About the fire protection design of wiring harness

In recent years, car fire accidents have frequently occurred. For example, in 2019, electric car fire news frequently appeared in major news, which not only caused personal and property damage, but also caused serious damage to car manufacturers' brands. According to statistics, about 70% of the fire problems are related to the wiring harness, so it is necessary to introduce the fire protection design of the lower wiring harness.

A. Fire mechanism

Fire requires three conditions, heat source, combustion aid, and air. The three are indispensable. The air element is basically difficult to control in the automotive industry. Therefore, it is important to consider the control of the two elements of high temperature heat source and combustion aid. In addition to the turbocharger, exhaust manifold, muffler, etc., the high-temperature heat source also includes abnormal heating caused by short-circuited wires. Combustion aids are mainly oils such as diesel, gasoline, engine oil, lubricants, etc. You must understand the concept of combustion aids. Generally, the surface temperature of the exhaust pipe can reach more than 800 °, and the short circuit of the wire can generate a high temperature of more than 1100 °. Copper wire has reached the ignition point of most substances, so the difference is only the speed of burning.


B. The fire protection design principles

Principles to keep in mind:

1. Prevention first, avoid fire from the source;

2. Reduce the ignition speed as much as possible;

3. No personal injuries can occur.


The following briefly introduces the above points:

1. Prevention is the main consideration, which is to eliminate the fire hazard from the design of the wiring harness.

2. Decreasing the ignition speed is mainly the application of flame retardant materials.

3. Safety-related escape devices can be opened normally at any time, such as car doors, sunroofs, etc., to avoid the situation where personnel cannot escape after a fire.


C. The wiring harness fireproof design

The wiring harness fire protection design mainly has two points, one is the wiring harness layout design, and the other is the matching design of the wire diameter and the insurance.

1.Wiring harness layout design

The wiring harness should avoid high-temperature heat sources more than 150mm, and at the same time, high-temperature heat sources need to increase insulation aluminum tiles.

The wiring harness should be arranged to avoid sharp edges and moving parts to prevent the wiring harness from being worn and short-circuited.

The wiring harness should avoid passing under the oil pipe, to prevent dripping on the wiring harness after oil leakage.



2.Matched wire and fuse design

This subject is basically metaphysics, and more is the accumulation of practical experience of various manufacturers. Here are two more words, why the maximum temperature resistance of the Japanese standard line insulation is only 120 °, and Japanese manufacturers generally implement lean production, the diameter of the wire is small, but there are not many fire accidents on Japanese cars. Personally I think there are two main points. First, the Japanese parts manufacturers are relatively single, basically Japanese-funded enterprises, coupled with the popularity of Japanese quality culture, the consistency of products is very high. The second is that Japanese manufacturers have done very good product verification and have a complete design verification process.



Closer to home, the so-called wire and fuse matching simply means that the wire smoke characteristic curve is lower than the fuse fuse characteristic curve. For example, by querying the Sumitomo AVSSX125f wire smoke characteristics, at 30A current, the insulation layer will emit smoke and fire for 100s. It is important to explain here that the smoking characteristics of Sumitomo are measured under a single wire in a freely ventilated environment. Now that there is MINI25A and 20A insurance, which one should I choose? The answer is 20A insurance. The reason is that the maximum fuse time of the MINI insurance at 135% of the rated load is 600s. The 20A insurance and the 25A insurance correspond to 27A and 33A. Obviously, the latter can no longer protect the wires.


D. The conclusion

Personally, I believe that the key to the fire protection design of wiring harnesses is the wiring harness layout, followed by the matching design of insurance. One thing to note is that don't pin your hopes on the insurance. The large-scale slow-melt insurance can only work if the wiring harness has a large area short circuit, but the actual situation is often a local short circuit heating and catching fire.