NO NO NO NO
On a negative ground system, the negative battery terminal cable should always be disconnected first and reconnected last.
If your socket/wrench/pliers/whatever you're using to loosen the clamping nut at the positive terminal contacts any metal part of the chassis it will cause a short directly through the tool. Automotive batteries have a couple hundred amps and will melt a wrench.
Use a voltmeter and measure from the positive battery terminal to any metal in the area, that voltage - with all the amps - will be flowing through whatever tool your using.
Conversely, use the meter and measure from the negative terminal to any metal and you will get 0 volts, which includes 0 amps.
Disconnecting the negative battery terminal breaks the circuit back to the battery and there will no voltage or current flow from the positive terminal to the chassis. You can verify this with a voltmeter after disconnecting the negative battery cable.
Now, if you're working on a rare, very rare, positive ground system the above will be reversed. But with 99.9999% of the vehicles on the road being negative ground the above is true.
I learned this in High School Auto Shop in the '70s. Never forgot it.
Bob
