Project Shop is keen on safety.
Both for the occupants of the car, and for the car itself! We like to fit a battery isolator to the terminal of a car battery, so that the owner can disconnect the electrics easily. Most often this is for convenience, when leaving the car in storage for periods of more than a few days.
But also, this can help if there is an electrical problem too. It’s easy to snap a master-switch over to isolate electrics. It’s hard to find spanners and undo a terminal if you are in an emergency situation.
For many cars it is simple to fit an isolator directly to the earth lead from the battery. But with cars such as MGBs, VW Beetles and Minis, it is often more useful to have a readily accessible master switch located elsewhere on the car than hidden by the rear axle, under the rear seat, or in the boot as these cars have.