Products > Test Equipment
Floating Scopes
tautech:
From another thread not far back that discussed safe measurement techniques:
https://www.eevblog.com/forum/beginners/dumb-oscope-question/
--- Quote from: jbb on May 22, 2017, 10:00:20 pm ---
--- Quote from: DanielSpokane on May 22, 2017, 08:47:27 pm ---...Does that mean if I take a tube amp and plug it in without the ground plug, the amp is isolated from the mains (ie the hot and the neutral are isolated via the power transform and the ground is not hooked up)?
...
Seems like that could be dangerous too since the amp is then not grounded. But for a few tests maybe ok?... assuming I test to make sure the grounded components aren't carrying any potential with respect to ground. Thoughts?
--- End quote ---
In general, removing/disconnecting a ground pin is a risky plan. There are some situations where it can help a lot with testing, but it's not something I'd do lightly. When you have gained some experience in electrical systems design you can make your own evaluation.
I suggest the following thought experiment:
* In the beginning, the chassis was grounded
* Maybe some internal circuitry - on the isolated side of the transformer - was grounded. Maybe the DC 0V (aka Ground).
* You disconnect (or 'lift') the chassis ground from the power plug
* As this point in time you're probably OK
* You want to measure something, so you connect your 'scope ground to DC 0V and measure.
* No problems yet
* You want to measure something else - maybe the voltage across the output transformer primary.
* You connect your scope probe to DC+ (could be well over 100V depending on amplifier).
* The '0V' rail is now forced down (because DC+ is connected to mains earth via your scope), so amplifier chassis is now live to negative(DC+) volts.
* You rest your hand on the chassis while poking around with the 'scope probe and get a shock.
When we consider the above scenario, we see that it takes 3 steps to shock yourself: steps 3, 8 and 10. Steps 3 and 8 were 'necessary' for your measurement plan. Step 10 was a mistake. Maybe you're tired. Maybe someone distracted you. Maybe you dropped the probe and went to recover it. What steps 3 and 8 did was put you one mistake away from danger.
And now I look like a safety nutter / the fun police... When you receive a significant electrical shock, several things can happen. You can suffer electrical burns at the point of contact (painful, not so bad). You can fall off your chair (or ladder for extra fun times) and injure yourself. You can suffer an immediate cardiac event (e.g. ventricular fibrillation), collapse and possibly die. Good thing you've got a buddy there keeping an eye on you, right?
You can even damage part of your heart and/or set up an irregular rhythm, feel OK, and then suffer a cardiac event when you go to sleep for the night, and your partner wakes up next to a dead body. So if you do receive a significant shock (static discharge onto a door handle doesn't count) you should probably go see your doctor or local emergency department to get checked out. It will likely take at least a couple of hours, or all bloody day depending on your triage level. (Pro tip: tell them you had an electric shock as soon as you get in because a) it's important and b) it'll increase your triage priority level.) It may cost a lot of money (depending on public health care, insurance or lack thereof).
So that's why I wouldn't float the ground.
--- End quote ---
Electro Detective:
I'm jumping ship (for once) and following the majority.. to recommend LEAVE WELL ALONE if not 100% sure,
or best practice is to dip into the piggy bank for a differential probe as front line measurement cannon fodder :-+
There are TOO MANY VARIABLES that are almost IMPOSSIBLE to document and break down here for the casual user wanting a 'fast fix'
which usually means a good chance for an earth/ground related BANG!
'Isolation' devices are no guarantee of safety, accuracy or performance once hooked up with faulty or suspect wired DUTs and other test gear powered by mains electricity, nor do they have crystal ball capability
BANGS can cause injuries, blindness, or facilitate an inexperienced or mis-informed prodder's early funeral,
or more important, cost MONEY due to damage :-[
FWIW I'm still a 'die hard' isolation transformer fan :clap: for many uses not just lab use,
but I have a good clue how the whole deal works,
check and and prep before I switch on, fail-safes galore,
and document working scenarios for future use, so no guessing games or 'impatience oopsies' :-[
IanMacdonald:
Slightly oftopic as it's not a test instrument, but everything said here also applies to earthed soldering irons. (I was going to say in spades, but actually it applies to all shapes of tip :groan:)
If you use an iron with a 240v element then earthing is essential for safety, since an insulation failure could put mains on the tip. No question about that, and it's one of the reasons that direct-mains irons are a poor choice for any serious work.
Most decent bench irons run from 24v though, and the question then arises, is earthing desirable or not?
Static discharge is necessary if working on MOSFETs, but that does not actually require a hard, zero-ohm earth.
The downside of earthing irons which don't actually require an earth for safety, is the risk of damage to equipment due to residual charge on electrolytics being dumped to earth through any sensitive component you are soldering on or near.
That, and if soldering on high current gear with an earthed iron you are in the same position, safetywise, as using a multimeter which does not have proper HBC fuses. If a terminal is unexpectedly live, that is much the same as forgetting to take your unprotected DMM off the 10A range before measuring voltage.
alm:
A hard connection to ground is obviously not optimal for ESD, since it would be causing a discharge with a large peak current. However, I do not agree that a hard connection to earth is obviously not required for safety or that it is similar to a DMM fuse. What is the insulation rating of the handle of a low voltage iron? The hand holding the solder and/or tweezers? Touching a node that unexpectedly has high voltage on it could easily shock the user if the iron is not grounded.
If the iron is grounded, then a GFI or fuse should trigger, protecting the user. So I would argue that a grounded iron is essential when working on circuits with non-SELV (separated extra-low voltage) voltages on them, unless you are fully isolated from the board/solder/tools, or you can insure that there are no dangerous voltages in your circuit. So basically quite similar to scopes ;). Only in circuits with only low voltages but a very high current capacity (e.g. battery banks) with no up converters could you argue that a tip with a larger resistance to ground is safer.
That a careless user may kill some semiconductors is not a safety issue.
Safar:
I have one solution else.
It's "semi-stupid" but anyway it can work. I use it for low voltage (devireg bus). Direct connection earth probe connector to any pole completely kill the signal. And I had no differential probe at this time. So I simple connected one signal pin of one channel to one pole, and second channel to another pole. And not connected earth connectors of probes to DUT. And had set math channel on scope A-B. So I get 1 differential channel from 2 scope channels.
Not sure how it safe for high voltage, but I guess that it possible to avoid danger voltage on BNC connectors if use 1:10 divider at least and not to disconnect probes. And you can still use eathed scope.
Of course I understand that HV differential probe is better.
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