Hi
ok, so looks like you are trying to detect the -28 volts with the arduino.
have a look at an opto isolator,
such as these
http://www.skpang.co.uk/catalog/optoisolator-breakout-p-665.html
there are plenty of types,
the led is driven off your 28 volts, via a resistor , such that the current through the led is around 2 mA.
the output of the opto isolator is powered off the same as your arduino '5v' and 'gnd'.
the logic output of the opto isolator goes to an arduino input pin.
no common earth to worry about, cheap and very reliable.