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Oscilloscope grounding question...

D

Default User

Jan 1, 1970
0
Hi,

How can you find out if a conductor is safe to ground?

For example, in a battery powered circuit since it is isolated from mains, I
should be able to ground it through the scopes ground lead and since it is
an isolated circuit, very little if any current should leak.

But what about various mains power supplies? How do you know if their
output is a ground that can be grounded safely?

I've been using a cheap meter I don't care that much about ($4) that does
current measurement. I have a cable that connects the mains ground pin only
to a wire and before I hook it up to the scope's ground, I've been using the
meter to test to see if any current flows through it to the ground first.

Is there a better method?

Thanks,

Alan
 
J

J.A. Legris

Jan 1, 1970
0
Hi,

How can you find out if a conductor is safe to ground?

For example, in a battery powered circuit since it is isolated from mains,I
should be able to ground it through the scopes ground lead and since it is
an isolated circuit, very little if any current should leak.

But what about various mains power supplies?  How do you know if their
output is a ground that can be grounded safely?

I've been using a cheap meter I don't care that much about ($4) that does
current measurement.  I have a cable that connects the mains ground pin only
to a wire and before I hook it up to the scope's ground, I've been using the
meter to test to see if any current flows through it to the ground first.

Is there a better method?

Thanks,

Alan

http://www.tek.com/Measurement/App_Notes/Technical_Briefs/tds3000-float/eng/51W_10640_1.pdf
 
P

Phil Allison

Jan 1, 1970
0
"Default User"
How can you find out if a conductor is safe to ground?

For example, in a battery powered circuit since it is isolated from mains,
I should be able to ground it through the scopes ground lead and since it
is an isolated circuit, very little if any current should leak.

But what about various mains power supplies? How do you know if their
output is a ground that can be grounded safely?


** Well, you have to know or figure out just what you are dealing with.
I've been using a cheap meter I don't care that much about ($4) that does
current measurement. I have a cable that connects the mains ground pin
only to a wire and before I hook it up to the scope's ground, I've been
using the meter to test to see if any current flows through it to the
ground first.

Is there a better method?


** 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
 
P

Phil Allison

Jan 1, 1970
0
<[email protected]
Running your mains powered test equipment (ie: scope) though an
isolation transformer would probably be a good idea.


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


YOU ASININE FUCKWIT MORON !!!




.... Phil
 
B

Bob Eld

Jan 1, 1970
0
Default User said:
Hi,

How can you find out if a conductor is safe to ground?

For example, in a battery powered circuit since it is isolated from mains, I
should be able to ground it through the scopes ground lead and since it is
an isolated circuit, very little if any current should leak.

But what about various mains power supplies? How do you know if their
output is a ground that can be grounded safely?

I've been using a cheap meter I don't care that much about ($4) that does
current measurement. I have a cable that connects the mains ground pin only
to a wire and before I hook it up to the scope's ground, I've been using the
meter to test to see if any current flows through it to the ground first.

Is there a better method?

Thanks,
Alan

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.
 
R

Rich Grise

Jan 1, 1970
0
How can you find out if a conductor is safe to ground?
(ammeters, etc...)...
Is there a better method?
Yes, if you're in the US, you can use an outlet checker:
http://preview.tinyurl.com/3e9jap

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

Have Fun!
Rich
 
P

Phil Allison

Jan 1, 1970
0
"Paul Hovnanian P.E."
Phil Allison wrote:

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.


** Huh ?

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



..... Phil
 
J

Jim Yanik

Jan 1, 1970
0
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.

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.
 
J

J.A. Legris

Jan 1, 1970
0
Phil Allison wrote:
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.

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.

http://www.tek.com/Measurement/App_Notes/Technical_Briefs/tds3000-flo...
 
J

Jim Yanik

Jan 1, 1970
0
in


Phil Allison wrote:

<[email protected]

Running your mains powered test equipment (ie: scope) though an
isolation transformer would probably be a good idea.

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

  YOU  ASININE   FUCKWIT  MORON   !!!

...  Phil

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.

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.

http://www.tek.com/Measurement/App_Notes/Technical_Briefs/tds3000-flo..
.

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.

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?
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

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!
 
J

Jim Yanik

Jan 1, 1970
0
On Fri, 25 Apr 2008 13:09:06 -0700 (PDT), "J.A. Legris"

in


Phil Allison wrote:

<[email protected]

Running your mains powered test equipment (ie: scope) though
an isolation transformer would probably be a good idea.

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

  YOU  ASININE   FUCKWIT  MORON   !!!

...  Phil

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.

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.


http://www.tek.com/Measurement/App_Notes/Technical_Briefs/tds3000-flo
.. .

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.

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?
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

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!

YOWZA !-)

...Jim Thompson

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. :cool:

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

legg

Jan 1, 1970
0
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.

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
 
P

Phil Allison

Jan 1, 1970
0
"legg"
How many scopes did go get with internal ground traces fused open?

** 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
 
L

legg

Jan 1, 1970
0
How many scopes did go get with internal ground traces fused open?
** 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.

My experience with this kind of fault pre-dates the TDS series by some
many years. It is part of my regular precaution, when visiting or
assuming position in an unfamiliar test site to do basic continuity
tests of this kind of internal equipment connection, digital ameter
fuses, wall power polarity, battery conditioning and source
interconnection, etc, etc, etc.

As the equipment dumped on a 'faringee' is usually dusty for some good
reason, this is just plain good sense. In practice, I've pointed out
open and dead circuits in these kinds of situations regularly, even in
equipment with fresh calibration stickers. How this calibration was
managed, I have no idea. Perhaps Yanik can illuminate inquiring minds.
Scope models exhibiting internal open circuits included, from memory,
Philips, Tek, Jiwatsu (sp?) and Nicollet.

In fact I actually own a TDS210 (since Y2K), and am listed to receive
all product and recall notices from Tektronix covering this and a
number of other models. I have no record of any recall being issued
for the unit and have not had the problem - probably because of normal
precautions previously mentioned in configuring a power electronics
test bench. I find it to be a usefull tool, once isolated in
conjunction with battery-powered laptops communicating through an
rs232 interface. I also communicate to non-isolated bench PCs through
various different isolated rs232 interfaces.

I have, however, seen similar models whose BNC sockets were wobbly
from simple brute force physical trauma. One of the problems with
light and portable equipment that should be dealt with at the design
stage.

In surviving a live wire short to ground, I wonder whether a 1mm
cross-section wire is stronger than BNC or scope shielding braid,
18AWG line cord, wall wiring or a wall breaker of random ampacity and
characteristics (not to mention all those iffy and worn socket, probe
tip and DUT screw terminals)? Why place a $2K+ piece of hardware in
the equation unneccessarily?

A mandartory ground connection is just another ignorant hand thrust
into a test circuit, when my own extranious appendages are tucked
away, where they should be.

RL

Excuse the odd posting format - original is copied from non-usenet
source, as your posting ID seems to get regularly '30-day plonked'on
my news handler. Possibly a lunar influence.
 
J

Jim Yanik

Jan 1, 1970
0
On a power electronics test bench it is often more practical to
isolate the test equipment than the source or the load.

RL

but dangerous.
anything or person that contacts the case of the "isolated" scope could be
harmed,the scope's line transformer may not be insulated for the voltage
across it,and there's also capacitive coupling across the xfmr.

much better to use isolated probes.
 
L

legg

Jan 1, 1970
0
but dangerous.
anything or person that contacts the case of the "isolated" scope could be
harmed,the scope's line transformer may not be insulated for the voltage
across it,and there's also capacitive coupling across the xfmr.

much better to use isolated probes.

You seem to ignore the fact that simple 'contact' with a hazardous
voltage or current source is not inherently harmful. It's the
completion of the circuit through a ground connection that produces
harmful effects.

Workng in areas with untreated concrete or earthen flooring, or
widespread grounding of hardware is the most likely scenario for such
a hazard to occur.

I go to great lengths to ensure that such a situation - including the
presence of untrained or improperly dressed persons - does not occur.

RL

RL
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
what is a ground? is it a potential what exactly is it? to me is a reference point. the best and only way is to use an islation transformer that can withstand some potential without breakdown.checking a battery 1.5v sitting 1000v above ground is the concerne. somebody sugested grounding the bench and so forth that only protects against ESD nothing else.isolated probes give a break. how are going to measure 1.5v dc sitting at 1000v isolated probes indeed.
 
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