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A Brief Discussion on Fire Protection Design of Wiring Harness

Jun 21, 2024

Introduction: In recent years, car fire accidents have occurred frequently. For example, in 2019, news about new energy vehicle fires frequently appeared in major news, which not only caused losses to people and property, but also seriously damaged the brand of automobile manufacturers. According to statistics, about 70% of fire problems are related to wiring harnesses, so it is necessary to introduce the fire protection design of wiring harnesses.

1. Fire mechanism:


Fire requires three conditions, heat source, combustion aid, and air. None of them can be missing. Among them, the air factor is basically difficult to control in the automotive industry, so the focus is on the control of high-temperature heat source and combustion aid. In addition to turbochargers, exhaust manifolds, mufflers, etc., high-temperature heat sources also include abnormal heating caused by short circuits of wires. Combustion aids are mainly oily substances such as diesel, gasoline, engine oil, and lubricating oil. Everyone should correctly 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°, which is enough to melt the copper wire and has reached the ignition point of most substances. Therefore, the difference is only the speed of combustion.


2. Fire prevention design principles


Principles should be kept in mind:


1. Prevention is the priority, avoiding fire from the source;


2. Reduce the speed of fire as much as possible;


3. No personal injury or death should occur.


The following is a brief introduction to the above points:


1. Prevention is the priority, which is to consider eliminating fire hazards from the wiring harness design.


2. The main way to reduce the speed of fire is to use flame retardant materials.


3. Devices related to safe escape can be opened normally at any time, such as car doors, sunroofs, etc., to avoid situations where people cannot escape after a fire.


3. Wiring harness fire protection design


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


1. Wiring harness layout design:


The wiring harness should be away from high-temperature heat sources by more than 150mm, and high-temperature heat sources need to add heat-insulating aluminum tiles.


The wiring harness layout should 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 oil from dripping on the wiring harness after leakage.


2. Wire and insurance matching design


This topic is basically metaphysics, and it is currently more of an accumulation of practical experience of various manufacturers. Here I would like to say a few more words. Why is the maximum temperature resistance of the insulation layer of Japanese standard wire only 120°? Moreover, Japanese manufacturers generally promote lean production, and the wire diameter specifications are relatively small, but there are not many fire accidents in Japanese cars. I personally think there are two main reasons. First, the parts suppliers of Japanese manufacturers are relatively single, basically Japanese-funded enterprises, and the Japanese quality culture is prevalent, so the consistency of products is very high. Second, Japanese manufacturers do a very good job in product verification and have a complete design verification process.


Let's get back to the point. The so-called matching of wires and fuses simply means that the wire smoke characteristic curve must be lower than the fuse fuse characteristic curve. For example, by querying the smoke characteristic of Sumitomo AVSSX125f wire, at a current of 30A, the insulation layer will smoke and catch fire in 100s. It needs to be explained here that Sumitomo's smoke characteristic is measured when a single wire is located in a free ventilation environment. Now there are MINI25A and 20A fuses. Which one should I choose? The answer is 20A fuse, because the longest fuse time of MINI fuse is 600s under rated load of 135%, and the corresponding fuses of 20A and 25A are 27A and 33A, which obviously cannot protect the wires.


4. Conclusion


I think the key point of wire harness fire protection design is the layout of wire harness, followed by the matching design of fuse. One thing to explain is that you should not put your hope on fuse. Large-sized slow-melting fuse can only work when the wire harness is short-circuited in a large area, but the actual situation is often that the wire harness is short-circuited locally and heats up and catches fire.


The above is my personal opinion, and I hope it can bring some inspiration to everyone.


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