K/5 Electrical help needed

[Leinie]

My truck wouldn't start after work Friday. Turn key- nothing
I figured starter solenoid. I crawled under and dropped the starter only to find the big + cable had rubbed raw on the starter's heat shield.
Since it was already off, I had the sterter tested ( OK ) and reinstalled with a new cable.
I started it up and backed out of the shop.
About 5 minutes later it died.
I looked and found the - battery cable arcing near the terminal. I pulled on it a bit and it broke off.
I replaced the neg cable and when I did, both new cables started melting off the insulation
What did I do ?
Any help appreciated, I gotta get this thing going today so I don't end up walking to work tommorrow. Thanks