Oscope ground lift

[johansen]

should be self explanatory.

40k ohms in series with the neon bulb, in series between the two grounds.
i have one on my hakko iron as well.


i'm sure this has saved my projects several times..

[Harvs]

Are you just using this with a linear PS based scope?  What's the voltage beakdown of the neon bulb? Will the neon conduct enough to open the mains fuse in case of fault?

Interesting idea.

[johansen]

i would rather have my oscope floating at 400 volts than my face burnt when the ground lead vaporizes...
its a digital standalone scope. i also lifted the ground for my analog scopes long time ago.

neon's typically light up at 40vac or more.

[Bored@Work]

 

[Harvs]

40k ohms in series with the neon bulb, in series between the two grounds.

Ok I miss-read that.  I thought you were using the Neon's breakover to keep the ground within a reasonable differential.  But you're just disconnecting the ground and using a neon as an indicator.

There's been many threads already arguing about this, so I'm not going to drag it up again.

[alm]

i would rather have my oscope floating at 400 volts than my face burnt when the ground lead vaporizes...

Grab one of the BNC connectors and tell me again how much you like that scope floating at 400 V.

I'm not particularly concerned if you want to compete for the Darwin award, just don't go around telling others to do the same.

[calmtron]

Get one of these puppies instead: Toroid ISB-060W Medical Grade Isolation Transformer

You get the ground lift and a survivable maximum of 100 µA leakage current.

[alm]

That's not the only problem with a scope, the problem is that a scope contains exposed metal that gets connected to external circuits. What is the current if you clip one of the probe ground leads to a ground-referenced +400 V rail, then touch the BNC socket with your other hand, while the scope is connected to the medical grade isolation transformer?

[Andy Watson]

This has to be a troll. Lifting the ground on an "ESD Safe" iron - "saves projects", from what?

[johansen]

This has to be a troll. Lifting the ground on an "ESD Safe" iron - "saves projects", from what?

lol, esd safe iron means the tip is grounded.

I don't solder on anything dangerously live, but lifting the tip sure prevents damage to cmos and stuff.. for example, trying to find the cold solder joint while the circuit is running.. its impossible to do otherwise...

[IanJ]

Ditto.

Ian.

[c4757p]

Am I the only one who really doesn't ever find the need to do something like this? It seems unnecessarily reckless when there are many other ways you could accomplish this without having test equipment with a 400V chassis.

for example, trying to find the cold solder joint while the circuit is running.. its impossible to do otherwise...

I guess you get an A for thinking outside the box... but dude, you can just climb out of the box, no need to dig a hole under it with a spoon...

[nctnico]

It seems we have another contestant for a Darwin award 
http://en.wikipedia.org/wiki/Darwin_Awards

[ivan747]

I don't solder on anything dangerously live, but lifting the tip sure prevents damage to cmos and stuff.. for example, trying to find the cold solder joint while the circuit is running.. its impossible to do otherwise...

I have had the same situation, this circuit could actually be of help if it was switchable. Otherwise, isolation transformers are more or less cheap, if you get them close to you. You can even make your own with 2 transformers back to back.

I guess you get an A for thinking outside the box... but dude, you can just climb out of the box, no need to dig a hole under it with a spoon...

This has to be the Darwin Awards smart quote of the year.

[crisr]

I agree that using an isolation transformer to power your DUT is a better option, but if you don't have one, and if you are using a scope to test something that is say >100V relative to earth ground, having you scope chassis (read: BNCs and GND cal tab, since most of the rest of the scope is plastic and you would not do anything like that with USB connected) at that voltage does not make what you are doing a lot more dangerous... c'mon, even if you are using a properly earthed scope with an isolated differential probe and you are careless you are far more likely to touch some live part of the DUT than some exposed GND on the scope!

As the OP said, your mains-earth referenced scope's leads vaporizing on your face of forming an arc if you touch the alligator clip to the wrong point in your DUT is a lot worse and a lot more likely than the touching a floating scope's BNC.

I never had to use a soldering iron on I live circuit, though, but I understand the point about finding / fixing cold joints, and while that solution may not be really ESD-safe, also follows the same principle.

[johansen]

I never had to use a soldering iron on I live circuit, though, but I understand the point about finding / fixing cold joints, and while that solution may not be really ESD-safe, also follows the same principle.

I don't consider hard grounded anything ESD safe, with regard to the maximum current discharge limited by the parasitic inductance of the 3 foot cord. but i don't work on hypersensitive stuff.

regarding the Oscope,  there is  two 4nF caps from line and neutral to ground, so that makes 8nF from the scope ground to the line. this is an issue with SMPS scopes, where as the old analog scopes just had the parasitic capacitance of the 60hz transformer to worry about.
I suppose i could put three inductors inside the scope to prevent the scope acting as a snubber, for example if i'm trying to measure the waveform at a switch that is floating at some high frequency waveform.

I have floating 0-13.8 vac available at a reasonable limit of 50 amps and i have another transformer boosting that back up to 120, so i have about 5 amps of 0-130vac available double isolated.
Another pair of transformers make isolated 40/80/120/160 vac available at about 3 KW continuous. (which i keep forgetting to use)

[alm]

I agree that using an isolation transformer to power your DUT is a better option, but if you don't have one, and if you are using a scope to test something that is say >100V relative to earth ground, having you scope chassis (read: BNCs and GND cal tab, since most of the rest of the scope is plastic and you would not do anything like that with USB connected) at that voltage does not make what you are doing a lot more dangerous...

Not all scopes are plastic toy scopes . Older and high-end scopes tend to have a metal chassis bonded to earth. The OP's scope may very well be a big honking Tek 465 with nice big metal knobs. And even in the case of plastic scopes, do you think any testing went into isolation between the user and ground? Any metal surface near the user will be connected to ground (eg. LCD bezel). Those rotary encoders are not CAT II+ 400 V (vaporizing a ground lead does not sound like CAT I to me) rated. Especially with those dodgy knobs that were used on some Rigol scopes. Why do you think all manufacturers recommend against this procedure for high-energy circuits?

c'mon, even if you are using a properly earthed scope with an isolated differential probe and you are careless you are far more likely to touch some live part of the DUT than some exposed GND on the scope!

Of course you shouldn't be careless, but safety is about making it hard to make fatal mistakes. Balancing on a cord 10 meters in the air can also be safe if you're careful, but that doesn't make it a good workplace.

As the OP said, your mains-earth referenced scope's leads vaporizing on your face of forming an arc if you touch the alligator clip to the wrong point in your DUT is a lot worse and a lot more likely than the touching a floating scope's BNC.

If those kinds of energies are involved, then you shouldn't be touching the DUT or probe will powered. You should also use face protection and probably get the HV diff probe out.

Another issue I don't get about floating your scope is that you lose all those advantages of a floating ground as soon as you clip on your first probe. Clipping the second probe to a different potential can still vaporize the ground lead and damage your DUT. If you're only using one scope anyhow, which not put the two channels in subtract mode for pseudo-differential operation?

I suppose i could put three inductors inside the scope to prevent the scope acting as a snubber, for example if i'm trying to measure the waveform at a switch that is floating at some high frequency waveform.

Capacitance to ground is going to kill your CMRR at high frequencies anyhow. Even the old scopes with linear power supplies could easily have 1nF capacitance to ground with the ground lead disconnected.

[crisr]

Not all scopes are plastic toy scopes . Older and high-end scopes tend to have a metal chassis bonded to earth. The OP's scope may very well be a big honking Tek 465 with nice big metal knobs. And even in the case of plastic scopes, do you think any testing went into isolation between the user and ground? Any metal surface near the user will be connected to ground (eg. LCD bezel). Those rotary encoders are not CAT II+ 400 V (vaporizing a ground lead does not sound like CAT I to me) rated. Especially with those dodgy knobs that were used on some Rigol scopes. Why do you think all manufacturers recommend against this procedure for high-energy circuits?

Yes, indeed. It's just when I read "scope" in this forum I automatically think "plastic DSO" such as Rigol DS1052E (which I own, by the way) 
But at ordinary mains-socket (<=240V @ 20A) voltage and power levels, mostly any piece of plastic however thin will provide enough isolation, even an eventual crack on the knob will probably not matter as long you do not touch exposed metal directly.

Of course you shouldn't be careless, but safety is about making it hard to make fatal mistakes. Balancing on a cord 10 meters in the air can also be safe if you're careful, but that doesn't make it a good workplace.

If those kinds of energies are involved, then you shouldn't be touching the DUT or probe will powered. You should also use face protection and probably get the HV diff probe out.

I didn't say it would be safe, what I meant is that in the specific situation of probing a live (relative to earth) DUT, having the scope chassis earthed will not improve safety, and might (in the event of a short) even be worse, regardless of the energy levels involved (I admit there were some exaggeration of the consequences, and as a CAT I device, an ordinary DSO should not be used to probe mains-connected circuits anyway). 

Another issue I don't get about floating your scope is that you lose all those advantages of a floating ground as soon as you clip on your first probe. Clipping the second probe to a different potential can still vaporize the ground lead and damage your DUT. If you're only using one scope anyhow, which not put the two channels in subtract mode for pseudo-differential operation?

Yes, the pseudo-differential operation is probably a (slightly) better idea, since using a second channel negates purpose of lifting the scope's ground.

Of course, all of what I said should be taken on a "hacking" perspective, based on self experience, and I don't mean any safety standard should be disregarded.