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'[SOT] RS232 communication blew up my computer...'
1999\02\15@070005 by D. Schouten

picon face
Hi All,

I have implemented a PIC in one of my power supply designs to monitor
and control several functions. I've also added a MAX232 transceiver
for communication with a PC running monitor software. A couple of days
ago I was busy testing the unit using the RS232 link for remote
monitoring and a oscilloscope to check out some waveforms. After about
one hour, something was smelling pretty bad and a resistor on my proto
type unit was developing some smoke. At the same time my PC shut down
too (fans were stopping and LEDs were out), and produced a bad smell.
The results : One blown PC mainboard (one IC was cracked in two
pieces), one blown PC power supply and the MAX232 on my proto type
unit had died too (possibly causing that resistor to smoke).

My theory is that some nasty ground loop currents from the
oscilloscope probes into the mains, into the PC and back to my
prototype unit via the RS232 link, caused all of this.
Is there anybody on this list who went through a similar situation?

I guess I have to build an isolated RS232 interface in my power
supplies from now on.

Thanks.

Bye,

Daniel...

1999\02\15@080619 by marcel

flavicon
face
Yes this is the reason that many of us disconnect the earth wire from the
mains plug of the scope or use
a transformer to separate the circuits if you do measurement on
television set's mains voltage is
sometimes on the chassis.

Marcel
spam_OUTM.v.d.BiltTakeThisOuTspamfibre.A2000.nl
Amsterdam

"D. Schouten" wrote:

{Quote hidden}

1999\02\15@113640 by dave vanhorn

flavicon
face
At 02:21 PM 2/15/99 +0100, marcel wrote:
>Yes this is the reason that many of us disconnect the earth wire from the
>mains plug of the scope or use
>a transformer to separate the circuits if you do measurement on
>television set's mains voltage is
>sometimes on the chassis.

Those of us who want to live past next week, use an isolation transformer
for the equipment being tested, and let the scope ground stay right where
it is.

1999\02\15@215651 by Glenville T. Sawyer

flavicon
face
Glenville T. Sawyer   ( the 48 year old )
Outback Communications.   South Australia
Theatre & Concert Lighting, Special Effects & Props. + more !
Also - Embedded Control systems.  http://www.gsawyer.mtx.net/emailpad.html



>"D. Schouten" wrote:
>


and I "Snipped bits" ...

>>... one hour, something was smelling pretty bad and a resistor on my proto
>> type unit was developing some smoke.


  No NO NO  Daniel, they do not   DEVELOP Smoke, they actually CONTAIN
Smoke, this is what makes all Electronic items function, and I can PROVE it
!

See when you let the smoke out of that resistor, it ceased to function -
need more proof,
what about that Audio I.C that "burned out" in the car radio, see again the
"burning" is proof that
once again Smoke was involved, once it all escaped from the I.C - it too
failed !

 :-)


Seriously, Yes, there are a number of products that do indeed have "Half
Mains" on the chassis,
the very part you would expect to be at ground potential, I have a 500Watt
(at 240V) Isolation Transformer
that I use in my 'shop, along with some R.C.D's etc - my life is more
important than a piece of Electronic
equipments' !

Regards,
                  Glenville.

1999\02\16@015714 by Nigel Goodwin

flavicon
picon face
In message <4.1.19990215113455.04664ed0@192.168.0.1>, dave vanhorn
<.....dvanhornKILLspamspam@spam@CEDAR.NET> writes
>At 02:21 PM 2/15/99 +0100, marcel wrote:
>>Yes this is the reason that many of us disconnect the earth wire from the
>>mains plug of the scope or use
>>a transformer to separate the circuits if you do measurement on
>>television set's mains voltage is
>>sometimes on the chassis.
>
>Those of us who want to live past next week, use an isolation transformer
>for the equipment being tested, and let the scope ground stay right where
>it is.

It's common practice to disconnect the earth wire to oscilloscopes, it's
far safer than leaving it connected, if connected you have a potential
230v (in Europe!) across your hands when you try to connect the earth
lead from the scope to the item under test. By disconnecting the earth
lead you remove this serious hazard, this only leaves the unlikely
breakdown of the scope mains transformer as a potential hazard.

I've been repairing domestic electronic equipment for 27 years, I
wouldn't use a scope with an earth lead, it's too dangerous!. Remember,
we are talking about professional engineers here, not members of the
public, we should be able to adopt safe working practices - and this
doesn't include earthed test equipment (at least in most domestic
cases).

BTW, we use isolation transformers for feeding all the service benches
at work (I ordered and installed them!), but I still consider an un-
earthed oscilloscope an essential safety feature.
--

Nigel.

       /--------------------------------------------------------------\
       | Nigel Goodwin   | Internet : nigelgspamKILLspamlpilsley.demon.co.uk     |
       | Lower Pilsley   | Web Page : http://www.lpilsley.demon.co.uk |
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1999\02\16@040715 by Dr. Imre Bartfai

flavicon
face
Hi,
it can happen e. g. if you connect the scope probe to a power line (e. g.
+5V) and the protective ground of scope and PC are correct. Or, if you do
all fine, but one of the two devices (scope OR pc) has its protective
ground floating. A small fast fuse built into output lines of the devices
to be tested can prevent such situation (my experience). A blown fuse is
far less expensive than a blown motherboard.
Imre


On Mon, 15 Feb 1999, D. Schouten wrote:

{Quote hidden}

1999\02\16@094318 by John

flavicon
face
Hello PIC.ers,

Glenville Sawyer wrote:-

{Quote hidden}

I ditto this in my own shop.
The setup I have is as good as identical, and for all the above reasons
PLUS
The isolation transformer is a very effective current limiter, a feature
which comes into its own
when applying power to a dead short!
Hum, slow glow & manufacturer's incorporated smoke, all develop at a more
leisurely
pace.
This allows you to disconnect on time, or, if it's dire, get out the door
ahead of the
pyrotechnics     :))

Best regards,   John

e-mail from the desk of John Sanderson, JS Controls.
Snailmail:          PO Box 1887, Boksburg 1460, Rep. of South Africa.
Tel/fax:            Johannesburg  893 4154
Cellphone no:   082 469 0446
email:                .....jsandKILLspamspam.....pixie.co.za
Manufacturer & purveyor of laboratory force testing apparatus, and related
products and services.

1999\02\16@104531 by dave vanhorn

flavicon
face
>It's common practice to disconnect the earth wire to oscilloscopes, it's
>far safer than leaving it connected, if connected you have a potential
>230v (in Europe!) across your hands when you try to connect the earth
>lead from the scope to the item under test. By disconnecting the earth
>lead you remove this serious hazard, this only leaves the unlikely
>breakdown of the scope mains transformer as a potential hazard.

Or grabbing a grounded lead from another instrument, or a lan cable...

Why would your item under test not be plugged into an isolation transformer?
So you're happy now with the DUT and the scope both at lethal voltage?
NO thanks.
Common practice dosen't make it safe.

>I've been repairing domestic electronic equipment for 27 years,

Careful, and/or lucky.  I repaired for 10 and have been designing for 15.
Never been shocked since I learned how to wire a bench.

>I wouldn't use a scope with an earth lead, it's too dangerous!. Remember,
>we are talking about professional engineers here, not members of the
>public, we should be able to adopt safe working practices - and this
>doesn't include earthed test equipment (at least in most domestic
>cases).

The only way this is safe, is if there's no ground anywhere.. How can you
predict which equipment is hard grounded, and which is not? Remember, you
bet your life on it. (and anyone else who might be in the lab.)  The last
place I worked, we had many labs, and engineers flying around like crazy.
Since all our benches run the same rule, nobody has to worry. You plug your
DUT into the isolation xfmr, and everything else is hard grounded.

>BTW, we use isolation transformers for feeding all the service benches
>at work (I ordered and installed them!), but I still consider an un-
>earthed oscilloscope an essential safety feature.

The whole point of an isolation transformer is to float the equipment under
test, so that anywhere you want to clip your grounded scope becomes
ground.. If you put the scope and the DUT on the isolation transformer
together, you defeat the whole purpose.  You'd need another transformer for
the DUT.

I'll concede that there are times when floating the whole bench is
waranted, and as long as EVERYTHING is floated, then you're ok.. but you
STILL need to float the DUT relative to the bench.  The <ONE> time I had to
float the scope, was for a very touchy measurement on the hot side of a
switcher, where there was enough current circulating in the bench ground to
fuzz the waveform.. I have a cheater plug for that. Still, it's safe, since
the DUT is on isolation.

The point is, that as long as the DUT is on an isolation transformer, you
don't have to remember any special systems, or make any lethal
assumptions.. You, and visitors who don't know any special rules are also
safe. Suppose I needed a power cable one evening, borrowed your scope power
cable, and accidentally replaced it with a different one with an intact
ground.. BZZZT.. Barbecue Mark.

1999\02\16@160335 by Gerhard Fiedler

picon face
At 23:37 02/15/99 +0000, Nigel Goodwin wrote:
>In message <4.1.19990215113455.04664ed0@192.168.0.1>, dave vanhorn
><EraseMEdvanhornspam_OUTspamTakeThisOuTCEDAR.NET> writes
>>Those of us who want to live past next week, use an isolation transformer
>>for the equipment being tested, and let the scope ground stay right where
>>it is.
>
>It's common practice to disconnect the earth wire to oscilloscopes, it's
>far safer than leaving it connected, if connected you have a potential
>230v (in Europe!) across your hands when you try to connect the earth
>lead from the scope to the item under test. By disconnecting the earth
>lead you remove this serious hazard, this only leaves the unlikely
>breakdown of the scope mains transformer as a potential hazard.

why do you have a potential across your hands when the item under test is
connected to mains through an isolation transformer? i thought (like dave)
this was the safest way to go, and disconnecting the ground on the scope a
work-around for those not concerned enough to use an isolation transformer...

BTW, i never saw any sparks when working this way :)

ge

1999\02\16@160822 by Gerhard Fiedler

picon face
At 10:39 02/16/99 -0500, dave vanhorn wrote:
>The point is, that as long as the DUT is on an isolation transformer

OF ITS OWN

>you don't have to remember any special systems, or make any lethal
>assumptions..

can we hang that somewhere on the wall?

ge

1999\02\16@164207 by Sean Breheny

face picon face
Hi all,

I think this may be a good place for me to ask a question that has always
bugged me:

I know its NOT a good idea (to put it very mildly) to touch a hot 120v or
230v line. However,I don't understand why anyone can possibly receive a
shock at all if they are wearing shoes and only touch the hot (and not a
grounded,conductive object at the same time). Or, for that matter,even if
they have bear feet but are on a non-conductive surface.

I know that the danger is MUCH greater when one contacts BOTH the hot and
ground with parts of their body,but is this the only (theoretically)
dangerous situation?

NO, I'M NOT  ADVOCATING EXPERIMENTATION IN THIS AREA!!!


Thanks,

Sean

At 01:00 PM 2/16/99 -0800, you wrote:
>why do you have a potential across your hands when the item under test is
>connected to mains through an isolation transformer? i thought (like dave)
>this was the safest way to go, and disconnecting the ground on the scope a
>work-around for those not concerned enough to use an isolation transformer...
>
>BTW, i never saw any sparks when working this way :)
>
>ge
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
shb7spamspam_OUTcornell.edu  Phone(USA): (607) 253-0315 ICQ #: 3329174

1999\02\16@165537 by dave vanhorn

flavicon
face
>I know that the danger is MUCH greater when one contacts BOTH the hot and
>ground with parts of their body,but is this the only (theoretically)
>dangerous situation?
>
>NO, I'M NOT  ADVOCATING EXPERIMENTATION IN THIS AREA!!!


It's "bird on a wire". Birds sit on 500kV transmission lines without a
problem because there is nowhere for the current to flow. Presumably as
they approach, the charge equalizes, and they may get only a very mild
shock on contact as their body capacitance comes into equality.

That's the whole idea behind using an isolation transformer. All of the
equipment is then operating without any earth ground, and you can safely
touch any point (high voltages excepted because they may break down the
isolation, and you still get a static shock)

This only holds true of you touch only ONE point. Edison's rule about
keeping one hand in your pocket isn't a bad idea.

As far as touching while wearing shoes, well... how much do you trust your
shoes (containing sweaty feet and nails?)
Linemen do this all the time, working on live circuits with gloves, but the
gloves are stringently tested, and they get a higher rate of pay if they
have to put the gloves on.

1999\02\16@165945 by dave vanhorn

flavicon
face
Addendum:

Most of what I know comes from a couple years in a volunteer fire
department, working under a chief who was a lineman.  He gave us loads of
fun electrical safety tips.  Like how to hose down a telephone pole when
it's got live HV on the top, and what happens when a powerline drops on a
fence..

1999\02\16@170213 by Gerhard Fiedler

picon face
At 16:40 02/16/99 -0500, Sean Breheny wrote:
>I know its NOT a good idea (to put it very mildly) to touch a hot 120v or
>230v line. However,I don't understand why anyone can possibly receive a
>shock at all if they are wearing shoes and only touch the hot (and not a
>grounded,conductive object at the same time). Or, for that matter,even if
>they have bear feet but are on a non-conductive surface.

if you make sure you're on pretty non-conductive stuff and don't touch
anything else, i guess you're safe to do so if you're in good health (i've
done it by accident, and it doesn't feel too horrible -- a long ways anyway
from touching power while being grounded --, and i've seen electricians --
with good dry boots -- use this as a "portable power indicator" :). but the
problem is that it is pretty easy not to be perfectly isolated, and you
don't need big currents to feel bad - some 10mA do it... so better get that
isolation transformer.

ge

1999\02\16@170821 by dave vanhorn

flavicon
face
>I know its NOT a good idea (to put it very mildly) to touch a hot 120v or
>230v line. However,I don't understand why anyone can possibly receive a
>shock at all if they are wearing shoes and only touch the hot (and not a
>grounded,conductive object at the same time). Or, for that matter,even if
>they have bear feet but are on a non-conductive surface.

The voltage has a lot to do with it too.. I've worked on a guy who took a
60kV line in the head, it blew holes in his shoes, and really caused him a
lot of problems. He's alive and walking around now, but not without problems.

It's not a game I'd choose to play.

1999\02\16@173134 by D. Schouten

picon face
>why do you have a potential across your hands when the item under
test is
>connected to mains through an isolation transformer? i thought (like
dave)
>this was the safest way to go, and disconnecting the ground on the
scope a
>work-around for those not concerned enough to use an isolation
transformer...
>
>BTW, i never saw any sparks when working this way :)

In my situation the power supply I designed containing the PIC, is a
stepup converter which converts low voltage DC power (<32VDC) into
higher voltage AC (230VAC) power. The DC input power is provided by a
big high current regulated (and earthed!) DC power supply. When I
connect the scope probe on the output of my stepup converter I see
small sparks jumping between the probes ground tip and stepup
converter output. This indicates quite some potential difference and a
relatively low impedance. It's so heavy that my PIC control circuit is
interfered too, resulting in distorted waveforms. This is confirmed by
the fact that when I touch the scope ground tip and the DC power
supply housing I'm receiving a big electric shock.
When I disconnect the scopes earth wire, I don't receive an electric
shock in this situation.

When I use an isolation transformer to supply my scope, what do I have
to do with the earth wire? Connecting this wire just to the mains
earth will bypass the isolation border IMHO. The scopes input EMI
filter Wye caps will conduct some current from the line and neutral
wire to the earth wire.

Daniel...
(who just installed a new PC mainboard today, and will never work with
non isolated RS232/485 communications anymore)

1999\02\16@174358 by dave vanhorn

flavicon
face
>In my situation the power supply I designed containing the PIC, is a
>stepup converter which converts low voltage DC power (<32VDC) into
>higher voltage AC (230VAC) power. The DC input power is provided by a
>big high current regulated (and earthed!) DC power supply. When I
>connect the scope probe on the output of my stepup converter I see
>small sparks jumping between the probes ground tip and stepup
>converter output.

If I understand you correctly, then your output should be isolated, unless
you have some components on the output that are coupling energy to the
chassis, or a transformer with a lot of interwinding capacitance. This may
indicate a safety problem that you need to address.

>When I use an isolation transformer to supply my scope, what do I have
>to do with the earth wire? Connecting this wire just to the mains
>earth will bypass the isolation border IMHO. The scopes input EMI
>filter Wye caps will conduct some current from the line and neutral
>wire to the earth wire.


Check your particular transformer. Some accept the ground pin, but don't
connect it, others connect it through, I guess a product of lawyer-ism..
For short term I'd use a cheater plug and a LOT of caution, but as above, I
think there may be another problem to solve.

1999\02\16@181551 by marcel

flavicon
face
Yes your right but impatient people like i am go for the quick way but for
share an isolation transformer is the way
to go a bit heavy if you are asked to check someone's television set at persons
home.
WATCH OUT WITH MAINS AND Eartloops
Marcel
@spam@M.v.d.BiltKILLspamspamfibre.A2000.nl
Amsterdam

dave vanhorn wrote:

{Quote hidden}

1999\02\16@222640 by Gerhard Fiedler

picon face
At 23:27 02/16/99 +0100, D. Schouten wrote:
>>why do you have a potential across your hands when the item under
>test is
>>connected to mains through an isolation transformer? i thought (like
>dave)
>>this was the safest way to go, and disconnecting the ground on the
>scope a
>>work-around for those not concerned enough to use an isolation
>transformer...
>>
>>BTW, i never saw any sparks when working this way :)
>
>In my situation the power supply I designed containing the PIC, is a
>stepup converter which converts low voltage DC power (<32VDC) into
>higher voltage AC (230VAC) power. The DC input power is provided by a
>big high current regulated (and earthed!) DC power supply.

ok, that's different. of course i'd prefer to put the dc power supply on an
isolation transformer, unearthed of course. if you can't do that, then of
course you better get your scope on a transformer -- again unearthed, of
course. an isolation transformer doesn't make a lot of sense if you connect
the ground wire through :)

{Quote hidden}

what kind of output do you have? is there a connection from the secondary
(i suppose a flyback circuit) back to the primary side? as i said, i'd
really prefer to hook up the dc supply to a transformer. or use one that's
transformer-based and has no connection of the ground (or the power lines
of course) to the output -- the ground may be connected to the case, but
the output voltage should be ground-free. this way you ain't got no sparks
nowhere... :)

>When I use an isolation transformer to supply my scope, what do I have
>to do with the earth wire? Connecting this wire just to the mains
>earth will bypass the isolation border IMHO.

of course; it's pretty rare that i use an isolation transformer with a
ground connection from primary to secondary. as you say, it kinda defeats
the purpose :)

ge

1999\02\17@015204 by Nigel Goodwin

flavicon
picon face
In message <4.1.19990216100449.03b16750@192.168.0.1>, dave vanhorn
<KILLspamdvanhornKILLspamspamCEDAR.NET> writes
>I'll concede that there are times when floating the whole bench is
>waranted, and as long as EVERYTHING is floated, then you're ok.. but you
>STILL need to float the DUT relative to the bench.  The <ONE> time I had to
>float the scope, was for a very touchy measurement on the hot side of a
>switcher, where there was enough current circulating in the bench ground to
>fuzz the waveform.. I have a cheater plug for that. Still, it's safe, since
>the DUT is on isolation.

I'll explain my situation a little further, all the benches in our
workshop are wired the same way. There are a number of different sockets
(to account for old style plugs - hardly ever seen now), these are for
the DUT and connected via an isolation transformer and a double pole
isolation switch. There is one extra 13A socket mounted seperately, and
not isolated or switched, and this is used for the oscilloscope or a
monitor TV if repairing a VCR or satellite receiver. Aerial sockets for
the benches are isolated ones to avoid any earth connections through
those, thus providing an earth free environment.

>The point is, that as long as the DUT is on an isolation transformer, you
>don't have to remember any special systems, or make any lethal
>assumptions.. You, and visitors who don't know any special rules are also
>safe. Suppose I needed a power cable one evening, borrowed your scope power
>cable, and accidentally replaced it with a different one with an intact
>ground.. BZZZT.. Barbecue Mark.


BTW, my scope doesn't have a plug-in mains lead :-), in fact out of the
seven scopes I have access to (both at work and home), only one of them
has a plug-in lead - that's a Philips one.
--

Nigel.

       /--------------------------------------------------------------\
       | Nigel Goodwin   | Internet : RemoveMEnigelgTakeThisOuTspamlpilsley.demon.co.uk     |
       | Lower Pilsley   | Web Page : http://www.lpilsley.demon.co.uk |
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       \--------------------------------------------------------------/

1999\02\17@033213 by Mark Willis

flavicon
face
The benches in my high school electronics lab were all nicely solidly
grounded, METAL benches underneath, so this thought makes me shudder
<G>  You can bet we got into the habit of not touching anything while it
was live (to make it worse, I'm tall enough that I was almost always
squeezed in against the metal underside of the bench.)  Birds on a wire
aside, you're safest if you assume you're grounded & be accordingly
PARANOID.  It's not that those electrons are out to get you - it's that
they just don't care whether they kill you or let you live.  (And yep,
I've worked on live HV, but I try not to, and make sure I PAY ATTENTION
to what I'm doing - I cherish my life, among others <G>)

 Mark, any benches I buy WON'T be hard grounded, 1 Meg is GOOD <G>

Sean Breheny wrote:
{Quote hidden}

1999\02\17@175523 by Nigel Goodwin

flavicon
picon face
In message <4.1.19990216125710.0099c1b0@mail>, Gerhard Fiedler
<TakeThisOuTlistsEraseMEspamspam_OUTHOME.COM> writes
>At 23:37 02/15/99 +0000, Nigel Goodwin wrote:
>>In message <4.1.19990215113455.04664ed0@192.168.0.1>, dave vanhorn
>><RemoveMEdvanhornspamTakeThisOuTCEDAR.NET> writes
>>>Those of us who want to live past next week, use an isolation transformer
>>>for the equipment being tested, and let the scope ground stay right where
>>>it is.
>>
>>It's common practice to disconnect the earth wire to oscilloscopes, it's
>>far safer than leaving it connected, if connected you have a potential
>>230v (in Europe!) across your hands when you try to connect the earth
>>lead from the scope to the item under test. By disconnecting the earth
>>lead you remove this serious hazard, this only leaves the unlikely
>>breakdown of the scope mains transformer as a potential hazard.
>
>why do you have a potential across your hands when the item under test is
>connected to mains through an isolation transformer? i thought (like dave)
>this was the safest way to go, and disconnecting the ground on the scope a
>work-around for those not concerned enough to use an isolation transformer...

You don't have a potential across your hands when the item is connected
through an isolation transformer, but if you should ever have occasion
to use the scope on an item where you don't have an isolation
transformer it's a lot safer if the earth lead isn't connected.

TV/Satellite repairs are particularly bad, as they almost always use
switch-mode PSU's, so it's quite common to want to connect the scope to
the 'live' side of the PSU - again via an isolation transformer it's no
problem with or without the scope earthed.

BTW, talking TV's, going back a LONG! time they used to use half wave
rectifers (usually a metal rectifier), this meant that when the mains
lead was inserted correctly the chassis was connected to the neural wire
- however if the lead was inserted incorrectly (50/50 chance with an old
two pin plug) the entire chassis of the TV was connected directly to the
incoming live connection!.

One final point, what does having the scope earthed protect you against
anyway?. Is there anything else apart from breakdown in it's internal
mains transformer), or some kind of short between incoming live and the
case.
--

Nigel.

       /--------------------------------------------------------------\
       | Nigel Goodwin   | Internet : nigelgEraseMEspam.....lpilsley.demon.co.uk     |
       | Lower Pilsley   | Web Page : http://www.lpilsley.demon.co.uk |
       | Chesterfield    | Official site for Shin Ki Ju Jitsu         |
       | England         |                                            |
       \--------------------------------------------------------------/

1999\02\17@190359 by Mike Keitz

picon face
On Wed, 17 Feb 1999 07:11:40 +0000 Nigel Goodwin
>One final point, what does having the scope earthed protect you
>against
>anyway?. Is there anything else apart from breakdown in it's internal
>mains transformer), or some kind of short between incoming live and
>the
>case.

If you do connect the ground lead by accident to a live circuit,
something will blow up.  This is much better than the case of the scope
becoming live, then you blowing up when you touch it.  With the safety
ground intact, it is always safe to touch the ground of the scope, unless
of course you are also toucing a live wire at the same time.

The answer for impromptu tests without using an isolation transformer is
simply DON'T.  If you can lug a scope around, take a transformer too.

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1999\02\17@195200 by Gerhard Fiedler

picon face
At 18:30 02/17/99 -0500, Mike Keitz wrote:
>If you can lug a scope around, take a transformer too.

i think that's a pretty valid point, with those "improvisation" arguments.
when you've got a scope with you, you're not that much "improvising" :)

ge

1999\02\17@195618 by Gerhard Fiedler

picon face
At 07:11 02/17/99 +0000, Nigel Goodwin wrote:
>One final point, what does having the scope earthed protect you against
>anyway?. Is there anything else apart from breakdown in it's internal
>mains transformer), or some kind of short between incoming live and the
>case.

well, the point is that often the scope's case is connected to its earth.
if you have to disconnect the scope's earth from the power earth (which is
approx. your potential), it is because you want to put the scope's earth on
a different potential -- which means, a different potential from =you=.
which would be right on the case, right? now i touch my scope every now and
then when working with it, don't know about others... so i definitely want
to be sure the scope's case it at approx. my potential. connect it to the
power earth is a good means to assure this (except when there is a short in
the scope, but that's another matter).

ge

1999\02\18@022101 by Lee Jones

flavicon
face
I'll probably regret keeping this thread alive, but...

>> One final point, what does having the scope earthed protect you
>> against anyway?

> If you do connect the ground lead by accident to a live circuit,
> something will blow up.  This is much better than the case of the
> scope becoming live, then you blowing up when you touch it.  With
> the safety ground intact,

Apparently not every large corporation agrees with this.  I
have a Philips oscilloscope.  I bought it brand new.  It has
never been modified.  It has a 2 wire line cord.  No ground.
It was designed using double insulation standards.

And how, on my portable battery-operated oscilloscope, do I
ensure my earth ground is connected when it's being operated
without _any_ line cord?

<flame shield on>
Sometimes the technician just needs to understand exactly
what they are doing with the test equipment they're using.
<flame shield off>

                                               Lee Jones

1999\02\18@023655 by Gerhard Fiedler
picon face
At 23:18 02/17/99 -0800, Lee Jones wrote:
>> If you do connect the ground lead by accident to a live circuit,
>> something will blow up.  This is much better than the case of the
>> scope becoming live, then you blowing up when you touch it.  With
>> the safety ground intact,
>
>Apparently not every large corporation agrees with this.  I
>have a Philips oscilloscope.  I bought it brand new.  It has
>never been modified.  It has a 2 wire line cord.  No ground.
>It was designed using double insulation standards.

you said it here: this scope ("double insulation" must mean something to
you :) was designed to operate safely without a ground wire...

>And how, on my portable battery-operated oscilloscope, do I
>ensure my earth ground is connected when it's being operated
>without _any_ line cord?

... and so was this scope.

the "method" in question is =removing= (not "using without") the ground
wire from scopes which were designed to operate safely =with= a ground wire
-- and possibly have touchable parts of the case connected to signal
ground. which probably is not the case with your two examples, and since
your scopes don't have any ground wire, this whole discussion (about
removing the ground wire) obviously doesn't apply to them.

ge

1999\02\18@092342 by dave vanhorn

flavicon
face
>Apparently not every large corporation agrees with this.  I
>have a Philips oscilloscope.  I bought it brand new.  It has
>never been modified.  It has a 2 wire line cord.  No ground.
>It was designed using double insulation standards.

>And how, on my portable battery-operated oscilloscope, do I
>ensure my earth ground is connected when it's being operated
>without _any_ line cord?

Then it is it's own isolation transformer. I bet you'll also find it
moderately difficult to come in contact with any metal.

><flame shield on>
>Sometimes the technician just needs to understand exactly
>what they are doing with the test equipment they're using.
><flame shield off>

Always. It's not just a good idea.

1999\02\18@120811 by John Payson

flavicon
face
At 07:11 02/17/99 +0000, Nigel Goodwin wrote:
>One final point, what does having the scope earthed protect you against
>anyway?. Is there anything else apart from breakdown in it's internal
>mains transformer), or some kind of short between incoming live and the
>case.

|well, the point is that often the scope's case is connected to its earth.
|if you have to disconnect the scope's earth from the power earth (which is
|approx. your potential), it is because you want to put the scope's earth on
|a different potential -- which means, a different potential from =you=.
|which would be right on the case, right? now i touch my scope every now and
|then when working with it, don't know about others... so i definitely want
|to be sure the scope's case it at approx. my potential. connect it to the
|power earth is a good means to assure this (except when there is a short in
|the scope, but that's another matter).

Would a GFCI be at all useful as a safety measure?  Would floating
the scope but plugging it and the device under test into seperate
GFCI's be reasonably safe?


Attachment converted: wonderland:WINMAIL.DAT (????/----) (0002A2C9)

1999\02\18@142442 by Nigel Goodwin

flavicon
picon face
In message <EraseME19990217.185530.9694.7.mkeitzspamjuno.com>, Mike Keitz
<RemoveMEmkeitzEraseMEspamEraseMEJUNO.COM> writes
>On Wed, 17 Feb 1999 07:11:40 +0000 Nigel Goodwin
>>One final point, what does having the scope earthed protect you
>>against
>>anyway?. Is there anything else apart from breakdown in it's internal
>>mains transformer), or some kind of short between incoming live and
>>the
>>case.
>
>If you do connect the ground lead by accident to a live circuit,
>something will blow up.  This is much better than the case of the scope
>becoming live, then you blowing up when you touch it.  With the safety
>ground intact, it is always safe to touch the ground of the scope, unless
>of course you are also toucing a live wire at the same time.

That's the point!, if your scope is earthed, touching it and the item
under test (if live) will give you a shock. It's MUCH easier to get a
serious shock in that way than if the scope isn't earthed and becomes
live when attached to the item under test - particularly if you try and
hold the item with one hand and clip the scope ground lead on with the
other - the worst possible result, straight from one hand to the other,
directly across the heart.

No doubt everyone else on this list (as well as myself) wouldn't do
that, but it's very easily done by inexperienced persons. Also I never
'grab' a metal chassis without first stroking it with the back of a
finger, if it's live you get a 'rough' feel to it, so know not to touch
it!.

But in all cases, it comes down to 'knowing what you are doing', taking
anything apart to work on should only be done with a good knowledge of
the principles and techniques involved.
--

Nigel.

       /--------------------------------------------------------------\
       | Nigel Goodwin   | Internet : RemoveMEnigelgspam_OUTspamKILLspamlpilsley.demon.co.uk     |
       | Lower Pilsley   | Web Page : http://www.lpilsley.demon.co.uk |
       | Chesterfield    | Official site for Shin Ki Ju Jitsu         |
       | England         |                                            |
       \--------------------------------------------------------------/

1999\02\18@142624 by Gerhard Fiedler

picon face
At 11:08 02/18/99 -0600, John Payson wrote:
>At 07:11 02/17/99 +0000, Nigel Goodwin wrote:
>>One final point, what does having the scope earthed protect you against
>>anyway?. Is there anything else apart from breakdown in it's internal
>>mains transformer), or some kind of short between incoming live and the
>>case.
>
>|well, the point is that often the scope's case is connected to its earth.
>|if you have to disconnect the scope's earth from the power earth (which is
>|approx. your potential), it is because you want to put the scope's earth on
>|a different potential -- which means, a different potential from =you=.
>|which would be right on the case, right? now i touch my scope every now and
>|then when working with it, don't know about others... so i definitely want
>|to be sure the scope's case it at approx. my potential. connect it to the
>|power earth is a good means to assure this (except when there is a short in
>|the scope, but that's another matter).
>
>Would a GFCI be at all useful as a safety measure?  Would floating
>the scope but plugging it and the device under test into seperate
>GFCI's be reasonably safe?

i don't know how these fit into the picture here...

they would probably increase the safety and i guess one of them would trip
when you touch eg. a scope case that's hot by connecting the ground to a
hot spot in the circuit, because the current has to come from somewhere. so
the shock would be limited to a short time.

but that's still possibly contact with mains... and their trip (why do i
want to write "tripp"? bad spelling or what? conditioning perhaps? :)
current is around 5mA or so, that's more than it sounds when it flows
through you, although i can't speak of experience here. all the hot wires i
touched have been safeguarded by fuses (from 8A upwards :) only, and none
of these tripped from the contact (i might add, gladly so... :)

yes, i think they increase the safety, maybe sufficiently. and in the cases
where you absolutely can't use a transformer, they're probably a good
second choice. but they don't provide by far the level of convenience a
transformer does by floating your DUT. and there are probably very few
reasons not to use a transformer -- as somebody mentioned: when you can lug
a scope around, you probably can take a transformer with you, too. and if
it's one of these battery-operated portable ones, that's in another
category anyway and all this discussion doesn't apply.

and when you have more than a scope involved (like serial comms with a
computer or so), it gets increasingly expensive not to use a transformer :))


OTOH, here in the usa you can go and do almost whatever you want with mains
voltage, it's only 115V anyway. (i grew up in germany, making all my early
childhood electrical experiences on 220V -- that's what =real= men use when
they need a charge :)

ge

1999\02\18@143926 by Gerhard Fiedler

picon face
At 07:10 02/18/99 +0000, Nigel Goodwin wrote:
>That's the point!, if your scope is earthed, touching it and the item
>under test (if live) will give you a shock. It's MUCH easier to get a
>serious shock in that way than if the scope isn't earthed and becomes
>live when attached to the item under test - particularly if you try and
>hold the item with one hand and clip the scope ground lead on with the
>other - the worst possible result, straight from one hand to the other,
>directly across the heart.

you seem to assume that the DUT has only one potential... what about a DUT
that has potential differences of 100 or 200 or 1500V in itself? that's
when it gets "important" to disconnect the ground wire of the scope (for
the ones who favour this method). in such a setting the "safety" in the
sense described seems not to be much worth.

and as long as you can touch the wires in your DUT, we're in pretty calm
waters anyway.

ge

1999\02\18@204125 by John Payson

flavicon
face
|and when you have more than a scope involved (like serial comms with a
|computer or so), it gets increasingly expensive not to use a transformer :))

What about designs where you need to connect your circuit to
something which is grounded, and (for various reasons) that
ground does not match your circuit ground [simple example: you
are building a battery-powered device with a serial output and
you want it to swing +/-5v with no extra components.  If your
power supply is floating relative to the computer (and you don't
need a serial input) you may simply connect one port pin to the
computer's ground and another to its serial data input.  This
will cause everything else in your circuit to bounce up and down
5 volts relative to earth, but that shouldn't pose a problem
unless you're trying to look at stuff with an earthed scope...

1999\02\18@221728 by Mike Keitz

picon face
On Thu, 18 Feb 1999 19:40:02 -0600 John Payson <RemoveMEsupercatTakeThisOuTspamspamCIRCAD.COM>
writes:
>|and when you have more than a scope involved (like serial comms with
>a
>|computer or so), it gets increasingly expensive not to use a
>transformer :))
>
>What about designs where you need to connect your circuit to
>something which is grounded, and (for various reasons) that
>ground does not match your circuit ground [simple example: you
>are building a battery-powered device with a serial output and
>you want it to swing +/-5v with no extra components.  If your
>power supply is floating relative to the computer (and you don't
>need a serial input) you may simply connect one port pin to the
>computer's ground and another to its serial data input.  This
>will cause everything else in your circuit to bounce up and down
>5 volts relative to earth, but that shouldn't pose a problem
>unless you're trying to look at stuff with an earthed scope...
>

A good way to do such a measurement would be to ground the scope to
computer ground and use two channels in a differential mode (A-B).  The B
channel would be connected to circuit ground so what you see on the
screen is actually the A probe voltage relative to PIC circuit ground.

Since the voltages involved are low, its not dangerous to lift the scope
ground; however the measurement may be affected by the high capacitance
from the scope case to ground.  Also there's a good chance of burning the
PIC or computer out if the scope ground gets shorted to something.  One
of the newer fully islated scopes would work great, but I'm assuming
that's not available.

In the example cited, it may be better to rearrange the circuit during
the test so that the computer and PIC grounds are connected, and the
serial input driven from just one pin.  In other situations, similarly
devise a test rig to provide isolation or rearrange the circuit so the
grounds are connected until the problem is debugged.

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1999\02\19@004352 by erik

flavicon
face
Hi,

I've been following this very long thread and waiting for it all to come
together for me.... No Such Luck.
I don't own an o-scope and haven't used one since school but would very
much like have one. So please forgive me if this comes across as a
stupid question. But I just don't understand.

Isn't the scope's probe isolated from the AC supply?
Isn't / shouldn't the scope's signal ground be isolated from the scopes
earth ground? After all, the signal ground should simply be a reference
and could be placed anywhere. (Right?)

If the above is true (which I suspect it isn't) how could there be an
appreciable potential anywhere that might be dangerous?

Aren't we just measuring a potential from the scope probe to the scope's
signal ground, irrespective of the supply of either the scope or the
item under test?

Again, I'm afraid someone will try to tell me how stupid I am but, this
is really bothering me. Could somebody please fill me in?

Thanks,
Erik

1999\02\19@020948 by Gerhard Fiedler

picon face
At 23:42 02/18/99 -0600, erik wrote:
>Isn't the scope's probe isolated from the AC supply?

from the =supply= lines, usually yes (more or less).

>Isn't / shouldn't the scope's signal ground be isolated from the scopes
>earth ground?

is not, but should be :) as one stated during the thread, there are various
(more expensive) varieties of scopes with floating grounds: the handheld
scopes (tek has a nice one, where even the two channels have separated
signal grounds), and some of the better big ones, like the one philips
scope mentioned, which has, instead of an earth ground for safety, a double
isolation.

but most scopes have their signal ground connected to the earth ground. i'm
not exactly sure why, but this is a fact. that the case is on earth ground
is obvious, but why exactly the signal ground often is connected to this --
i'd like somebody to please jump in here and tell me why. after all, this
seems to be a major cause of trouble, so it seems to me there must be a
really important reason for it -- and i should know it :)

>After all, the signal ground should simply be a reference
>and could be placed anywhere. (Right?)

if it is only a reference, right. but see above -- often it isn't "only" a
reference.

>If the above is true (which I suspect it isn't) how could there be an
>appreciable potential anywhere that might be dangerous?

the (possibly) dangerous potential would be in your circuit. say you have a
circuit that operates directly from the power line, and you want to scope
something in there -- it might just be that you want your scope's signal
ground on the hot side of the circuit.

now either you disconnect the scope's earth ground and hook the signal
ground to the hot wire -- which obviously gets some kind of dangerous
potential to everything this signal ground is connected to (and the fact
that you had to remove the earth ground indicates that there is a
connection between the two, and of course the case is hooked to the earth
ground, too) --, or you put your circuit on an isolation transformer, and
hook the earthed signal ground to whatever point in your circuit -- which
then puts exactly this point at earth ground potential (becasue of the
transformer the circuit is floating freely in potential space).

hope this clears the discussion up a bit. i've seen really cheap scopes in
surplus shops; a scope is highly instructive, even if it's not a good one.
seeing waveforms is nothing you should miss if you can get it. it's also
real FUN... :)

ge

1999\02\19@021818 by Gerhard Fiedler

picon face
At 19:40 02/18/99 -0600, John Payson wrote:
>What about designs where you need to connect your circuit to
>something which is grounded, and (for various reasons) that
>ground does not match your circuit ground [simple example: you
>are building a battery-powered device with a serial output and
>you want it to swing +/-5v with no extra components.  If your
>power supply is floating relative to the computer (and you don't
>need a serial input) you may simply connect one port pin to the
>computer's ground and another to its serial data input.  This
>will cause everything else in your circuit to bounce up and down
>5 volts relative to earth, but that shouldn't pose a problem
>unless you're trying to look at stuff with an earthed scope...

i guess as a quick fix you could just do it, with the possible caveats mike
talked about. or, since in this case the computer is somehow part of the
"DUT," it therefore gets to run on an isolation transformer :)  otherwise
you could maybe put the scope on a transformer.

ge

1999\02\19@143445 by Nigel Goodwin

flavicon
picon face
In message <EraseME36CCF9CF.B6FCC4EspamspamspamBeGonenetwurx.net>, erik <RemoveMEerikKILLspamspamNETWURX.NET>
writes
>Hi,
>
>I've been following this very long thread and waiting for it all to come
>together for me.... No Such Luck.
>I don't own an o-scope and haven't used one since school but would very
>much like have one. So please forgive me if this comes across as a
>stupid question. But I just don't understand.
>
>Isn't the scope's probe isolated from the AC supply?
>Isn't / shouldn't the scope's signal ground be isolated from the scopes
>earth ground? After all, the signal ground should simply be a reference
>and could be placed anywhere. (Right?)

It's usual for the signal ground and the earth ground to be the same,
they are both connected to the metal case of the scope.
--

Nigel.

       /--------------------------------------------------------------\
       | Nigel Goodwin   | Internet : nigelgSTOPspamspamspam_OUTlpilsley.demon.co.uk     |
       | Lower Pilsley   | Web Page : http://www.lpilsley.demon.co.uk |
       | Chesterfield    | Official site for Shin Ki Ju Jitsu         |
       | England         |                                            |
       \--------------------------------------------------------------/

1999\02\23@161142 by John Payson

flavicon
face
|but most scopes have their signal ground connected to the earth ground. i'm
|not exactly sure why, but this is a fact. that the case is on earth ground
|is obvious, but why exactly the signal ground often is connected to this --
|i'd like somebody to please jump in here and tell me why. after all, this
|seems to be a major cause of trouble, so it seems to me there must be a
|really important reason for it -- and i should know it :)

[light bulb goes on]

I wonder if the connection of the scope ground to earth is to
avoid hazards which could otherwise result from connections
*BETWEEN* the ground leads on multiple probes?

Example: Bob is using one of the probes on his 'scope to look at
his television set.  Since the TV has a hot chassis, Bob puts a
"cheater" on the scope and is careful not to touch anything he
shouldn't.  After working on the thing for awhile, Bob decides to
go to lunch.

While Bob is at lunch, Joe needs to check out what signals are
present on a serial data cable.  Joe sees that probe #1 is some-
where in the TV set and doesn't want to interrupt what Bob was
doing with it, so he simply uses probe #2.  Unfortunately, when
he connects the probe #2 ground lead to the data cable, that now
makes the entire data cable and everything connected to it "hot"
(if it was floating) or blows stuff up (if it was ground-based).

Obviously, the operative words here are "don't do that", but this
type of situation may be what scope designers were trying to avoid
when they earthed the scope ground.

1999\02\23@170451 by Gerhard Fiedler

picon face
At 15:12 02/23/99 -0600, John Payson wrote:
>Obviously, the operative words here are "don't do that",

wow, it sure needs some phantasy to come up with this one! :)

>but this
>type of situation may be what scope designers were trying to avoid
>when they earthed the scope ground.

but they sure couldn't avoid that somebody blows his head off because he
connects the scope ground to a hot wire -- that would be the same joe who
uses bob's scope :))

hm... instead of cheating with the earth pin on the plug, has anybody tried
to disconnect a scope's circuit ground from the (earthed) case? for the
"single user mode" safe user? :)

ge

1999\02\23@170650 by Gerhard Fiedler

picon face
At 15:12 02/23/99 -0600, John Payson wrote:
>Obviously, the operative words here are "don't do that",

wow, it sure needs some phantasy to come up with this one! :)

>but this
>type of situation may be what scope designers were trying to avoid
>when they earthed the scope ground.

but they sure couldn't avoid that somebody blows his head off because he
connects the scope ground to a hot wire -- that would be the same joe who
uses bob's scope :))

hm... instead of cheating with the earth pin on the plug, has anybody tried
to disconnect a scope's circuit ground from the (earthed) case? for the
"single user mode" safe user? :)

ge


and a PS: the "better" scopes, with double isolation and no earth or
battery-operated and therefore earth-free, may still have the common signal
ground between channels (most do). your argument would apply to these as
well, so i guess it's at least not the whole story.

1999\02\24@071137 by paulb

flavicon
face
Gerhard Fiedler wrote:

> hm... instead of cheating with the earth pin on the plug, has anybody
> tried to disconnect a scope's circuit ground from the (earthed) case?
> for the "single user mode" safe user? :)

> and a PS: the "better" scopes, with double isolation and no earth or
> battery-operated and therefore earth-free, may still have the common
> signal ground between channels (most do).  Your argument would apply
> to these as well, so i guess it's at least not the whole story.

 Oh come on!  How easy do you suppose it is to design an isolated
input module including an isolation coupler with a DC to 200 MHz
bandwidth?  Do you even have any idea how to do it?  Easy enough for an
ECG machine with 300 *Hertz* bandwidth of course.  Otherwise my estimate
is the scope would cost about *five times* the usual price.

 Has anyone ever *seen* a 200 MHz scope with probe ground isolated from
frame to 1 kV spec.?
--
 Cheers,
       Paul B.

1999\02\24@073624 by Andy Kunz

flavicon
face
>  Has anyone ever *seen* a 200 MHz scope with probe ground isolated from
>frame to 1 kV spec.?

Check out the Tek THS-72x scopes.  My THS-720 is 600 V isolation probe
ground-to-probe ground, and to wall wart.

Andy

  \-----------------/
   \     /---\     /
    \    |   |    /          Andy Kunz
     \   /---\   /           Montana Design
/---------+   +---------\     http://www.montanadesign.com
| /  |----|___|----|  \ |
\/___|      *      |___\/     Go fast, turn right,
                              and keep the wet side down!

1999\02\24@133427 by Gerhard Fiedler

picon face
At 23:05 02/24/99 +1000, Paul B. Webster VK2BZC wrote:
>Gerhard Fiedler wrote:
>> and a PS: the "better" scopes, with double isolation and no earth or
>> battery-operated and therefore earth-free, may still have the common
>> signal ground between channels (most do).  Your argument would apply
>> to these as well, so i guess it's at least not the whole story.
>
>  Oh come on!  How easy do you suppose it is to design an isolated
>input module including an isolation coupler with a DC to 200 MHz
>bandwidth?  Do you even have any idea how to do it?  Easy enough for an
>ECG machine with 300 *Hertz* bandwidth of course.  Otherwise my estimate
>is the scope would cost about *five times* the usual price.

yes, i do have some ideas, but that's not the issue here. obviously =they=
know how to do it, since scopes like that are on the market.

but the question i was writing about is that obviously your light bulb idea
on why a normal scope's signal ground is connected to earth ground would
apply even more to those expensive double isolation barrier and battery
operated earth-free scopes (with the exception of the ones which have
isolated channels). note that this has nothing at all to do with completely
separated input modules (as eg. the tek-scope has andy mentioned, and
cheaper as 5 times the usual price, albeit still =somewhat= more
expensive), only with the whole scope's circuit (and therefore the signal
ground) being earth-free.

for these, the designers obviously thought it wasn't necessary to have
signal ground connected to earth in order to protect the equipment from the
incidents you thought of. suppose your bill (it was bill,wasn't it? :) has
an expensive philips scope with double isolation and no earth connected to
signal ground, or he is working with a fluke portable scope, also without
earth connection (but with the same signal ground on both channels), and
then joe comes along and takes the second channel and does what you meant
him to do -- and bum! something blows up.

i figure there is a reason why they think that it's necessary in one case
to connect signal ground to earth and in the other case not. it might be
that it's too difficult/expensive to isolate the whole circuit from the
case in order to pass regulatory test for 1kV or so, even if it's earthed,
i don't know -- but i'm pretty sure your scenario isn't part of the
thoughts they have :)

ge

1999\02\24@170941 by John Payson

flavicon
face
|and a PS: the "better" scopes, with double isolation and no earth or
|battery-operated and therefore earth-free, may still have the common signal
|ground between channels (most do). your argument would apply to these as
|well, so i guess it's at least not the whole story.

If you can come up with a reliable way for a scope manufacturer to
earth a battery-powerd scope you oughta be famous. >:*3

Perhaps what might be good would be to have a scope with a weakly-
earthed floating chassis (e.g. earthed through 1meg) and an indicator
which would show if the chassis/signal ground was at a significantly
different potential from earth ground?


Attachment converted: wonderland:WINMAIL.DAT 2 (????/----) (0002A93E)

1999\02\24@175316 by Gerhard Fiedler

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At 16:10 02/24/99 -0600, John Payson wrote:
>|and a PS: the "better" scopes, with double isolation and no earth or
>|battery-operated and therefore earth-free, may still have the common signal
>|ground between channels (most do). your argument would apply to these as
>|well, so i guess it's at least not the whole story.
>
>If you can come up with a reliable way for a scope manufacturer to
>earth a battery-powerd scope you oughta be famous. >:*3

i could come up with a rather simple and reliable way to earth the ground
of the double isolated ones driven from mains voltage, but this doesn't
make me famous -- at least not as of yet :)

>Perhaps what might be good would be to have a scope with a weakly-
>earthed floating chassis (e.g. earthed through 1meg) and an indicator
>which would show if the chassis/signal ground was at a significantly
>different potential from earth ground?

i guess that's a =whole= lot better than just disconnecting the earth wire.
and not too difficult to do either. but i guess it's not what safety rules
(who's that here in the usa? UL?) would require, that's only something you
can do to your scope at your own risk.

but before i go any long way to work around this, i rather hook either the
DUT (preferred, in any case) or the scope (if hooking the DUT is not
possible) or both (if some third device is involved, which has to be
earthed) on to their (separate) isolation transformers :)

ge

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