Select Wiring Harness Twisted Pair of Parameters

The selection of twisted pair cables for wiring harnesses needs to be considered comprehensively in combination with application scenarios, performance requirements, and cost factors. The specific analysis can be conducted from the following dimensions:
1. Application scenarios and functional requirements
‌Anti-electromagnetic interference scenarios‌
Twisted pair cables are preferred, and the twisted characteristics of the two wires are used to offset common-mode signal interference. They are suitable for scenarios with high signal stability requirements such as automotive wheel speed sensors and CAN buses.
‌
High temperature or high voltage environment‌
Twisted pair cables made of high temperature resistant materials (such as aluminum foil layer + protective layer) should be selected. The temperature range should cover -40 to 200°C, and the withstand voltage should be above 600V. They are suitable for wiring harness assemblies of new energy vehicles.

‌Complex wiring environment‌
If you need to save space or reduce the difficulty of wiring, you can use twisted pair cables with injection molding fixation process. The static area is fixed by cable ties, staples, и т.н., and the dynamic area is enhanced by injection molding process.‌

2. Structural design and performance parameters
‌Twisting density and pitch‌
The smaller the twisting pitch (such as 38.1~140mm), the stronger the anti-interference ability; the difference in pitch between adjacent pairs should be controlled within 12.7mm to reduce crosstalk‌.
‌Conductor material and insulation layer‌
The conductor is preferably a high-purity copper wire (No. 22~26) to ensure conductivity and flexibility‌26.
The insulation layer must take into account high temperature resistance and wear resistance, such as polyethylene (PE) or polyvinyl chloride (PVC) materials‌.
‌Shielding requirements‌
‌Unshielded twisted pair (UTP): low cost, suitable for ordinary electromagnetic environments (such as home or office networks)‌.
‌Shielded twisted pair (STP): add aluminum foil or braided copper mesh shielding layer, suitable for high-interference scenarios such as industrial control and vehicle-mounted knock sensors‌.

3. Transmission performance and standard specifications
‌Bandwidth and transmission rate‌
For general scenarios, you can choose Category 5e (1000Mbps) or Category 6 (Cat6a, higher bandwidth) twisted pair cables.
Automotive wiring harnesses must meet specific signal transmission standards (such as CAN bus rate requirements).
‌Industry standard adaptation‌
In the automotive field, German standard (FLRY), Japanese standard (AVSS) or American standard wires are preferred. German standard wires have become mainstream due to their wide applicability.
Verify wire performance with reference to national standards (GB/T8139) and Japanese standards (JASO_D608) 3.

4. Cost and maintenance considerations
‌Cost optimization‌
Unshielded twisted pair cables are cheaper than shielded cables and are preferred on the premise of meeting anti-interference requirements.
‌Installation and maintenance‌
Check the outer sheath identification of the twisted pair cables (manufacturer, implementation standard, cable length, и т.н.) to ensure that they meet application requirements.
High-quality cables with uniform twisting density and clear color distinction can reduce installation errors and failure rates.

V. Recommended selection process
1. Identify environmental conditions (temperature, voltage, interference source) → 2. Determine transmission performance requirements (bandwidth, signal type) → 3. Select conductor material and insulation layer type → 4. Evaluate shielding requirements and costs → 5. Verify compliance with industry standards → 6. Test sample reliability.
Through the above steps, you can systematically screen out cost-effective wiring harness twisted pair solutions that are suitable for specific scenarios.

There are many systems that use twisted pairs in automobiles, such as electronic injection systems, audio and video entertainment systems, airbag systems, CAN networks, и т.н. Twisted pairs are divided into shielded twisted pairs and unshielded twisted pairs. The shielded twisted pair cable has a metal shielding layer between the twisted pair cable and the outer insulating envelope. The shielding layer can reduce radiation, prevent information leakage, and also prevent external electromagnetic interference. Using shielded twisted pair cable has a higher transmission rate than similar unshielded twisted pair cable.
Shielded twisted pair wires, wire harnesses are generally used directly with finished shielded wires. For unshielded twisted pairs, manufacturers with processing capabilities generally use a twisting machine for twisting. During the processing or use of twisted wires, two important parameters that need special attention are the twisting distance and the untwisting distance.

Twisted pair length
The twist length of a twisted pair refers to the distance between two adjacent wave crests or troughs on the same conductor (it can also be seen as the spacing between two twists in the same direction). See Figure 1, lay length = S1 = S2 = S3.
Figure 1, twist length S of twisted pair
The lay length directly affects the signal transmission capability. Different lay lengths have different anti-interference capabilities for signals of different wavelengths. However, except for the CAN bus, relevant international and domestic standards do not clearly stipulate the twist length of twisted pairs. In GB/T 36048 Passenger Car CAN Bus Physical Layer Technical Requirements, the CAN wire twist size range is 25±5mm (33-50 twists/meter). It is the same as the CAN lay length specified in SAE J2284 250kbps high-speed CAN for vehicles.
Общо взето, each car company has its own twisting distance setting standards, or follows the requirements of each subsystem for the twisting distance of twisted wires. For example, Foton Motor uses a winch length of 15-20mm; some European OEMs recommend selecting the winch length according to the following standards:
1. CAN bus 20±2mm
2. Signal cable, audio cable 25±3mm
3. Drive line 40±4mm
Generally speaking, the smaller the twist pitch, the better the anti-interference ability of the magnetic field. However, it is necessary to consider the diameter of the wire and the bendable range of the outer sheath material, and determine the most appropriate twisting distance based on the transmission distance and signal wavelength. When multiple twisted pairs are laid together, it is best to use twisted pairs with different lay lengths for different signal lines to reduce interference caused by mutual inductance. The damage to the wire insulation caused by too tight a twist length can be seen in the figure below:
Figure 2, Wire deformation or cracking caused by too tight lay length
In addition, the twist length of twisted pairs should be kept even. The twisting pitch error of a twisted pair will directly affect its anti-interference level, and the randomness of the twisting pitch error will cause uncertainty in the prediction of twisted pair crosstalk. Twisted-pair production equipment parameters The angular speed of the rotating shaft is a key factor affecting the size of the inductive coupling of twisted-pair lines. It must be considered during the production process of twisted pair cables to ensure the anti-interference ability of twisted pair cables.

Untwisting distance of twisted pair
The untwisting distance refers to the size of the untwisted part of the twisted pair end conductors that need to be split when installed into the sheath. See Figure 3.
Figure 3, Untwisting distance L of twisted pair
The untwisting distance is not specified in international standards. The domestic industry standard QC/T29106-2014 “Technical Conditions for Automotive Wire Harnesses” stipulates that the twisting distance should not be greater than 80mm. See Figure 4. The American standard SAE 1939 stipulates that the twisted pair of CAN lines should not exceed 50mm in untwisted size. Therefore, the domestic industry standard regulations are not applicable to CAN lines because they are larger in size. Currently, various car companies or wiring harness manufacturers limit the untwisting distance of high-speed CAN lines to 50mm or 40mm to ensure the stability of the CAN signal. For example, Delphi’s CAN bus requires an untwisting distance of less than 40mm.
Figure 4, Untwisting distance specified in QC/T 29106
In addition, during the wire harness processing process, in order to prevent the twisted wires from loosening and causing a larger untwisting distance, the untwisted areas of the twisted wires should be covered with glue. The American standard SAE 1939 stipulates that in order to maintain the twisted state of the conductors, heat shrink tubing needs to be installed at the untwisted area. Domestic industry standard QC/T 29106 stipulates the use of tape encapsulation.

Summarize
As a signal transmission carrier, twisted pair cables need to ensure the accuracy and stability of signal transmission, and they should have good anti-interference capabilities. The twist pitch size, twist pitch uniformity and untwisting distance of the twisted wire have an important impact on its anti-interference ability, so it needs to be paid attention to during the design and processing process.