Why do almost every automotive procedure instructions start by saying:

Disconnect the cable from the battery negative terminal.


  • Your answer is here – Zaid Aug 18 '15 at 22:04

Two reasons:

  • To reduce the possibility of short-circuiting the battery by removing the connection to 'ground'

  • To make sure everything is switched off and unable to function while you're working on it. You don't want, for example, the electric fan to suddenly kick in while you've got your hands in there...

  • 5
    Also, the reason for not removing the positive terminal (even though the effect is the same in the end) is to avoid touching the grounded chassis with your tool while it's connected to the pole connector. – Allman Aug 19 '15 at 8:43
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    Yeah, electricity should never ever come in contact with your tool ;) – Captain Kenpachi Aug 19 '15 at 9:36
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    Learned this the hard way when I welded one of my Dad's Craftsman wrenches to the strut tower under the hood of my first car, a 1992 Chevy Lumina. Luckily I have come a long way since that 16-year-old was outside striking that wrench with a hammer to try to break it loose from under the hood. Took a lot of careful filing and emery paper to clean up that inadvertent tack weld from the wrench. – Tedwin Feb 6 '17 at 23:29

In general the first step in working on a dangerous machine is to "make it safe". This applies to a car just as much as any piece of industrial machinary.

Most hazards in a car are mitigated by disabling the electrical system. With no electrical power the engine can't start, electric accessories can't operate, the ignition system can't create high voltages and wiring shorts cannot create dangerous arcs.

The safest way to disable the electrical system is to first turn off the ignition so that minimal currents are flowing, then disconnect the "earth" lead from the battery. On most cars the "earth" is the negative lead.

Disconnecting the non-earth lead (positive on most cars) first is risky because of the potential for short circuits if a tool touches chassis and terminal at the same time.

  • Good to see your name somewhere besides NE Stack Exchange! – Tedwin Feb 6 '17 at 23:25

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