Floating the 'scope

Sat Mar 06, 2010 5:05 am   

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.

Sat Mar 06, 2010 5:05 am   

"George Hairoil"




** 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

Sat Mar 06, 2010 5:05 am   

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


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

Sat Mar 06, 2010 5:05 am   

"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

Sat Mar 06, 2010 5:16 am   

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


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.

Sat Mar 06, 2010 5:41 am   

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

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...)

Sat Mar 06, 2010 7:31 am   

"John Larkin" <jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote in message
news:lfj3p5p2vk92ani4ti7bg9uqjd0n5uahud_at_4ax.com...

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

Sat Mar 06, 2010 8:36 am   

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


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.

Sat Mar 06, 2010 10:09 am   

On Sat, 06 Mar 2010 18:36:27 +1100, John Tserkezis
<jt_at_techniciansyndrome.org.invalid> 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

Sat Mar 06, 2010 12:51 pm   

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


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.

Sat Mar 06, 2010 1:03 pm   

On 6 Mar 2010 03:09:01 -0600, The Phantom <phantom_at_aol.com> 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.

Sat Mar 06, 2010 1:17 pm   

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


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

Sat Mar 06, 2010 1:22 pm   

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


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?

Sat Mar 06, 2010 1:58 pm   

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


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.

Sat Mar 06, 2010 5:01 pm   

"Hammy"



** Another recipe for dead technicians.

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



...... Phil