Originally Posted by Thermo

Good point about the colors. That does give a hint as to how hot it got at some point in its life. Also note how there is a very thin line that is colder/silvery. This is telling me that you had a very small contact area where the metals were pushed together, not simply very close. A piece of 22 gauge wire will attempt to flow 50+ amps. It may very well do it plus more. But, it comes at a cost. The cost is looking like a toaster heating element (ie, the current is so much, the wire gets hot). Same thing with a connection that has very little surface area making good contact. That fraction of an inch is like putting a piece of 22 gauge wire in a length battery cable. yeah, most of the wire is battery cable that can easily handle 100's of amps. But that small piece is working its butt off and as a result is getting very hot and causing a voltage drop.

As for the "50 A" stamp, sure, the metal is designed to only handle a max of 50 amps, but it will handle a lot more, but again, it comes at a cost (heat). Also of note, you don't want to put a 10 amp fuse on a load that runs at 9.9 amps. It may surge, or change slightly over time. You normally give it a margin to work in. The general rule is to put the fuse roughly 10% above its max. But, there are times where a higher rated fuse may be used. An example would be say HID head lights. They are 35 watts each (so, in a 12V system, they pull roughly 3 amps). So, you would think a 5 amp fuse would be more than enough to handle the bulb. NOPE. The starting surge (only lasts for a second or two) is up near 20 amps per bulb. IE, the starting of the HID pulls almost 250 watts (about 8 times the rated power). In a lot of cases, you will find each HID bulb with a 25 or 30 amp fuse. Now imagine your dash fan. It pulls roughly 25 amps when running on high speed. Now, imagine you are jumping in your car on a hot day and your A/C is immediately kicking into high gear. That fan is pulling a ton of power (in reality, it will be up over 100 amps, yet on a 40 amp fuse). This is where a fuse will take an over current for a finite amount of time. The higher above the fuse rating, the shorter the time period the fuse will take it. Now imagine the same situation where you are starting your car but now you have the dash fan, seat fan, engine coolant fans, etc all starting and trying to pull that initial surge. Yep, that bar is going to have to grunt and let that current pass and some engineer figured out that you have to have a mega fuse of 60 amps or 80 amps in various locations, even though under constant conditions, it is only going to a max of 50 amps. The newer XJ's have a 400 amp maxi fuse. My god, what pulls that much power? The car does when starting. It may be very short lived. Things may not get hot in that time. But the current happens. In your case, you had long term "near max" current pulls (you do live in Arizona, I am sure that dash fan gets a work out), now you introduce an area of less contact, the heat generated will now increase. You make some heat, now you start oxidizing the metal, that introduces more resistance, making even more heat.

Getting a little off topic here, but it will help explain my point, I work in the power production field. We routinely check how good electrical connections are using FLIR cameras. So, now imagine looking at a 500KV line and seeing the camera showing a hot spot. That is not something that you could measure really any other way. The voltage is just too high to do that. Or, we look at buss bars (metal bars 8 inches wide by an inch or so thick). What is the resistance of those? (hint, pretty much 0 ohms). BUt, you start passing 1,000's of amps through them, where they get bolted together will get warm. This is where you have to establish "what is warm but tolerable" and "what is too warm". This is your metal bar, but on a smaller scale. But ,the ability to measure a "high resistance connection" is just the same. You are talking on the scale of a tenth of an ohm if not hundredths of an ohm. Your back yard mechanics just don't have the tools to measure stuff like this. It would almost be interesting to have you get one of these FLIR cameras and take a picture of your 2 fuse boxes with the same load applied, just to see the heat difference. I know it would be a lot of work, but if you have the time.......... This is where I learn some of my things that I try and explain to others. Most can't imagine working around something pulling 1,000's of amps. Me, I touch pumps pulling 400 amps on a 4KV circuit every day of the year and don't give it a second thought. But, sometimes you have to think about how current can affect things and make them do very interesting things.