Floating the 'scope
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From: John Tserkezis on 5 Mar 2010 23:16 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.
From: Paul E. Schoen on 6 Mar 2010 01:31 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:lfj3p5p2vk92ani4ti7bg9uqjd0n5uahud(a)4ax.com... > On Fri, 5 Mar 2010 19:05:54 -0800 (PST), George Herold > <ggherold(a)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
From: John Tserkezis on 6 Mar 2010 02:36 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.
From: The Phantom on 6 Mar 2010 04:09 On Sat, 06 Mar 2010 18:36:27 +1100, John Tserkezis <jt(a)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.
From: John Tserkezis on 6 Mar 2010 06:51
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. |