two voltages, one kill switch

This is for a robot project. I've got two sets of two 12v batteries for power. Two are 12v and two are 24v systems. I have a 12v relay and a 24v relay I want to wire a kill switch from both to cut power to main components. I was told to use a common ground but this proved disasterous as I discovered by accident. Long story short, I need at least one new battery now. I had grounds from both systems connecting across the two relays. Could someone help with an easy to follow guide as to how to wire these up correctly in keeping the two systems seperate and safe? Thank you kindly.

Why did the common ground not work?  It should.  Are you sure you hooked everything up correctly?

I would use a DPDT or DPST switch - one switch, two isolated sets of contacts to keep the different voltages seperate.

Honestly I don't quite follow what you are doing with the relays.  Possibly a diagram that is a bit more detailed would help.

Any DPDT switch that has contacts rated to hanle the current you are working with should do - I would connect them as close to each power supply as possible and cut the power at each set of batteries.

Gotcha, I thought the switch was the actual power supply.

What you have in the drawing should work fine.

If you go with a DPDT switch you would just wire the 24v line though the switch just like you did the 12v line.  The DPDT switch is basically two isolated sets of switch contacts operated by the same switch lever.  You should be able to wire the 24vdc in to the common of the switch and use the NO contact (or output) to feed the 24dc to the distribution board.  You should be able to do the exact same thing with the 12vdc line, just like you have shown above.

If the relays are only there for isolation then you can replace them with the switch - the switch will provide the isolation.

The only thing I can think of as to why the grounds would spark if you touched them is if the 12vdc and the 24vdc outputs were tied together somewhere.  As long as you only tie one side of the power supply together (+ or -) you shouldn't have any problems.  However, when you tie them both together with no load between them you'll have issues.

Its very common to have a common ground system when working with multiple voltages - for example computers have a 5vdc and a 12vdc voltage going to all the optical drives and these voltages share a common ground.