[narrobb]

To figure out whether DC ground is isolated from the machine body, bring out a multimeter and measure resistance between the two.

*If they are isolated*, then that's for safety; in the case some hot wire comes loose and touches the shell, it will provide a return path for the current, and if you have working GFCI, it will blow, else after a short while the fuse will blow.
Depending on the resistance in the path between the possible short, and the DC ground, this MAY cause a DC offset voltage that could be big enough to hurt the DC electronics in the system. There's really no way of knowing without careful study of the internals of the system.
In this case, as long as you don't have a stray high-voltage wire loose (or blown capacitor bending out, or fraying wire, or somesuch,) then it "should" be fine, and once the protection blows, it "should" be fine again. However, you will also want to measure the voltage potential between pin 4 and the chassis when the machine is on and idle, because sometimes electrical designs aren't as "natural" as you'd expect, for whatever reason. If there's a significant potential between pin 4 and chassis ground in idle-on state, it would not necessarily be safe to tie the two together at all.

*If they are electrically connected* with the multimeter, then there's zero additional risk in using it for this signal path.

Back when I did the Passive Probe circuit I've checked the connection; I recall that the resistance was infinite, and the potential was less than 50mv.

[jwatte]

So, actual isolation, but at least not built dumb :-)

If you're willing to take the risk on some possible fraying wire or exploding component blowing out your DC power side (which honestly is not big) then this sounds like a good way to go.

[narrobb]

So, actual isolation, but at least not built dumb :-)

If you're willing to take the risk on some possible fraying wire or exploding component blowing out your DC power side (which honestly is not big) then this sounds like a good way to go.

Dont think there is any risk. Or at least extremely low. For any major issue, the AC side protection would have to be offline also.

Been using the custom Passive probe for over a year now. All good.

On the other hand, it’s is much better using it as Active type similar to this. Now I plan to convert the Poor’s man 3D probe to Active type this way no need to keep changing the Tormach settings