layer within the conductor). The work of breaking up the shielding layer (including cutting off the aluminum foil and expanding the shielding wire) is generally completed by the high-voltage wire harness assembly supplier before the wires and connectors are connected and installed.

(3) Magnetic ring
The connection between the high-voltage wire harness and the high-voltage connector will receive serious EMC interference, so the interface of each high-voltage connector needs to be shielded. For example, the front and rear motor interfaces are shielded snap rings that are crimped to the electrical box guide rails, and the controller and battery box connectors use structural parts with shielding functions.
It is a common and efficient practice to generally add magnetic rings to high-voltage wire harnesses and high-voltage equipment.
The magnetic ring is a ring-shaped magnetic conductor. The magnetic ring is a commonly used anti-interference component in electronic circuits and has a good inhibitory effect on high-frequency noise.

Magnetic ring material
Depending on the frequency at which interference is to be suppressed, ferrite materials with different magnetic permeabilities are selected. The higher the magnetic permeability of the ferrite material, the greater the impedance at low frequencies, and the smaller the impedance of high-aluminum materials.

Magnetic ring performance
The effect of the magnetic ring is related to the impedance of the circuit. The lower the impedance of the circuit, the better the filtering effect of the magnetic ring. The greater the impedance of the ferrite material, the better the filtering effect. When capacitive filter connectors are installed at both ends of the wire, the impedance is very low and the effect of the magnetic ring is more obvious.
The installation position of the magnetic ring is generally as close as possible to the source of interference. For the high-voltage wire harness of the high-voltage system, the magnetic ring should be as close as possible to the inlet and outlet of the high-voltage wires of the motor and controller.
The greater the difference between the inner and outer diameters of the magnetic ring and the longer the axial direction, the greater the impedance. The inner diameter of the magnetic ring must be tightly wrapped around the wire. Therefore, in order to obtain a large attenuation, try to use a larger magnetic ring on the premise that the inner diameter of the magnetic ring is tightly wrapped around the wire.
Increasing the number of magnetic rings on the cable can increase the low-frequency impedance, but due to the increase in parasitic capacitance, the high-frequency impedance will decrease accordingly.

The above is about the classification and composition of high-voltage conductors, as well as the organization and sharing of the composition of the insulation layer and shielding layer of high-voltage conductors. In the next article, we will continue to share the deshielding design of high-voltage wires and the most important parts-the wire core. We look forward to your attention and welcome your communication.