Testing the TK Tar Baby

[picowatt]

Beyond what circuit cellar wizards do, Lecroy, a company that designs and makes the equipment, do they recommend ditching the ground termination during any time of use, and if so, what do they have to say about that?
This company must understand these measurement protocols and also understand the necessity for isolation in given cases such as this. These are the people to ask. What they say in the matter should be golden. Otherwise, what could we say about the quality of the equipment?

Mags n The Moonies

Magluvin,

Not sure what you mean by "circuit cellar wizards", go talk to pro equip installers.  And by "pro" I mean "real pro", not some basement studio or night club.  In multi-suite interconnected installs, system grounding is an art in itself.       

No manufacturer will likely recommend bypassing the AC ground due to code and liability issues, but, this can be done by an end user and still meet code and safety requirements if done properly.  It has little to do with measurement accuracy or equipment operation (except where floating is required to break a loop for an accurate measurement), it is a safety related issue.

Also, as I said, I would not mess with the LeCroy to begin with.  I would float the FG as described.  Even doing so can still produce issues with regard to stray capacitance at the frequency of oscillation, and this must be considered as well for high accuracy measurements.

The only other alternative is the use of an isolation transformer on the AC supply to the FG so that in the event of an AC to chassis fault, the AC supply is not ground referenced and therefore reduces any shock hazard, or just use an isolated power supply instead of the FG.


PW 

[TinselKoala]

@PW: This is separate from the line cord ground issue (sort of...) but consider the following scenario.

A six-year old child, having just learned how to solder, is in your water-heater closet using your DSO to make complete measurements on her Little Miss Mosfet Oven baking set, which is powered by a 72-volt lead acid battery stack. She has several scope probes scattered around the system, most of them grounded to the same point, which is connected to the battery negative pole. Now she decides to take a measurement "across the load", a quite logical thing to do. So she unclips a probe and hooks it to one side of the load... the transistor drain side... and she takes that probe's ground lead and hooks it to the other side of the load.

Of course she is wearing safety glasses and gloves like any six-year old should when playing in the laboratory...er... water closet. But her batteries catch fire anyway, the probe shield leads melt down and the probe BNC connectors weld themselves to the scope chassis connectors.



You are walking through the desert and you see a tortoise......

[picowatt]

@PW: This is separate from the line cord ground issue (sort of...) but consider the following scenario.

A six-year old child, having just learned how to solder, is in your water-heater closet using your DSO to make complete measurements on her Little Miss Mosfet Oven baking set, which is powered by a 72-volt lead acid battery stack. She has several scope probes scattered around the system, most of them grounded to the same point, which is connected to the battery negative pole. Now she decides to take a measurement "across the load", a quite logical thing to do. So she unclips a probe and hooks it to one side of the load... the transistor drain side... and she takes that probe's ground lead and hooks it to the other side of the load.

Of course she is wearing safety glasses and gloves like any six-year old should when playing in the laboratory...er... water closet. But her batteries catch fire anyway, the probe shield leads melt down and the probe BNC connectors weld themselves to the scope chassis connectors.



You are walking through the desert and you see a tortoise......

Well yeah... that would be a problem!

PW

[Magluvin]

Lol Pico, I should have put a  after the wizards comment. It was meant to be a complement of sorts. Like Tk. Look at all his vids. Amazing stuff. But most of it seems to be done in a cellar. And he is a Wizard at this stuff. So dont take it badly.

I just figured if Lecroy had ideas, they would be good, and this part of the arguments could be long over.

Lets say we had 3 or 4 pieces of equipment going on this project, and a couple of them would be as you describe that you wouldnt disable the gnd.  Maybe Lecroy has encountered this and has some recommendations. Certainly they have encountered more complex circuits than this and have been asked this before.

I would lean also to ungrounding the gen before the scope if need be. Do they make isolated scope probes? Or like a self powered(9v) preamp box with isolation built in. Dunno.

Mags

[MileHigh]

One thing for sure:

Little Miss MOSFET has fallen silent about the grounding issue because she read PW's comments and realizes that she is hopelessly outclassed and it has dawned on her for the 100th time that she barely knows what she is doing.

So the solution is to sign off and run away from the grounding issue and make no more comments.  She needs to lick her psychological wounds and willfully forget about this episode and then simply come back and soldier on pushing her proposition.

She knows next to nothing about grounding issues?  Who cares!  Just forget about it and keep pushing forward.  Brute force will get her there!!!  For sure!

MileHigh