Oscilloscope grounding question...

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51W_10640_1.pdf

"Default User"

** Well, you have to know or figure out just what you are dealing with. ** Yep.

Inspection should tell if you are dealing with an * isolated * supply - ie one with a transformer inside it. It may be a switching supply or a iron transformer. I you find it has neither - then BEWARE as the output is NOT able to safely be connected to ground.

You could also use the ohms or continuity test ranges on your meter to see if there is a direct ( few ohms or less) connection from one of the output terminals to the AC safety ground of the supply.

If there is, then that terminal can be grounded to the scope but not any others.

If there is none ( as with wall warts and the like) you can safely connect the scope ground to any of the output terminals.

...... Phil

Check voltage, not current.

Even with a $4 meter, it probably has an internal fuse. Odds are that when you check a 'hot' chassis for safety, you'll blow this fuse without noticing. From that point on, the meter will read zero, which you will assume to be a safe condition.

Think through what it is you are hooking up before doing so. That little 'gotcha' probably means you haven't considered other ways to fry something.

--
Paul Hovnanian	paul@hovnanian.com
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Procrastinators: The leaders for tomorrow.

Hi,

Thanks for the help!

Alan

Running your mains powered test equipment (ie: scope) though an

** They all have one of them inside already !!!

YOU ASININE FUCKWIT MORON !!!

... Phil

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I ALWAYS use an isolation transformer on the scope or on the circuit under test when working with the AC supply mains. Scopes are usually grounded to the mains ground so connecting a scope ground into the circuit without isolation creates two grounds. This can be dangerous if one of the "grounds" is not a proper ground but a higher voltage. That is easy to test for as you mentioned. Touching the scope probe to the point without the ground clip connected will tell if there is a significant voltage difference between that point and the scopes internal power line ground.

A bigger deal is that the double ground creates a ground loop. A ground loop will have currents induced in it from stray magnetic fields (always present) and also possible ohmic sources with in the circuit. These currents create hum and noise in the measurement of the circuit under test and can throw off your measurement and interfere with waveforms you are trying to see.

Be careful to avoid ground loops as they are very troublesome especially in audio circuits. Use isolation wherever possible.

Yes, if you're in the US, you can use an outlet checker:

If this verifies the outlet, then it's very probably safe to ground ground to ground. ;-)

Have Fun! Rich

My Tektronix 'scope chassis is tied to the AC supply ground. Since I'm not always certain of how the unit under test is wired, I always check voltage differences before connecting it.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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"There\'s something vewy scwewy going on awound here." -- Elmer Fudd

** Huh ?

Why the f*ck did you post that facile remark to me ?

.... Phil

"Paul Hovnanian P.E." wrote in news: snipped-for-privacy@hovnanian.com:

You want to isolate the DUT,not the scope. That way,the scope chassis stays grounded,and you don't get fried if you or someone else lays their hand on the scope case.

TEK used to print a nice booklet on isolation measurement techniques and do's and don'ts.I used to include one with every scope I serviced that had the ground pin removed,after replacing the power cord[mandatory]. TEK also made some nice isolated probes.

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Jim Yanik
jyanik
at
kua.net

That's a better approach, if you can do it.

It depends on what sort of DUT you are working with. If I'm poking around in the metering/protection circuits of a BPA substation, odds are that I don't have a suitable isolation transformer sized for that application handy. :-)

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Paul Hovnanian	paul@hovnanian.com
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Procrastinators: The leaders for tomorrow.

hovnanian.com:

r

innews: snipped-for-privacy@hovnanian.com:

From '77 to '87 I was using a TEK accessory set of probes that provided high voltage isolation between DUT and scope, so grounds were never an issue.

I also had a wall-mounted, hit-it-with-your-palm, panic button which would disconnect ALL power to the lab, if an emergency arose ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona  85048                          |             |
|  Voice:(480)460-2350  Fax:Available upon request  |  Brass Rat  |
|  Web E-mail at http://www.analog-innovations.com  |    1962     |
             
         America: Land of the Free, Because of the Brave

Jim Thompson wrote in news: snipped-for-privacy@4ax.com:

A6902,IIRC.

I could never figure out why they stopped making them. I wonder if it was a liability matter,or just low sales volume?

Our cal lab had one,in the middle of the room. (ours left the lights on.) Some places also had an insulated hook-pole to safely drag a body off a hot hookup...Yowza!

--
Jim Yanik
jyanik
at
kua.net

YOWZA !-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048                          |             |
| Voice:(480)460-2350  Fax:Available upon request  |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
         America: Land of the Free, Because of the Brave

Jim Thompson wrote in news: snipped-for-privacy@4ax.com:

one funny incident was when another tech(older,not wiser..) tried to change out a dead metal-vapor halide lamp in a fixture way over his head,that was open on the top and directed the light to reflect off the ceiling.Because shutting the lights off meant that it would take 15 minutes for them to come back on,he decided to change it with the power on.He didn't know the bulb envelope had shattered,he hit the exposed wires,screamed like a girl,then ripped his hand up extracting it. He hit a very high pitch. 8-)

He was a brown-noser,trying to impress the boss by not forcing us to stop work while he changed the bulb.

--
Jim Yanik
jyanik
at
kua.net

How many scopes did go get with internal ground traces fused open?

A bit more expensive to repair than a frigging line cord, I'm thinking.

On a power electronics test bench it is often more practical to isolate the test equipment than the source or the load.

RL

"legg"

** That should not happen with any scope that meets even basic electrical safety requirements - one of which is that external metal parts ( like the BNC sockets) must be connected to the AC supply ground terminal by conductors of at least 1 sq. mm cross section.

Precludes the use of PCB traces alone to do the job.

Tek broke this rule with their very popular TDS210 / 220 series and had to recall the whole lot.

..... Phil