When to ground scope leads

I'm 45 and decided to take the plunge into learning the scope and have found this subscription the best I have found as far as learning...everything. I picked up a mid-range (cost-wise) 4-channel, the Autel MP408. Mainly because the software is already on my Autel Maxisys and makes for a more compact package when I do mobile work.

With that said, I have one question. (Please don't shoot me...I'm new to the scope):

When do I ground my test leads (and would there be an example of when I would not)? Is there a rule of thumb? ALWAYS ground them or only during certain tests?

Many thanks for helping this old guy learn a few new tricks.

SRBerg wrote: When do I ground my test leads

The simple answer is "Always". The black lead is a "reference". Usually, you will want to reference "ground" and there is no better ground than the NEGATIVE battery post. But, there are times when you may want to reference a voltage supply. Such as when doing a voltage drop test, from one end of a circuit, to the other end. (When referencing a power supply, be mindful of a scope that shares a ground with all channels.) As with a normal volt meter, the labscope measures the DIFFERENCE in potential (voltage) between the "reference" lead and the "other" lead(s).

(and would there be an example of when I would not)? Is there a rule of thumb? ALWAYS ground them or only during certain tests?

Without something to reference, there can be no "difference" in potential (voltage). The scope output would be meaningless.

Many thanks...that helps tremendously.

Cylinder block or battery terminal that way you get the best possible ground.
If testing multiple sensor best to use a single ground an on most scopes akk grounds are common and you avoid danger of burning your scope with a ground loop which could put a dead short or starter current through the ground side of your scope
If looking at a signal from a sensor (for example a MAP using single channel grounding to the sensor ground will give a cleaner signal.