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Old 10-06-2012, 01:03 PM   #11
roger 04 rt
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Joined: Nov 2011
Location: Massachusetts
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Quote:
Originally Posted by 1200gsceej View Post
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.






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?




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?

As you might imagine, these type of connections use compression to make the connection. With no compression, the resistance is high (maybe even open circuit). As you add compression, the resistance quickly drops. With enough compression the resistance is effectively zero. The problem arises when there is only enough compression to connect with a few ohms of resistance. Then the current can flow but the heat comes from the equation: current times current times resistance. So it is the partly-loose situation which causes heating.

If it were me I wouldn't put two wires into one connection unless the wires were soldered together. Inevitably one wire shifts due to vibration, allowing the compression of the joint to lessen. Then you know what happens.

I would connect the wire to a fused connection. The problem you're trying to prevent is not the normal current flow, but the current demand that can occur if something shorts out at the other end of the wire.

Although these compression connections aren't my favorite since they can loosen, even with the problems of tinning, I would do it, and dress/route the wire so there was no strain put on the tinned to untinned point.
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