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Old 10-06-2012, 10:51 AM   #10
1200gsceej OP
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Thanks everyone! Straightforward responses with information I (mostly) understand. But I have a few comments and follow-up questions:

First, if I read correctly, several of you indicated that the red wire probably overheated, either due to fewer strands carrying load (why? Aren’t they all touching?) and/or that the terminal connection became loose (that did seem to be the case since I was able to pull the wires out w/o loosening the terminal screw). Can someone explain how these cause the resistance to rise? Is it that the effective wire size gets larger (i.e. the diameter gets smaller)? I am not an EE, but I can follow you if you write slowly.

Originally Posted by def View Post
{snip} It is likely over the years that some of the strands in your #12 red wire became isolated and no longer carried current. As such, fewer strands were sharing the load and overheated even though the circuit was not overloaded. I would tin each wire inserted into the fuse block.
Originally Posted by Marki_GSA View Post
as above your connection became loose causing it to heat up. As said you can either tin the ends or use a bootstrap crimp.
Originally Posted by roger 04 rt View Post
+1, this is a compression fitting that came loose over time. When you don't tin the wires, vibration can cause some strands to escape the screw's compression, lessening the compression of the rest. Once the resistance starts to build up the result is what you see. Personally I'm not a fan of that type of connection in applications where there's vibration.
Originally Posted by aschnell View Post
{snip} You probably had the small blue wire loose since it is a smaller gauge wire. The screw terminal touched the larger gauge wire first and you didn't get any compression on the blue wire. That wire connection had high resistance and heated.

Next, I did some research and found references to the issues about tinning mentioned below. So the alternative is to crimp a wire ferrule to the end of the stranded wire? The crimp compression is better? because it is less likely to loosen over time?
Originally Posted by aschnell View Post
Actually tinning the wire isn't good unless you have spring terminals. The solder will cold flow from the force and will loosen up the screw terminal connection.
Originally Posted by ragtoplvr View Post
years ago the US military conducted many tests on making reliable connections. Tinning the wires is bad for 2 reasons. The tin will cold flow as previously stated. The tinning ends at some point, making a transition from stiff to flexible. {snip}

Finally, no one said that my connecting the blue wire with the red wire was wrong, or that it was the (direct) cause of the problem. So it is OK to attach relay-triggering wires to the switched terminals of the Centec? But aren’t you supposed to “fuse the wire”? Meaning that the fuse should fail before the wire does? Or do these draw so little current that it is not important?
Originally Posted by aschnell View Post
Get wire ferrules to keep the wire ends neat instead of solder and be careful putting different gauges in the same spot. If you can stack them vertical they will both get compressed, but side by side won't work.
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