Quote:
Originally Posted by CThunder-blue
It's the reason why people always tell you to disconnect the negative cable from the battery before the positive.
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Actually, it's because the frame/body is used as a ground. Since the negative terminal is already connected to the metal of the vehicle, it's not a big deal if your wrench bridges the negative terminal to the vehicle while you're removing it.
If you disconnect the positive first and happen to touch your wrench against the vehicle, you'll be directly shorting it out. If you disconnect the negative terminal from the vehicle first, then you're not completing any circuit if you happen to bridge the positive terminal to the vehicle while you're removing it.
Disconnecting either cable from its terminal breaks the circuit. Doing the negative first removes all the nearby metal from being a conductor in the circuit though, which then makes it safer to do the positive.