I have perused miles of threads pertaining to possible damage to persons, circuits & 'scopes from improper placement of the probe ground clip.
As a novice with a brand new 'scope, I am especially aware of this problem.

I seek a detailed explanation of how one might use a DMM to find a point on the PCB which is at (or near) the same potential as say, the BNC shield on the 'scope, and therefore safe to clip to.

Am I even correct in thinking that I want the probe's ground clip to go to earth ground, or is chassis ground sufficient?

 

One method is to use a 1k resistor from the point being tested and earth GND.
Measure for any voltage across the resistor, If one exists, then there is a potential difference between the point and GND indicating that it is not isolated.

 

I don't know if there is a safe test to determine that any circuit node is a proper GND connection, except perhaps knowing well your test equipment and the circuit under test.

I would use a DMM set to read the lowest range of resistance. You want to see close to zero ohms between any and all points that are considered proper GND points.

 

I have perused miles of threads pertaining to possible damage to persons, circuits & 'scopes from improper placement of the probe ground clip.
As a novice with a brand new 'scope, I am especially aware of this problem.

I seek a detailed explanation of how one might use a DMM to find a point on the PCB which is at (or near) the same potential as say, the BNC shield on the 'scope, and therefore safe to clip to.

Am I even correct in thinking that I want the probe's ground clip to go to earth ground, or is chassis ground sufficient?

What PCB though? The negative rail of any circuit might or might not be connected to the electrical supply's ground. The device under test might even be galvanically isolated if the device uses a transformer or if you power it from an isolation transformer.

 

The test in #2 is to confirm as to whether there is a potential between the circuit GND and Earth GND.
Which where the danger lies.

 

is chassis ground sufficient?

Possibly only if you are looking at low frequency signals.
For proper rendering of higher frequency signals, the ground clip should on a ground (common) near the measuring point.

 

When in doubt, don’t connect the grounding clip.
When you probe around you will soon discover what is not ground.
The reverse is not always true.

 

You are getting good advice here, especially MrChips' suggestion of measuring across a 1K resistor to see whether the current is low enough out ignore. I report that I have been measuring voltages on printed circuit boards as well as hand-wired circuits for more than 50 years and by just being aware of what was hooked to what, have never had a problem.

Clearly circuits like those floating on the AC line need special care, mainly via an isolation transformer and separate grounding, but at least in the circuits I have been working with there have not been any ground loops that damaged anything.

Relax.

 

One method is to use a 1k resistor from the point being tested and earth GND.
Measure for any voltage across the resistor, If one exists, then there is a potential difference between the point and GND indicating that it is not isolated.

Thanks for this -- this is the only reply which actually addressed my question

 

You are getting good advice here, especially MrChips' suggestion of measuring across a 1K resistor to see whether the current is low enough out ignore.

Do you mean post #2?

 

Yep. Max takes the credit, not MrChips.

 

Sorry, my mistake, yes your post #2.

 

Typically circuit ground (0V) is NOT mains Gnd that all the exposed metal parts of a scope are.....but not always < Easy to check with a DMM.
Most devices under test (DUT) have a supply neutral and mains ground inside them and separate however the PE (protective earth) is bounded with neutral back at the switchboard to be near the same potential in the DUT.
Some DUT can have their 0V rails bonded to PE with the probe Gnd lead < again easy to check with a DMM.
Good practice is to refer to the probes ground clip as the reference lead in that any waveform is referenced to mains Gnd/PE just as the probes reference lead is.

When disparities arise between where you need to reference a signal and the probe reference lead you have 2 options on how to best provide a viable waveform reference point.
They are:
Using 2 channels and probes with reference leads removed < this is why they are easily removed from the probe and then engage Maths and Minus while probing the POI and the other probe connected to the desired reference point.
Buy a Differential probe < they can be used with any scope and remove any issues with connecting a probe reference point to an elevated voltage.

Play smart and safe.