Floating the 'scope

[George Herold]

Why do they make 'scope inputs ground referenced to the third wire of
the AC plug? Today I smoked the 10 ohm resistor that was separating
The AC ground from the 'ground' of my circuit. I'd forgotten to float
the 'scope and connected -15V to the ground clip of the scope probe.

George H.

 

[Phil Allison]

"George Hairoil"

Why do they make 'scope inputs ground referenced to the third wire of
the AC plug?

** For obvious safety reasons.

It helps reduce the number of technicians getting electrocuted.

But YOU go right ahead and float yours - by all means.




..... Phil

 

[John Larkin]

On Fri, 5 Mar 2010 19:05:54 -0800 (PST), George Herold
<ggherold@gmail.com> wrote:

Why do they make 'scope inputs ground referenced to the third wire of
the AC plug? Today I smoked the 10 ohm resistor that was separating
The AC ground from the 'ground' of my circuit. I'd forgotten to float
the 'scope and connected -15V to the ground clip of the scope probe.

George H.

That's a safety thing.

Our TPS2024 has four truly floating channels and floating trigger
input. You can hook a scope ground clip anywhere.

John

 

[Phil Allison]

"George Hairoil"


I've been floating 'scopes for.... say since the early '80's. I
haven't been biten by it yet. (Though I've made plenty of other
mistakes.) What are the obvious safety reasons?


** My god you are stupid.



..... Phil

 

[John Tserkezis]

On 6/03/2010 2:49 PM, Phil Allison wrote:

"George Hairoil"
I've been floating 'scopes for.... say since the early '80's. I
haven't been biten by it yet. (Though I've made plenty of other
mistakes.) What are the obvious safety reasons?

** My god you are stupid.

Welcome to an entirely new class of stupid, Phil.

At a place I used to work at, the cro on my bench was floated by the
previous "tech", he just forgot to tell anyone about it. And forgot to
label it too. No clues at all unless you measured it, or took it apart
to actually look.

Made worse by the fact it was a dual trace cro, and could have done the
job without floating it.

I can only presume these idiots have been doing for so long, they stop
caring about the fact you hadn't had to do it since two trace (or more)
CROs appeared on the market.
Hey, I don't care if they want to kill themselves, but their idiocy
will continue to kill others well into the future.

I'm starting to think that being dropped as a baby isn't enough to
explain this level of stupid.

 

[George Herold]

On Mar 5, 10:10 pm, "Phil Allison" <phi...@tpg.com.au> wrote:

"George Hairoil"

Why do they make 'scope inputs ground referenced to the third wire of
the AC plug?

** For obvious safety reasons.

It helps reduce the number of technicians getting electrocuted.

But  YOU go right ahead and float yours  - by all means.

....  Phil

Hi Phil, now don't yell at me too much, it's Friday night here and
I'm half in the bag.

I've been floating 'scopes for.... say since the early '80's. I
haven't been biten by it yet. (Though I've made plenty of other
mistakes.) What are the obvious safety reasons?

In today's mistake I was looking at the voltage from a heater
circuit. The heater runs off +15 and -15 but knows 'nothing' about
ground. (Well nothing except for the bypass caps, but no DC
connection.) Things weren't quite working right and I worried about
some oscillation. I hooked up the 'scope to look.... opps.

George H.

(who believes that you make progress only as fast as you can make
mistakes...)

 

[Paul E. Schoen]

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:lfj3p5p2vk92ani4ti7bg9uqjd0n5uahud@4ax.com...

On Fri, 5 Mar 2010 19:05:54 -0800 (PST), George Herold
ggherold@gmail.com> wrote:

Why do they make 'scope inputs ground referenced to the third wire of
the AC plug? Today I smoked the 10 ohm resistor that was separating
The AC ground from the 'ground' of my circuit. I'd forgotten to float
the 'scope and connected -15V to the ground clip of the scope probe.

George H.

That's a safety thing.

Our TPS2024 has four truly floating channels and floating trigger
input. You can hook a scope ground clip anywhere.

Handheld and portable scopes often have floating inputs, and it is
convenient when measuring circuits that have voltages which are referenced
at other than earth ground. But line powered scopes are usually in metal
enclosures that need to be at earth ground potential for shielding purposes,
and thus the probes are connected to the chassis and the shields are an
extension of this ground. You can float the scope by disconnecting the
ground at the AC plug, and placing the scope on an insulated surface, but
the capacitance of the scope enclosure will likely affect the measurement by
inducing noise currents into the circuit where the ground clip is connected.

Hand held scopes have less material connected to the ground, but the ground
clip can still inject noise and it will have a much lower impedance than the
tip, which ideally will have a 9 Mohm resistor close to the tip so that
minimal capacitance is applied. The best way is to use differential mode
between two identical input channels, ie, A-B, but there is a limit to how
much common mode voltage can be tolerated before the signal will be
distorted.

Paul

 

[John Tserkezis]

On 6/03/2010 5:31 PM, Paul E. Schoen wrote:

Handheld and portable scopes often have floating inputs, and it is

Even if a portable scope is double insulated, that's not always safe.

If you clip the "ground" of the scope probe to a live line, that shield
and any metal bits all the way to the scope are also live.

If the ground clip is exposed, it'll bite (I've frequently seen it, and
on some probes, that's the way it's built) , if the insulation is
damaged, it'll bite.

Also note that none of the cable insulations I've seen are rated for
high voltage, only the centre conductor is good for the specified
voltage, not the outer to the outside world.

Your only avenue is to completely isolate yourself from ground, which
is going to prove to be a bit difficult, considering the capacitance you
can't really avoid..
Forget about wearing an antistatic strap. I've worked in some
environment where it's mandatory, so that's going to be a bit of a
challenge.

Or use an earth leakage breaker, and have the thing cut out on you a
dozen times a day even under normal use.




There are too many things that can go wrong. There are no procedures
to ensure all the avenues are covered and enforced to prevent shocks
from this method.

 

[The Phantom]

On Sat, 06 Mar 2010 18:36:27 +1100, John Tserkezis
<jt@techniciansyndrome.org.invalid> wrote:

On 6/03/2010 5:31 PM, Paul E. Schoen wrote:

Handheld and portable scopes often have floating inputs, and it is

Even if a portable scope is double insulated, that's not always safe.

If you clip the "ground" of the scope probe to a live line, that shield
and any metal bits all the way to the scope are also live.

If the ground clip is exposed, it'll bite (I've frequently seen it, and
on some probes, that's the way it's built) , if the insulation is
damaged, it'll bite.

Also note that none of the cable insulations I've seen are rated for
high voltage, only the centre conductor is good for the specified
voltage, not the outer to the outside world.

Tektronix has probes designed to solve these problems:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13471&lc=EN

They are specifically intended for floating use with the TPS2000 series scopes,
which, as John mentioned, have true isolated inputs.

Or, you can use these probes with an ordinary scope:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13415&lc=EN

Your only avenue is to completely isolate yourself from ground, which
is going to prove to be a bit difficult, considering the capacitance you
can't really avoid..
Forget about wearing an antistatic strap. I've worked in some
environment where it's mandatory, so that's going to be a bit of a
challenge.

Or use an earth leakage breaker, and have the thing cut out on you a
dozen times a day even under normal use.




There are too many things that can go wrong. There are no procedures
to ensure all the avenues are covered and enforced to prevent shocks
from this method.

 

[John Tserkezis]

On 6/03/2010 8:09 PM, The Phantom wrote:

Tektronix has probes designed to solve these problems:
http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13471&lc=EN

They are specifically intended for floating use with the TPS2000 series scopes,
which, as John mentioned, have true isolated inputs.

Or, you can use these probes with an ordinary scope:
http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13415&lc=EN

Ah, I stand corrected.

Though it still needs to be said, if someone is of the mentality that
they can freely dismantle a piece of equipment that says "do not
dismantle" to intentionally remove a safety mechanism (against the law
the last time I checked), what is the likelyhood they're going to use
the right probes for the job?

After all, if they obviously don't care about the now live metal bits
of the casing, what makes you think they're going to worry about a probe
in the first place?

What if they have to make an adjustment?
Make sure they just touch the plastic bits and avoid the metal bits?
Make sure they unclip the probe first?

More importantly, if they live through all those don't cares, what
makes you think they're going to care enough to warn other users?

Yeah right, I see that happening.

 

[Hammy]

On 6 Mar 2010 03:09:01 -0600, The Phantom <phantom@aol.com> wrote:

Tektronix has probes designed to solve these problems:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13471&lc=EN

They are specifically intended for floating use with the TPS2000 series scopes,
which, as John mentioned, have true isolated inputs.

Or, you can use these probes with an ordinary scope:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13415&lc=EN

Those are nice expensive but nice.

You should use an isolation transformer on any line powered device
your testing. You can usually find a 500VA one for under 100 bucks. I
got a TEMA 500VA one for 45 bucks from Newark. Well worth the money.

Its not really a good idea to float your scope.

 

[Hammy]

On Sat, 06 Mar 2010 07:03:56 -0500, Hammy <spam@spam.com> wrote:

On 6 Mar 2010 03:09:01 -0600, The Phantom <phantom@aol.com> wrote:



Tektronix has probes designed to solve these problems:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13471&lc=EN

They are specifically intended for floating use with the TPS2000 series scopes,
which, as John mentioned, have true isolated inputs.

Or, you can use these probes with an ordinary scope:

http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13415&lc=EN

Those are nice expensive but nice.

You should use an isolation transformer on any line powered device
your testing. You can usually find a 500VA one for under 100 bucks. I
got a TEMA 500VA one for 45 bucks from Newark. Well worth the money.

Its not really a good idea to float your scope.

Anyone use any of PICO's differntial probes?

This ones only $350.00 Canadian.

http://canada.newark.com/pico-technology/oscilloscope-x10-x100/active-differential-oscilloscope/dp/02M0836

 

[John Tserkezis]

On 6/03/2010 11:03 PM, Hammy wrote:

You should use an isolation transformer on any line powered device
your testing. You can usually find a 500VA one for under 100 bucks. I
got a TEMA 500VA one for 45 bucks from Newark. Well worth the money.
Its not really a good idea to float your scope.

Unfortunately, it was the cost of the transformer that makes people do
things like float their scopes.

"It does the same job, and it's a lot cheaper".
Till someone dies. How much does liability cost?

 

[Hammy]

On Sat, 06 Mar 2010 23:22:59 +1100, John Tserkezis
<jt@techniciansyndrome.org.invalid> wrote:

On 6/03/2010 11:03 PM, Hammy wrote:

You should use an isolation transformer on any line powered device
your testing. You can usually find a 500VA one for under 100 bucks. I
got a TEMA 500VA one for 45 bucks from Newark. Well worth the money.
Its not really a good idea to float your scope.

Unfortunately, it was the cost of the transformer that makes people do
things like float their scopes.

"It does the same job, and it's a lot cheaper".
Till someone dies. How much does liability cost?

Well the one I paid $45 on sale last year is still only $70. Cheaper
then a new scope or a funeral.;-)

http://canada.newark.com/tenma/72-6670/isolation-transformer/dp/97C7982

If you look around there's usually always a decent one for under $100.

 

[Phil Allison]

"Hammy"

You should use an isolation transformer on any line powered device
your testing.

** Another recipe for dead technicians.

The horrendous fallacy in the thinking is way beyond the likes of retards
like Hammy.



...... Phil

 

[Jamie]

John Larkin wrote:

On Fri, 5 Mar 2010 19:05:54 -0800 (PST), George Herold
ggherold@gmail.com> wrote:


Why do they make 'scope inputs ground referenced to the third wire of
the AC plug? Today I smoked the 10 ohm resistor that was separating
The AC ground from the 'ground' of my circuit. I'd forgotten to float
the 'scope and connected -15V to the ground clip of the scope probe.

George H.


That's a safety thing.

Our TPS2024 has four truly floating channels and floating trigger
input. You can hook a scope ground clip anywhere.

John

So does my cheap crappie OWEN 20 mhz hand held. That also includes

isolation between the DMM inputs.

 

[Hammy]

On Sun, 7 Mar 2010 03:01:46 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"Hammy"


You should use an isolation transformer on any line powered device
your testing.


** Another recipe for dead technicians.

The horrendous fallacy in the thinking is way beyond the likes of retards
like Hammy.



..... Phil



Really and I thought that an isolation transformer removes the earth

ground reference making it safe to connect your probe on say the
primary side of a DC/DC converter? That is what kills you isn't it you
are connected to ground and the line voltage is referenced to ground?
There still exist the high voltage but it no longer has an earth
ground reference. So it's not to say you can't still kill yourself
particularly if you don't follow the one hand rule but it reduces the
risk and it makes it safe to hook your probe up to anywhere reasonable
without sparks flying.

If this isn't the case then I guess I've been pretty fortunate over
the past ten years. Because following that line of thought I've yet to
fry a scope or myself.

Ideally you would want a wide BW active HV differential probe but they
are usually in the thousands of dollars so an ISO tranny will at least
provide some additional safety and be cheaper on your test equipment.
You should probably still use an ISO tranny with the diff probes for
safety anyways.

Some people prefer to hook there scope to the iso tranny because the
tranny will effect the accuaracy or alter the measurement I think
that's stupid. If you need to make measurements like that on a regular
basis in a professional environment tell your employer to buy you the
right dam equipment like the diff probes.

And over to you Philly.

Oh I almost forget the obligatory profanities

You fucking ASD fuckwit!!!!!!!!!!

 

[krw@att.bizzzzzzzzzzzz]

On Sat, 06 Mar 2010 22:51:11 +1100, John Tserkezis
<jt@techniciansyndrome.org.invalid> wrote:

On 6/03/2010 8:09 PM, The Phantom wrote:

Tektronix has probes designed to solve these problems:
http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13471&lc=EN

They are specifically intended for floating use with the TPS2000 series scopes,
which, as John mentioned, have true isolated inputs.

Or, you can use these probes with an ordinary scope:
http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13415&lc=EN

Ah, I stand corrected.

Though it still needs to be said, if someone is of the mentality that
they can freely dismantle a piece of equipment that says "do not
dismantle" to intentionally remove a safety mechanism (against the law
the last time I checked),

I'd like to see a citation for the above. Sure, such modifications would
leave one exposed to litigation and it may not be legal to SELL a modified
widget in some jurisdictions, but I'd like to see a cite that says it's
illegal for one to modify one's own property.

what is the likelyhood they're going to use
the right probes for the job?

A competent engineer certainly would. These things *are* designed for
engineers to use.

After all, if they obviously don't care about the now live metal bits
of the casing, what makes you think they're going to worry about a probe
in the first place?

Who, exactly, is "they"?

What if they have to make an adjustment?
Make sure they just touch the plastic bits and avoid the metal bits?

When working with dangerous voltages, that's my plan.

Make sure they unclip the probe first?

Huh?

More importantly, if they live through all those don't cares, what
makes you think they're going to care enough to warn other users?

Other users should know something. Obviously not a good assumption in some
cases.

Yeah right, I see that happening.

Can I watch you?

 

[John Tserkezis]

On 7/03/2010 5:27 AM, krw@att.bizzzzzzzzzzzz wrote:

I'd like to see a citation for the above. Sure, such modifications would
leave one exposed to litigation and it may not be legal to SELL a modified
widget in some jurisdictions, but I'd like to see a cite that says it's
illegal for one to modify one's own property.

So your legal advice is that it's ok to disable a safety device?
More so, and to not replace it with something else after you've
disabled it?

what is the likelyhood they're going to use
the right probes for the job?

A competent engineer certainly would. These things *are* designed for
engineers to use.

Use yes, butcher no.

After all, if they obviously don't care about the now live metal bits
of the casing, what makes you think they're going to worry about a probe
in the first place?

Who, exactly, is "they"?

Hacks, keep up with it, the questions are only going to get harder.

What if they have to make an adjustment?
Make sure they just touch the plastic bits and avoid the metal bits?

When working with dangerous voltages, that's my plan.

Good luck, it's been nice knowing you.

Make sure they unclip the probe first?

Huh?

Yeah, thought as much.

More importantly, if they live through all those don't cares, what
makes you think they're going to care enough to warn other users?

Other users should know something.

So you're saying that it's MY fault that YOU'VE disabled a safety device?

Yeah right, I see that happening.

Can I watch you?

Guessing I'll be living longer, that will be *me* watching *you*.

 

[krw@att.bizzzzzzzzzzzz]

On Sun, 07 Mar 2010 06:46:19 +1100, John Tserkezis
<jt@techniciansyndrome.org.invalid> wrote:

On 7/03/2010 5:27 AM, krw@att.bizzzzzzzzzzzz wrote:

I'd like to see a citation for the above. Sure, such modifications would
leave one exposed to litigation and it may not be legal to SELL a modified
widget in some jurisdictions, but I'd like to see a cite that says it's
illegal for one to modify one's own property.

So your legal advice is that it's ok to disable a safety device?

You have a real reading comprehension problem, don't you?

More so, and to not replace it with something else after you've
disabled it?

See above.

what is the likelyhood they're going to use
the right probes for the job?

A competent engineer certainly would. These things *are* designed for
engineers to use.

Use yes, butcher no.

Your uninformed opinion.

After all, if they obviously don't care about the now live metal bits
of the casing, what makes you think they're going to worry about a probe
in the first place?

Who, exactly, is "they"?

Hacks, keep up with it, the questions are only going to get harder.

Obviously they are, since you can't answer. Although, since you can't read it
does make it hard to answer.

What if they have to make an adjustment?
Make sure they just touch the plastic bits and avoid the metal bits?

When working with dangerous voltages, that's my plan.

Good luck,
Luck has nothing to do with it. I've been around long enough so I'm not

afraid of my shadow, unlike some here.

it's been nice knowing you.

I can't say the same.

Make sure they unclip the probe first?

Huh?

Yeah, thought as much.

If you're not touching anything "hot", who cares if you unclip the probe?

More importantly, if they live through all those don't cares, what
makes you think they're going to care enough to warn other users?

Other users should know something.

So you're saying that it's MY fault that YOU'VE disabled a safety device?

Apparently you're too stupid to live - not my problem.

Yeah right, I see that happening.

Can I watch you?

Guessing I'll be living longer, that will be *me* watching *you*.

You must be DimBulb's brother.