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Discussion on the fire protection design of wiring harness

release time:2021-07-08publisher:Howdy

   Principle of Fire

   Fire requires three conditions: heat source, combustion-supporting material, and air. The three are indispensable. Among them, air is basically difficult to control in the car industry. Therefore, the focus is on the control of the two elements of high-temperature heat source and combustion-supporting material. In addition to turbochargers, exhaust manifolds, vents, etc., high-temperature heat sources also include abnormal heating caused by wire short circuits. Combustion-supporting substances are mainly diesel, gasoline, engine oil, lubricating oil and other grease substances. We must correctly understand the concept of combustion-supporting substances. Generally, the surface temperature of the exhaust pipe can reach over 800°, and the short circuit of the wire can produce a high temperature of more than 1100° which is enough to melt. The copper wire has reached the ignition point of most materials, so the difference is only the burning speed.

   Fire protection planning principles

  Principles should be kept in mind:

   1. Prevention is the first priority, avoiding fire at the source;

  2. Reduce the fire speed as much as possible;

   3. No personal injury or death occurred.

   Here is a brief introduction to the above points:

   1. Prevention is the first to consider eliminating fire hazards from wiring harness planning.

  2. The main reason for reducing the ignition speed is the use of flame-retardant materials.

  3. Equipment 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.

   Harness fire prevention planning

   The wiring harness fire prevention plan mainly consists of two points, one is the wiring harness placement plan, and the other is the wiring diameter and the proper matching plan.

  1. Harness placement planning:

  The wiring harness should avoid the high temperature heat source by more than 150mm, and the high temperature heat source needs to add heat insulation aluminum tile.

  The wiring harness should be placed away from sharp edges and moving parts to avoid wear and short circuit of the wiring harness.

  The wire harness should avoid passing under the oil pipe to avoid dripping on the wire harness after oil leakage.

  2. Wire and secure matching planning

   This subject is basically metaphysics, and now it is more about the accumulation of practical experience of various manufacturers. Let me say a few more words here. Why is the insulation layer of the Japanese marking line with a temperature resistance of 120°, and Japanese manufacturers generally promote lean production, the diameter of the wire is relatively small, but there are not many fire incidents in Japanese cars. I personally think that there are two main points. First, the parts suppliers of Japanese manufacturers are relatively single, and they are basically Japanese-funded enterprises. In addition, Japan's quality culture is prevalent, and the consistency of products is very high. The second is that Japanese manufacturers have done very well in product verification and have a complete planning verification process.

To get back to the subject, the so-called wire and safe matching simply means that the wire's smoking characteristic curve is lower than the safe fusing characteristic curve. For example, by querying the smoking characteristics of Sumitomo AVSSX125f wire, at a current of 30A, the insulating layer will emit smoke and fire for 100s. Here needs to focus on explaining that Sumitomo's smoking characteristics are measured under a single wire in a freely ventilated environment. There are MINI25A and 20A safe, which one should I choose after all? The answer is that 20A is safe. The reason is that the longest fusing time of MINI is 600s when the rated load is 135%. The 20A and 25A correspond to 27A and 33A. Obviously, the latter can no longer protect the wire.

  I personally think that the focus of wiring harness fire protection planning is on wiring harness placement, followed by sound matching planning. One thing to note is that do not place your expectations on security. Large standard slow melting and security can only be effective when the wiring harness is short-circuited in a large area. The actual situation is often that the wiring harness is partially short-circuited and heats up.