Motor Vehicle Maintenance & Repair Asked on December 17, 2020
Why do almost every automotive procedure instructions start by saying:
Disconnect the cable from the battery negative terminal.
?
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...
Correct answer by Nick C on December 17, 2020
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 in the process of removing the non earth terminal a tool touches both the chassis and non-earth terminal at the same time.
Answered by Peter Green on December 17, 2020
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