Isolation transformer earth pin

[winboxes]

I am trying to wrap my head around this YT video.

Can someone help me comprehend this. I realize that he was talking about having the scope on the isolation transformer but what if the device under test is plugged via the isolation transformer? This would mean that the device under test LIVE and NEUTRAL is isolated from the mains. Even if the scope is then referenced to mains earth how could it cause a short or be an additional issue? I would understand if NEUTRAL and EARTH is tied together on the secondary side or if the device under test has the Earth wire connected to the Ground for some reason.

I am confused as it seems some are advocating to have the Earth wire that is connected to the output outlet on the isolation trasnfromer removed. I think I can see at least one good reason as to why the Earth is connected to the output outlet.

[amirm]

He is talking about the case where the isolation transformer does not isolate ground.  In that case, your isolated Line is still referenced to ground.  If you then hook up your scope ground to Line in your circuit (by accident or otherwise), you will create a short.

[FlyingHacker]

He is talking about the case where the isolation transformer does not isolate ground.  In that case, your isolated Line is still referenced to ground.  If you then hook up your scope ground to Line in your circuit (by accident or otherwise), you will create a short.

An isolation transformer can isolate the mains (hot and neutral) but still have the ground connected. Many argue this is the "proper" wiring for an isolation transformer, as the ground, for metal cases, etc. is still earth grounded. With this setup you can still hook a (properly earth grounded or floating) scope's ground lead to the line or the neutral. This type of isolation transformer is isolated in that neutral is in no way tied to ground (like it is in your building breaker box).

Generally, never float your scope. Float the DUT.

[helius]

The danger in these scenarios is always with the oscilloscope ground clip. If you remove the ground clip from the probe completely, then you are safe probing anything, as long as its peak voltage is within the probe specs. In fact, there is no purpose in "floating the scope" at all, and you should never ever do that.
You absolutely do not refer to the ground clip as "the negative probe".

[CoffinDodger]

How do you go about setting your reference then for the probe without the ground clip? Ch1 on the test point and Ch2 on the board's ground reference and use the math function to get a differential or some such?  I ask because my instructors are hell bent on making me use an isolation transformer because we have some specialized protective relays (I believe a Basler BE1-851 digital overcurrent relay) that have a certain board with a different ground potential some such that'll cause some arcing and such if we don't use the ungrounded iso-transformer.

[helius]

That's just it, the ground clip does not "set the reference," because it is already set to GROUND!
The only thing it can do is to improve signal integrity when attached to a known GROUND point.

When you float the DUT with an isolation transformer, the ground clip can be attached to one of the floating nodes of the DUT circuit. Please note that this does not "set the reference", since the reference is already set to GROUND. What it does do is to reference that node in the floating circuit to ground, which makes the circuit no longer floating (meaning that the operator is no longer protected from electrocution).

[CoffinDodger]

hmm i'm seeing the danger here.  So if the device is floating in it's own little world of potential and you suddenly clip your ground clip to it you just shorted it to ground and either overcurrent your scope and/or overcurrent the DUT.  Is that about right?

[helius]

To be "shorting something to something", there needs to be a circuit between those points, in other words a loop.
A transformer isolates its primary side from its secondary side: no galvanic connection exists between them, therefore there is no loop. You can't "short" a single wire without a return path.

Try this experiment: take a linear DC power pack, one of the big heavy square ones with 2 prongs. plug it in, then put one probe of a DMM into the barrel. Put the other probe into the electric outlet: hot, neutral, or ground, it doesn't matter. There will be no current through the DMM in either case. Did you "short" the power supply to ground?

[KerryW]

There is NEVER a reason to plug a scope into an isolation transformer!  The scope already has a transformer inside that isolates the DC circuits from Hot/Neutral.

If the isolation transformer has Ground pass-through then plugging it into the wall is exactly the same thing.
If the isolation transformer doesn't have Ground pass-through then plugging it into the wall with a 3 prong adapter is exactly the same thing.  It is no safer, and no more dangerous.

Mains voltage is pretty simple, it is amazing how few people understand it (but think they do).  It's not so much what people don't know as it is what people know that just ain't so.

[amirm]

An isolation transformer can isolate the mains (hot and neutral) but still have the ground connected.

I know and that is the case I covered in my post.  I am unclear why you are quoting me here.

[Ian.M]

I knew I'd seen and commented on this horrible video before: https://www.eevblog.com/forum/projects/isolation-transformer-and- variac-safety/  There's also a link from user 'Shock' in that topic that's worth following.

There are two problems with floating a scope with normally grounded shell input connectors.:

• If not done correctly, it stresses the insulation in its PSU, and one mains spike later you risk having a rather crispy ex-scope
• The case controls and connectors aren't rated to provide safety isolation so its a hazard to the user and if metal cased, to anything next to it on the bench.

The second issue is why its now generally frowned upon to float a scope.  In the days when men were men, high voltage switchgear consisted of 4' tall open frame knife switches, and PPE for Arc flash was a pair of  leather gloves and a heavy woollen overcoat with a tall collar, floating normally grounded metal chassis test equipment wasn't unusual - I've done it my self with the scope carefully placed on a sheet of plate glass and a severe shock hazard tag hung from the test loop on the front panel.  However it was a PITA as one had to shut down to make any adjustments.  Mind you, this was in an establishment that regarded it as acceptable practice for students to clean the commutators of 630V 240HP DC motors or slip rings of similar size 415V three phase induction motors while they were running to get stable readings for their lab work!   Nowadays, OSHA, HSE or whatever your national health & safety body is will throw the book at you for unsafe working practices if you float normally grounded test equipment without a full insulating shield round it blocking access to the controls and a full risk assessment that proves its impossible to make the required measurement without floating the test equipment, and if anyone is injured, the fines will force the company board of directors to sell their first-born into slavery!