DC earth fault detection

[justinjohney]

i have a isolated 24DC power supply with earth fault indication.(ie,is the +ve or -ve line touch the earth)

for that i used 2 relays and connected the earth to coil and other pin to +ve and -ve lines
Quote:

-12V .. RELAY1 ...... RELAY2 .. +12V
-VE__/\/\/\/\/\____/\/\/\/\/\__ +VE
................ ... |
.................. EARTH


the problem is that the voltage 24 is divided into half and get a 12V across the earth and power lines.

later i used comparator which compare the earth and power lines then also i get the same problem.
then i use optocoupler with a microcontroller (atmega168 which i have now). i gave a isolated powersupplay to uC and connect the earth to gnd of uC.
then i pull up one pin(D,4) and using two optocoupler ,i swithched the +VE and -VE to that pin alternately(15 sec)
my logic is that when +VE or _VE line touch the earth ,the pin (D,4 )become pulldown and i make two outputs,the system works. but the problem is
that when i swiching the +ve lined to pin (D,4) i get a voltage around (12-24)V between -ve and earth.
likewise ,when i switch -ve next time i get same between +ve and earth.
is any body has to any idea to solve this problem.
atleast reduse the voltage below 3V while checking.
how can it possible.