When is the probe ground clip required to be connected when taking oscilloscope measurements?

Hi,

It's generally required at all times, but there is a problem though with most o'scopes, and that is that the o'scope's ground connection is also connected to AC ground through the AC power plug.

The best way to circumvent the possible shorting of the circuit you're testing, you should use an AC isolation transformer on the o'scope. This removes the AC ground from the scope but allows you to retain the signal and AC ground on the device being tested.

Hope this helps,

Cubby