Oscilliscope and Ground

The ground clip on the scope probe should have a direct connection to the case and safety ground prong of the scope. (check this with an ohm meter with the scope unplugged.) So it is safe to connect the probe ground only to nodes that are either isolated from ground (any node in your low voltage circuits that have been isolated from ground by the small supply transformer) or to nodes that already have an an earth ground connection by some other route.

If you connect it to any node that has some stiff voltage with respect to earth ground, then the connection will constitute a short circuit across that voltage and cause large current to pass through the probe shield and scope ground conductor, possibly destroying the probe and possibly injuring you. If in doubt, make an AC and DC voltage measurement between any node you are thinking of grounding (with the probe clip) and the clip. If more than a fraction of a volt shows up (there can be slight ground noise differences between points that have a connection to ground), investigate further before making the direct connection.

The purpose of the clip is to provide a high frequency path for the signal across the scope front end amplifier, back to ground, without having to go out the scope safety ground prong, around the distribution wiring and back up the power cord of the equipment being tested. Or, for isolated equipment, to provide a ground reference point for the scope, with which to measure some voltage.

Correct, the Earth is the ground. Even though your plug doesn't have the ground connection, the 'live' or 'hot' side of the supply is still referenced to it. The oscilloscope is also connected to the Earth ground through its plug, so you already have a ground connection. Trying to connect the 'scope's ground in this situation to anything other than the Earth ground (which it already is anyway) would be hazardous. You did it right.

The reason you need the 'scope's ground lead on the other side of the transformer is that there is no return path for the current otherwise, no 'zero-reference'. Without it, the 'scope will measure AC voltage induced onto the wires with reference to Earth (you can touch just a large piece of metal for the same effect). Not much use.

A last tip - never 'float the 'scope' as you may see advised in some places. That means removing its own Earth connection from the plug. It can be extremely hazardous, putting the whole body of the scope at mains potential.

Rick.

On 10 Mar 2005 08:08:46 -0800 in sci.electronics.basics, snipped-for-privacy@fflax.net wrote msg :

Excellent experiment. Even without a ground prong on your oscope power cord (how old is that thing?) the scope will sense your building's earth ground through the neutral lead. Your transformer primary sees the same ground through its neutral. Thus, they are common grounded, and you can measure a good ac signal. The DC side is isolated from that common ground via the transformer, so you need that second reference to make a measurement. You saw an AC signal on the output with the ungrounded probe, because that's the voltage the scope could see with reference to the building earth ground. Hope that makes sense.

Ground is a DEFINITION, not a PLACE. People often (mis)use (IMHO) the term ground to mean earth. But that only makes sense if you're sitting on the dirt. Where there's distance, there's impedance. Your measurement problem gets much more difficult as the frequency increases. And there's also mutual coupling of all kinds...but I digress.

The best you can hope for is to measure one specific point relative to some other specific point. You put the probe ground clip on the point you've defined as the reference using the shortest possible wire. This is often zero volts nominal relative to system ground...which may also be nominally the same as building wiring ground...which may also be nominally the same as earth ground at the dirt outside. I say nominally because if there's current and distance, there's always differential voltage. IF that voltage gets really big, you blow out the ground wire on your probe and (maybe) electrocute yourself. If it's not so big, you get measurement errors.

Now, if you're measuring low frequencies and don't need extreme accuracy and can stand some noise on your reading, you can increase the length of the wire on the ground clip to make probing more convenient. For rough measurements, you can even clip a wire from scope chassis ground to the device chassis ground and skip the probe ground altogether...again assuming they're both at the same nominal potential (ground). This is NOT always the case. If you care what the waveshape looks like, use the probe ground wire.

You almost always want a wire from somewhere in the scope (ground) system to someplace in the (ground) system in the device under test. It's possible to make measurements where this is not the case, but it's usually an unsafe measurement condition.

Even if you do have the ground clip on the proper reference node, there will also be multiple parasitic return paths that can disturb the measurement. It's not uncommon to use common mode measurement techniques where you put one probe on the signal, another on the reference (ground) and subtract the two. In this case, you can safely measure differentials that are not ground referenced.

If you don't have a defined common (ground) return path for the signal, the laws of physics will define one for you...but you have no idea where that is or how it affects your measurement.

While I'm on a tirade, let me address the probe issue. Good scope probes are not just a coax with a resistor. They're complex networks that often include coaxial wire with a resistive center conductor. Depending on vintage, your scope vertical input will probably say something like 1Meg, 10pf to maybe 45pf. If you put a 1X probe onto the scope, the input capacitance at the tip gets HUGE. It can cause significant circuit loading and misinterpretation of the waveform. It also makes the grounding problem much worse because you have to charge all that capacitance.

You almost always want to use a 10X probe that's properly compensated to the scope input capacitance.

mike

On Fri, 11 Mar 2005 01:35:01 -0800 in sci.electronics.basics, mike wrote msg :

It appears we are in violent agreement. Terms, nevertheless, persist, and a building's earth ground might just be called that because it is sitting in the dirt. Something to consider.

Don't you think you're adding a bit more complexity to the issue than it deserves? The man's question was pretty simple. If you recall,

"I figured I would need to use the ground clip when measuring my DC voltage output, but why didn't I need it for measuring the AC voltage?"

Rich did, and so would I for the OP. His questions indicated that he does not yet know enough to make sound judgments as to when that safety precaution should be set aside and what additional precautions to take once the o'scope is floated.

I agree that 'never' is a strong statement, and that with appropriate caveats and precautions, floating an o'scope can be done safely. But I would advise the OP to walk away with 'never' for now and look into it sometime later when he may have the need to float his o'scope. There are usually better ways to solve the problem anyway.

No, no, Fred was asking who the hell would tell somebody to float a scope, not who would advise somebody not to do this.

On the other hand, I've heard about people who planned to do this. However, oddly, I've never gotten a report back from them on how it went... ;)

Communication is always difficult in a half-duplex medium like the internet. My experience has shown that people often ask simple questions about complex issues because they don't know enough to ask the question that gets them the answer they really need. Then they get many fragments of information that range from correct to wrong to downright dangerous. I tend to overkill the answer so that there's a consistent message in one place. Since I'm always right ;-) this is best for all ;-) ;-)

Scope probing is a very complex issue and deserves treatment in a basics newsgroup.

mike

Who the hell "advises" this?

Precisely.

I know a few people who have (and some who still do), including people working on big industrial equipment where the thinnest piece of metal between them and the power station is a hundred amp fuse. You can't tell some people anything. Sadly, they're the ones that survive. It's the people who are scrupulously careful and have one fleeting moment of aberration who have to be scraped off the floor.

I gave this advice mainly because when I bought my first 'scope (off ebay) it had been floated. The earth wire was cut off inside the plug (UK plug).

Guess what? Finding out about it *was* a shocking experience.

Rick.

Actually, I have "floated the scope", but in very controlled conditions, and we (it was a whole class in tech school) followed all of the safety rules.

It's doable, but not recommended, for all of the obvious reasons. Then again, I've seen some pretty badly pitted 'scope ground clips! i.e., if the scope is properly grounded, what you ground the clip to has to be very, very close to zero potential from earth.

Cheers! Rich

If the ground connection gives us an absolute reference, absolutely free from noise and absolutely stable within microVolts, there is no reason to float the 'scope.

If that is not the case the noise and voltage changes from the ground connection will be added to all measurements.

Maybe the former owner did not trust his ground connection to be perfectly stable and noise free.

More likely he needed to measure above ground and either didn't know about add-and-invert, or had only one probe.

Fault finding on the input side of switchmode PSUs tends to encourage that sort of misbehavior.

That's one advantage of molded plugs. Tampering is obvious.

What I was commenting on was on the lines of "surely nobody actually advocates floating 'scopes".

Plus the revelation of the sort of home it had come from.

Tek actually used to make a device for floating 'scopes up to 20 or so volts. Anything over, it would trip to ground. I've seen one.

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I have always used an isolation transformer for the entire test station to avoid such problems, but even this precaution requires care and use of save techniques.