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Here is an example of a scenario that describe the question using a car electrical wiring harnest as the complete and complex system unit overall. The tail lights, brake lights and reverse lights are each seperately wired using different fuse ratings individually. To create a "kill switch" that can turn off and disable all three lights simultaneously, i wonder if the following approach would work. First, i would identify and consolidate negative wires of each circuit and terminating it to a common grounding point after connecting a simple SPST switch between the connection. This switch can allow the ground connection to be disconnected off all three circuits and reconnected it back on simultaneously. If the wiring connection are done correctly and soundly, will this method (disconnecting and reconnecting ground connection of multiple circuit from a single point using only a switch) create any potential harm to the components, bettery or the overall system such as the ECU? If it does, i would like to know how and if there is a better way to tackle this.

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  • \$\begingroup\$ Your idea works, but the return path in a car is usually the frame, so you might find difficult to find and isolate the three return wires. You can use three separate relays on the 12V line, and connect their coils in parallel. \$\endgroup\$ – Vladimir Cravero Oct 7 '16 at 8:26
  • \$\begingroup\$ Make sure your insurance is still valid after your modifications (brake lights especially are considered safety functions). \$\endgroup\$ – Dmitry Grigoryev Oct 7 '16 at 8:41
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    \$\begingroup\$ Are you planning a getaway car? \$\endgroup\$ – Spehro Pefhany Oct 7 '16 at 8:42
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There are a couple of problems with this approach:

  1. You will find that the main part of lamp holders are metal and connect directly to the chassis and are shared between the tail, brake, reversing and directional indicator lights. Isolating the negative will be a problem.
  2. You will find that you create backfeeds. e.g., If you open the ground connection on all lamps and press the brake pedal you will feed out through the brake lamps to the now floating ground plate of the tail lights, back through the indicator lamps and light up the dashboard indicators for example. You will also backfeed the flasher unit (but this is unlikely to cause trouble).

It would be much better to add series switches / relay contacts on each circuit.

enter image description here

Figure 1. Image from SaabNet. Ignore the auxiliary ground wire but note the common negative around the edge of the assembly.

If you have ever seen a vehicle whose tail lights vary in brightness when the indicators are turned on then you are witnessing a similar phenomena due to poor earth connection.

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You might instead try a triple pull, triple throw switch on the hot side of the wires. All three lines stay electrically isolated, but are controlled by the same switch. This means the switch can come after the fuse on all three lines. A simple google search for 'tptt switch' pulls up enough usable options. Just be careful that you don't short any two together, it'll wreak havoc on your system if they do get shorted (blown fuses, reverse lights coming on when you turn on the tail lights, etc.)

Example switch I would go for:
http://www.electro-tech-online.com/attachments/3pdt-jpg.8375/
I'm a big fan of toggle switches, especially when you attach a missile switch cover.

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