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I was about to pull the car out of the garage for a little work, so I turned on the cut off switch form the battery and I noticed a tiny little sizzle sound, so I turned it off. Turned it back on but didn't hear it this time. When I turned the key, it turned over just for a second and quit. I popped the hood and immediately saw a huge ball of small coming from the battery. I ran to turn the battery switch off and it was hot as a firecracker. I grabbed the **** and it came off in my hand, burning the crap out of my thumb and finger. I ran for the pliers for the spring clamp removal. By the time I got it off, the negative side of the cable had burnt into from the switch. LUCKILY the only damage was to the two cables. The cause? The positive cable had gotten up against the back of the exhaust manifold. I don't know how long it had been that way, but guessing not long. I checked out all the other wiring, and it was all good. Installed a old pair of regular cables and it was good as new.Thank God for cut off switches, because it could have shorted out at any time without it. Whew!
You are very fortunate that you managed to get your positive lead disconnected. In the late 70's, I had a '67 Camaro that I only had for a short period of time, and before I had a chance to really make any of the multitude of repairs it was in need of, I was taking a short trip and went across some railroad tracks. The minor bouncing caused the positive battery cable to come in contact with one of the primary tubes on the passenger side header which ultimately melted the insulation and became a direct short. The car's electrical system went dead, and I hopped out and opened the hood to find that the positive cable was red hot. I pulled the cable off the header with some difficulty and gave myself some minor burns in the process. I was not fortunate to have had any tools with me at the time. The battery was not only in the tray backwards, but the cable was not secured or correctly routed (obviously). The situation would have been a non issue if I had taken the time to check things out a bit more carefully. No one I knew of at the time was using battery disconnects but I'm not surprised that the type of disconnect one uses would potentially wend its way into this discussion. It is possible that with the kind of current flowing through the circuit on a complete short like you or I experienced, a knife blade switch could weld itself shut making it difficult to open. There was definitely metal transferred to the header on my car as the cable tried to weld itself to the header. I currently have a disconnect on my Corvette but not for the intention of eliminating the kind of experience you or I had. Although I know there are fans of both of the more common types, it's hard to say whether one type of disconnect would have survived any better than the other. It all comes down to preference. I would however, recommend checking the battery out. Have a load test done. You can show sufficient voltage potential and still have a weak battery. It may or may not need replacing.
It could have been so much worse. Quick reaction and clear thinking I'm sure kept the damage to a minimum. Also enlightening information about what type of battery cut off switch to choose.
This post makes me think about adding a high amp fuse at the bat pos terminal, similar to but heavier than the 200 amp fuse used for power lift gates on trucks. Figure out max amp load for cables and get a fuse a little lower rated.
Chip
St. Jude Donor '14-'15-'16-'17-'18-'19-'20-'21-'22-'23-'24
Originally Posted by Frankie the Fink
A knife blade disconnect would have required one swift single action....
I'm just sayin'
BUT, glad you dodged a bullet...
that, to my thinking, the knife blade disconnect would have been faster, but, to the other point someone else made, even the knife blade might 'weld' itself shut. In any case, if this happens to me, I'll try, in the heat of the moment, to remember to use a handkerchief or rag to open the knife blade, so as to avoid burning my fingers. Gotta think, though, that I'll probably be panicking, and just grab it, and get burned, like the Thread Originator did.
When I replaced all the original wiring on my '65 I installed this Hella battery cutoff through the firewall just below the heater box on the passenger side. The red key is accessible (by the driver too) inside the car as it's just above where a passenger's right foot would be. It fully isolates the ground and is handy when parking or working on the car. I have one of those "Remove before flight" keychain lanyards on the red key, so it's super easy to pull out, yet out of sight. In fact, with the red key removed, you'd never see the small socket poking through the black carpet. A little extra security from theft, but a LOT of extra security in case of electrical concerns.
that, to my thinking, the knife blade disconnect would have been faster, but, to the other point someone else made, even the knife blade might 'weld' itself shut. In any case, if this happens to me, I'll try, in the heat of the moment, to remember to use a handkerchief or rag to open the knife blade, so as to avoid burning my fingers. Gotta think, though, that I'll probably be panicking, and just grab it, and get burned, like the Thread Originator did.
This is a good reason to have a knife blade cutoff switch on the negative battery lead.
Yep. By disabling the negative side of the battery, your circuit will be open and a negative cable is a lot less likely to burn you or cause issues. I like the Hella switch AND the high capacity fuse link. Good ideas, will probably never need them, but IF you do, will save the car. And we all know or have experienced lost cars due to fires.
I'm a fan of this disconnect. I have mine mounted between the door and passenger seat, in my 62. I have welding cable running from the battery to the switch and them to ground. You can't see the switch when the key is removed and it's very hard to see when the key in not removed. It's very easy to reach across and ffli the switch, so I shut it off when ever I get out of the car. My wife shuts it off if she is riding. The knife switch may be great but if something happens when you are driving, you have to get out and open the hood to disconnect, where the switch inside the car can be switched off in a second. My first choice for safety switches.]
When I replaced all the original wiring on my '65 I installed this Hella battery cutoff through the firewall just below the heater box on the passenger side. The red key is accessible (by the driver too) inside the car as it's just above where a passenger's right foot would be. It fully isolates the ground and is handy when parking or working on the car. I have one of those "Remove before flight" keychain lanyards on the red key, so it's super easy to pull out, yet out of sight. In fact, with the red key removed, you'd never see the small socket poking through the black carpet. A little extra security from theft, but a LOT of extra security in case of electrical concerns.[/QUOTE]
Any battery disconnect should be put at the negative post if possible. It would not matter what type of disconnect was installed in the op's car with the issue he had. The cable insulation was melted through by the heat from the exhaust manifold and as soon as enough insulation was gone so that the conductor in the cable came in contact with the manifold, a direct short was established. At that point, there was an immediate attempt by the battery to completely discharge. The initial current flow would be somewhere well North of the batteries CCA, most likely 1200 to 1700+ amps at a minimum. Enough to make the cable become red hot and further melt the insulation as was demonstrated by the op's picture of his cable. Once a discharge condition of this nature occurs, there is no way to save the cable. Also, remember that current flows from negative to positive and that is a reason to put the disconnect at the negative terminal at the battery. You can locate it away from the battery, but you're going to want to make sure that the negative cable is secured so that there is no chance of it coming in contact with something that has the capability of abrading the insulation. A fuse would be a good idea and offer protection from a direct short. The fuse capacity would probably need to be just above CCA capacity and would protect against a direct short, but would not protect against a direct short somewhere else in the car where the conductors have a lower current capacity. A fuse, although a good idea, still wouldn't save the cable, however.
Yes - I mounted the switch to where I can reach it even with a seatbelt still fastened.
I know it has thwarted uninvited users and it hasn't once been a problem since I installed it nearly 20 years ago. I winter the car in a great location except no plug for a trickle charger but with the switch off my battery has zero parasitic drain. Even after as much as 6 months storage, no problem starting.
BTW, Its not the 12Vdc that generates heat, its the short circuit current/high amperes that generates the heat!
The high current also is flowing through the return/ground cable and will be just as hot as the high side cable (as long as they are the same size)
Also, concerning the Hella battery cutoff switch, what can the switch handle during a short circuit condition? There's a big diff between continuous carry and break/arcing current ratings!
I have the Hella style on my car trailer. No key, no power.
Speaking from my own experience with my '67 GTO shorting out many years ago, I would be pulling the fuse panel to confirm no visible heat damage on the back side. And I would lay my hands on as much of the wiring harness as possible to confirm no "crunchy" sections. Especially around the ignition switch and distributor.
06-08-2019, 04:11 PM
Close Call Today
that, to my thinking, the knife blade disconnect would have been faster, but, to the other point someone else made, even the knife blade might 'weld' itself shut. In any case, if this happens to me, I'll try, in the heat of the moment, to remember to use a handkerchief or rag to open the knife blade, so as to avoid burning my fingers. Gotta think, though, that I'll probably be panicking, and just grab it, and get burned, like the Thread Originator did. 
