Selection and Design of Automobile Low Voltage Harness

Wire harness is the carrier of transmitting electric signal and current, and wire selection is related to the normal operation and safety of vehicle electrical components. This paper explains the wiring harness selection of automobile low-voltage cable harness in detail, including the selection of wire type and wire diameter, the selection method of fuse model and capacity, the matching check between fuse and wire, the voltage drop check of wire, etc., which provides support for the early development of vehicle Wiring Harness.

Outline

It is the network main body of automotive wiring harness automobile circuit. It can transmit signals and realize functions through reasonable design and arrangement, connection of power supply and various electrical devices such as controllers, actuators and displays. If a certain functional circuit fails, it will lead to the failure of the corresponding electrical appliances. automotive wiring harness is one of the key systems of automobiles.

With the improvement of people's requirements for automobile performance, automotive wiring harness becomes more and more complex, and the selection of conductor is reasonable and reliable, which can effectively save the comprehensive cost of wiring harness, reduce the overall quality of wiring harness and ensure the safe operation of automobile function.

Classification of Wires

According to different standards, low-voltage automobile wires are mainly divided into four series: national standard, Japanese standard, German standard and American standard.

Reference Standards Are as Follows.

1) national cable reference standard: GB/t8139.

2) Reference standards of Japanese standard cables: JASO D608, D609 and D611.

3) Reference standard of German standard cable: ISO 6722.

4) American standard cable reference standards: SAE J1128, SAE J1678, US-CAR-21.

3.1 Calculation of Current of Electrical Equipment

Calculate the rated current of the electrical equipment after collecting the parameters of the electrical equipment of the whole vehicle:

Electrical equipment current = (rated power/nominal voltage) × (actual voltage/nominal voltage)

Rated power: the rated power of electrical equipment;

Nominal voltage: usually refers to the voltage without any load, generally 12V;;

Actual voltage: the voltage actually loaded on the electrical equipment, generally 13.5V or 14v.

3.2 Fuse Selection

When choosing the fuse, firstly, the worker of our company, Kable-X, choose the corresponding type of fuse according to the type of the whole vehicle load. Generally, the electrical load with small starting current or short impact time and resistive load choose fast-melting fuse, while the load with large current impact or long impact time and inductive load choose slow-melting fuse.

When the load current is steady state current: If=In/ load characteristic coefficient× RR

Where If: the ideal working current of the fuse, in: the rated current of the electrical equipment, and the load characteristic coefficient is generally 0.75, RR: the environmental temperature loss coefficient.

For pulse current or surge current, I2t should be calculated according to the load I-T characteristic curve to verify that the service life of fuse meets the requirements of electrical equipment. Different manufacturers of fuses have different calculation methods. For example, in the fuse calculation method of a manufacturer, the service life of the fuse is judged according to the relative I2t, that is, the ratio of current accumulated I2t to fuse fusing I2t. Choose the fuse whose relative I2t is less than and closest to 30%, and the number of times of bearing impact is not less than 105 times.

3.3 Wire Selection

The selection of wires is divided into wire types and wire diameters. Generally, the wire types are selected first and then the wire diameters are selected. The selection of types of wires usually refers to the following contents.

1) As the current carrying capacity of the conductor will decrease with the increase of the ambient temperature, the conductor should be selected according to the working environment of the Wire Harness, and the conductor has eight temperature grades, refer to ISO 6722.

2) The vibration resistance, wear resistance, corrosion resistance and moisture resistance of the wires in the parts with high frequency vibration, harsh working environment, high humidity and easy corrosion shall meet the relevant test requirements.

3) For weak signal circuits or signal circuits susceptible to interference, twisted-pair or shielded wires shall be used for circuits with high performance requirements, such as airbags, ABS and audio-visual systems.

4) Refer to QC /T 414 "Colors of Low Voltage Wire for Automobile" for the selection of conductor color.

Wire diameter selection refers to the following methods:

According to the rated current of electrical equipment, inquire the rated current carrying capacity of the wire under different environmental temperatures, select the principle that the rated current carrying capacity of the wire is greater than the load, and preliminarily select the wire diameter.

3.4 Fuse and Wire Matching Check

Matching principle: Verify whether the selected wire diameter matches the fuse according to the comparison between the fuming time-current curve and the fuse fusing time-current curve, and ensure that the fuming time-current curve of the wire is above the fuse fusing time-current curve without crossing, that is, the fuming time of the wire is longer than the fusing time of the fuse.

If the smoke curve of the wire is above the fuse's fusing curve, it means that the fusing time of the fuse is less than that of the wire, and the two match. If the two curves intersect or the smoke curve of the wire lies below the fuse's blowing curve, it means that under certain working conditions, the fuse's blowing time is longer than the smoke time of the wire, and the wire and fuse are not matched well, so there is a risk of ablation, and the wire diameter needs to be readjusted.

In order to simplify the verification process, short-time fuse current and long-time fuse current are usually used for verification. Calculation formula:

lz=lf×N×[1-0.15%×(T-23℃)]

Where lz: fuse current lf: rated current of fuse.

For fast-melting fuses, the short-time fusing current is 200% of the rated current, n = 200%; Long-term fusing current is 135% of rated current, N=135%. For slow-melting fuses, the short-time fusing current is 600% of the rated current, n = 600%; Long-term fusing current is 200% of rated current, N=200%. Through the above calculation, it is verified whether the fusing current of the fuse is less than the smoke current of the wire, and if not, the wire needs to be re-selected.

3.5 Check the Voltage Drop of Conductor Length

If there are strict requirements for circuit resistance and voltage drop in the electrical equipment parameter table, the voltage drop of the whole circuit should be checked. Formula:

U1=R1× load current× loop length

Where U1: actual voltage drop of conductor loop; R1: Resistance value per unit length of conductor at working environment temperature.

Calculate the loop voltage drop ≤ the allowable loop voltage drop. If it is not met, it is necessary to re-select the conductor.

To sum up, the wire selection method is introduced. Wire selection is very important in the principle design of automobile, and each circuit needs accurate calculation and check, so as to realize the safety and lightweight design of wiring harness.