Ground impedance monitor circuit design
Not only is the example circuit insensitive, it is mildly unsafe.  For example if the supply side ground were to open the other ground (possibly connected to the equipment) is put at an AC "hot" potential.  In this situation, even with the 47k resistors, someone touching the left side ground could receive a "mild" tingle, (a light AC shock).

A better idea would be to use a very small low voltage (safety agency rated) transformer across the Hot1 and HOT2 points, then take the secondary winding and use this to create a low voltage DC supply for the opto-isolator LED.  Now use the two AC ground points to short out the LED.  That way if either of the ground points ever becomes open you get a light pulse from the LED.  Select the series resistor going to the LED low enough so that an extra 100 ohms across the LED (in a defective GND to GND connection) allows a voltage greater than the LED's turn on voltage.  So with this setup any signal coming from the opto-isolator indicates a bad ground.

(Ideally the one ground side should be a separate known good ground - such as an Earth ground).