Isolated grounds are common at the output of an On-Board Power Supply
or an Optical Coupler, sometimes a Hot Plug Circuit. The problem created for ICT, is that
all of the devices connected to this isolated ground will fail their vector based test
because the pin system is open to the isolated ground.
There are a couple ways to overcome this. You could simply short the
main ground to the isolated ground with a wire. But this will eliminate the detection of
this short if it were in fact a fault on your UUT.
A relay can be added to the fixture that will connect the main ground
to the isolated ground. This may be required if the device on the isolated ground is a
complex part that has lots of test vectors, or something you need to ISP. But in most
cases, only small simple logic is out on the isolated ground. In this case a relay is just
over-kill, and another moving part in your fixture that could fail down the road.
My favorite solution to this problem is a pair of (front to back)
shottly diodes that connect the main ground to the isolated ground. You can still detect a
short across these grounds, and your isolated ground will never wander more than a
200-300MV from your main ground. All of your simple vector based tests will work just
fine. I have also done this with a 15-ohm resistor, that works too.