A battery cut-off switch is very handy for routine electrical maintenance or for emergency disconnect. Gotta love it. I recommend a bulkhead switch, not one on the battery post, avoiding requirement to lift the battery cover to access the switch.

I recommend installing the switch in the ground cable. When switched off it will then isolate the battery from the frame. When wrenching on cable terminals on the battery you can touch the tools on the frame without causing short from hot terminal to ground. Otherwise always disconnect the ground cable first. If the switch was in the hot cable, touching wrench from battery hot terminal to frame will give VERY undesirable results (melted wrench, burned hands, blown up battery).

I have the Lucas SSB103 switch. Permanently affixed round knob, no removable key. I cut the larger hole, screwed the flange to front of panel. The knob sticks out enough to get a good grip on it. I suppose you could drill the smaller hole and screw the flange to back of panel, but might have a tough time working the set screw to attach the knob. See picture. You might find prices from $46-USD to $186-USD (some of them are outrageous). Repro's are likely to be cheaper than originals, although the white lettering and arrow may have different appearance.



I installed mine when I still had 2x6v batteries in the car. I put the switch low on the rear plywood bulkhead behind the drivers seat (LHD), accessible sitting in the car (with a bit of a stretch), and easily accessible from outside, no need to lift battery cover for access to the switch. Disconnect battery ground cable from frame and connect it to the switch. Install a standard bare engine grounding strap from switch to frame.

When changing to 1x12v battery, install battery in right side carrier using original hot cable, and buy/install a (cheap) longer insulated ground cable, from battery to switch, leaving switch in same location. If you already have 1x12v battery on passenger side (LHD), then you might install the switch behind the passenger seat.

Many race sanctioning groups require the switch to be in the hot cable and on the outside of the car for easy outside access (generally not relevant for a street car). For supplementary wiring instructions see 145-770.pdf

At 05:00 PM 1/20/2021, Tony Hanks in Macquarie, NSW, Australia wrote:
"I have solved this question – with the help of some MG owners on Facebook.
When using the Lucas SSB103 switch, some owners prefer to drill a smaller hole and to mount the switch assembly behind the plywood bulkhead. The challenge then is to remove the knob for installation, as it appears to be permanently attached. The solution to this is to remove the knob and shaft simply by releasing the C-clip at the back of the switch (see photos).