Why is my fuse holder melting? - Answered!

[KiLaHuRtZ]

Why is my fuse holder melting?

It's simple really,...Ohm's Law!

Let's take a look at an example. John Doe has a 1/0 AWG cable run with a 300A ANL fuse installed. He is only drawing 100 amps on a 12 volt system, yet his fuse holder melts. We take a closer look at the fuse holder and discover that there is a resistance of 0.012 ohms between each wire connecting to the fuse holder. Not much, so John ignores this. However, if we use Ohm's Law, we discover that a resistance of 0.012 ohms yields a voltage drop of 1.2 volts; V=IR or 100 amps times 0.012 ohms equals 1.2 volts. Using Ohm's Law once again, we find that John's fuse holder is generating 120 watts of power lost in heat; W=VI or 1.2 volts times 100 amps equals 120 watts. Let's put this in to perspective, take a 120 watt light bulb, we all should know how hot that gets, well John's fuse holder is roughly that same temperature! Therefore, the plastic begins to melt! When this happens, we can only assume the connections are now getting weaker, thus yielding more resistance and generating more heat. And so on, and so on until something has to give; hopefully the fuse holder does and John's car does not burn down. One might ask, why didn't the fuse blow? Well, that is a valid question and here is the answer. Fuses are rated for current not wattage. So back to John's 300A fuse, doing some more math, it would take in excess of around 3600 watts (12 volts times 300 amps) to cause John's fuse to blow. Even then, this all depends on the rated open time of that particular fuse (I^2*t rating of the fuse). On average, it takes about 10 seconds for a fuse to blow at double its rated current or 0.1 seconds to blow at ten times its rated current! That's at least 7200 and 36000 watts, respectively. In a dead short, the current climbs to infinity in relatively no time at all. So the open time is a moot point and the fuse blows almost instantaneously. I might also mention that this scenario is taking into account that John is drawing 100 amps continuously. In music, that is almost never the case, even with short bass burps. Check the specs on your fuses, because most fuses can handle double their current draw for a brief time. John could band-aid this by using a smaller fuse, however when the bass hits a long note, he might be blowing fuses all the time. The moral of this story is to check your fuse holders post-installation and pre-powerup for any resistance (be sure to adjust for any test lead resistance on your DMM). If there is, you need to pinpoint the problem and remove it, or replace the fuse holder. An easy way to do this is to probe your battery positive to battery negative and see what voltage you have. Next, probe the battery negative to various points along your fuse holder. If you get anything lower than the previous voltage, you have resistance!

It should be noted that this resistance issue is not as important on high voltage systems. For example, on a 120 volt system with the same total power of 1200 watts generated, John would only draw 10 amps. With the same resistance of 0.012 ohms, it would only yield a 0.12 volt drop which would generate 1.2 watts of power lost in heat. This is why higher voltage systems are better. Plus you can get away with smaller cable and save money. Although, high voltage DC systems come with their own risks as well.

I hope this explanation helps some people out and if I made any errors (I did this at 2AM), please let me know so that I can edit with the correct information to benefit all!