Hello Jeroen,
Thanks for the long and interesting reply. There are a lot of questions and I will try and answer all of them. This is going to be long and might be a bit technical.
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Originally Posted by Jeroen I was simply responding to some members believing that fuses should prevent this from happening. You are in the automotive industry, so you know what I am talking about. |
Yes, fuses can prevent this sort of thing from happening and that is exactly why fuses of so many different ratings can be found in one fuse box of any vehicle.
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Originally Posted by Jeroen A poorly executed modification, say too thin wire, might mean that wire will heat up, long before the fuse pops. If you have a 15 Amp fuse in a 12 V circuit that means you can generate 180Watts of energy. Have you ever tried holding a 40w bulb after it has been switched on for more than half a minute. |
Yes, this is true, however, copper wires have very low resistance and will not heat up very much even with a fairly high current, for example, a 16AGW copper wire will have a resistance of approx 20mOhm/meter and as we know, Power=I^2xR, so even if 100 Amps (short circuit current) is flowing in a 16AWG cable of 10cm length, the total power dissipated = 20W which is not enough to heat up the wire enough to burn through its insulation. A fuse, for example, will not be made of pure copper, it will be made of an alloy that has defined fusing characteristics and will be made of a specific cross section that will be calculated to melt when a certain power is dissipated in the link. There are standards that are available for fuse designs that will specify the fusing characteristics of each fuse. A fuse manufacturer will publish a Time Vs Amps chart for their fuses which will tell us how long will a fuse open for a certain over current, a min and max time is specified clearly for each fuse rating. One point that is usually important is that they all have a requirement that in case of a short circuit condition, the fuse must blow without damaging its socket and its surroundings.
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Originally Posted by Jeroen If somebody made a modification and used half the gauge wire as the original what do you think would happen. The circuit would not draw any more current, but that piece of thin wire would heat up real quick! |
As above, not much would happen if a thinner wire was used, yes, it would be hotter than before but not by much. Certainly not enough to cause a fire.
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Originally Posted by Jeroen The same is true for a wire being chaffed and causing a ground. It might not be enough to cause the fuse to pop. But it will certainly heat up things!
Same agin, if a wire got pinched in between two parts. |
Yes, this is true, but, I don't think that it would become hot enough to catch fire. However, if it does get grounded, then a short can happen causing a very large current to flow. but, if the ground is happening in a way that it causes an intermittent short or just sparking, it will not draw a lot of current continuously.
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Originally Posted by Jeroen I would also like to draw attention to the many, many test that have been carried out on automotive fuses over the years. They don’t pop very precisely do they? In fact, most pop at a higher value than their rating. To some extend this can be explained that it is not unusual to have more than 12V in some circuits.
Very few fuses pop at lower than the stated value. The one thing everybody around the world seems to agree on, is that as a rule of thumb, always buy the most expensive fuses you can get. The cheap ones, are extremely unreliable and will almost always pop at way too high values! |
I would encourage you have a look at the fusing charts by one of the biggest fuse manufacturer Little Fuse.
https://www.littelfuse.com/products/...fuses/ato.aspx
Again, a fuse will not and should not open at the rated current, it is designed to carry the rated current indefinitely. It will only fuse when the rated current is exceeded by a fair margin.
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Originally Posted by Jeroen Now, I don’t say I agree with the conclusion of the dealer. They have shown no evidence at all. Going by many threads and even many more posts on TeamBHP I would say it is fair to say that aftermarket electrical work on cars in India is not necessarily up to Industry standards.
Again, that does not make the explanation any more credible, just statistically more likely. I must admit, if they HD shown evidence I a quite sure some of our members would have responded that the evidence was fabricated.
So, I am not saying anything about the root cause as such. Just a few words and thought about what you can and can not expect from fuses.
This is coming from guy who ended up with two melted wall mounted sockets in his apartment in Delhi and no popped/tripped fuses either!
As you are in the electrical automotive industry I would really like to understand how you guys design and test fuses and what tolerance and repeatabillity you allow for?
Jeroen |
As far as design of the electrical system goes.... every manufacturer will do a design FMEA and a process FMEA to determine where failures can occur and how to mitigate these. If it was possible to have a failure due to a short circuit, then they would ensure that a protection would be available to break the circuit.
Now, as far as burning, a manufacturer would normally use a fire retardant or even a fire resistant material (plastic) at locations where there could be a chance of fire occurring.
As far as tolerances go, every certified and approved fuse manufacturer, will sample test each batch to make sure that the fusing characteristics are as per the data sheet. Maybe there are fly by night manufacturers who skip such quality checks but any reputed automotive manufacturer will not take such risks.
As far as your experience of melting sockets in Delhi, I would think that someone installed copper wires in your fuse socket (pretty common occurrence in India) when a fuse opened up and the right gauge of fuse wire was not available, this is more common than you would think. Unscrupulous electricians do this all the time. Thankfully this is becoming less common now due to the use of MCB's on the fuse boards. There are also a lot of poor quality plugs and sockets over here and this kind of problem is not uncommon. A lot of builders cut corners in these items.
Another thing repair men do often is to just twist together wires and wrap them with insulation tape, rather than use a connector or even solder wires and then insulate them, this can easily cause problems down the line.
My point was that the press release at minimum sounded like they were putting the blame somewhere easy and not going deeper.