This is my understanding anyway, if I'm wrong I'd dearly love to understand why!

That's correct.
So how is that different from what I said?

 

That's correct.
So how is that different from what I said?

I said

"Touching a "live" wire in the DUT will lead to electric shock, whereas if the scope were isolated that could not happen."

I said that because a grounded, non-isolated scope when attached to an isolated DUT will ground the DUT, but if the scope too is isolated (neither scope nor DUT now have a connection to ground) then you cannot get a shock.

But you then said:

"Yes it can. The scope may be isolated, but you are not. "

So I guess I don't understand what you're saying, I don't see how you can say "Yes" - that one can get an electric shock if both the DUT and the scope are isolated, no connection at all to ground on the secondary side of the isolation transformer.

 

that one can get an electric shock if both the DUT and the scope are isolated,

Okay, I see the misunderstanding.
I was replying to the question of whether the the equipment or the UUT should be isolated and I stated the UUT.
If both are isolated, then obviously there's no path to ground.

 

So as long as the secondary winding has no connection to safety ground then having a secondary safety earth ground is allowed?

 

So as long as the secondary winding has no connection to safety ground then having a secondary safety earth ground is allowed?

Since the isolated secondary can't generate any voltage to the safety earth ground, that would seem to be true.

 

Seems like I have a repair to do then... Thx

 

Just be careful. Isolation transformers for utility line operated test and measurements are banned at many industrial sites. Your instrument might be floating but you as the operator are not. The operator panel and knobs and buttons in the test instrument are not usually designed for large differentials between chassis 'ground' and the operator.

https://www.newark.com/pdfs/techarticles/tektronix/FFM.pdf

 

variac (why I needed one) - to bring up a repaired audio amplifier.

Isolation transformer: (why I needed one): TV's that had a 120V filament. Usually 2-prong cord.

Isolation transformer (when I really needed one and used it): the primary of a transformer was shorted to ground somewhere in the middle of the winding, so metal parts were at 60VAC off of ground. With the isolation, It didn't cause me issues.

Isolation transformer: Older tube type stuff has capacitors connected to ground from line. They are not the x/y capacitors of today. They can cause excessive leakage currents.

Isolation transformer (work) see lots of videos by www.powervar.com about power conditioning They re-reference the secondary to clean up the ground. They claim you need both an isolation transformer with the refereneced ground and a surge suppressor to do any real good. I believe them. Powervar used to be called OneAC.

I think I have a re-referenecd ground on the one I built, but i have only used an isolation xformer on ungrounded stuff.

You will find it useful working on line operated switching power supplies.

 

I want to get an isolation transformer, in my mind this eliminates the need to ground connected appliances and or if they are grounded (as in their plug has a ground pin), there's no path to ground if I accidentally touch something I shouldnt

In my mind this would eliminate a respected contributor of AAC.

They claim you need both an isolation transformer with the refereneced ground and a surge suppressor to do any real good. I believe them

I do not think your loved ones would find solace in your untimely demise, because of your beliefs.
One must possess a command of electrical theory and the physics behind ones endeavors or be condemned by them. So do what I do... get the undergraduates to flip the switch!

 

No matter what you do about isolation, the outlet you work from should have GFCI.
You still may get a shock (as has been my experience when I tripped one) but it won't be fatal.

 

the outlet you work from should have GFCI.

The isolated transformer's outlet or wall outlet?

 

Just be careful. Isolation transformers for utility line operated test and measurements are banned at many industrial sites. Your instrument might be floating but you as the operator are not. The operator panel and knobs and buttons in the test instrument are not usually designed for large differentials between chassis 'ground' and the operator.

https://www.newark.com/pdfs/techarticles/tektronix/FFM.pdf

That article from Tektronix has to be trusted, but I do not understand something and this is what's coming up time and again as we discuss this, here a snapshot from that article:




Surely, if neither the UUT nor the scope are grounded (e.g. they are both fed by the secondary of an isolation transformer and that secondary has no concept of a grounded side) then V1 will always be zero volts?

There is simply no "galvanic" path between an operator touching the scope/UUT and ground. The ground only has a galvanic path to the transformer primary, because the outlets neutral is - to all intents and purpose - tied to ground, but the secondary has no connection whatsoever to ground.

So how can V1 not be zero volts?

(The diagram and explanation makes sense (to me) only if the UUT is grounded, but they don't call that out clearly in that article).

 

The isolated transformer's outlet or wall outlet?

I expect it can only apply to the wall outlet, because the GFCI is sensitive to leakage currents (to ground) and relies on a ground referenced outlet, I don't think a GFCI would do anything on the secondary side, no leakage (to ground) is possible, there's no way that the current into one side of the secondary can ever differ from the current emerging from the other side.

 

The isolated transformer's outlet or wall outlet?

Wall outlet, which will protect from any fault to ground in the isolation system.
Of course, obviously nothing will protect you if you touch both sides of the isolation transformer output.

the GFCI is sensitive to leakage currents (to ground) and relies on a ground referenced outlet

No, the GFCI requires no ground connection to work.
It measures the difference in current between hot and neutral, and trips if there's more than a few mA difference, indicating current to ground.

 

No, the GFCI requires no ground connection to work.
It measures the difference in current between hot and neutral, and trips if there's more than a few mA difference, indicating current to ground.

That's what I meant, it detects (by measuring current delta) leakage (to ground), leakage has to be a possibility so requires a ground referenced outlet, yes the GFCI itself cares not about ground, but unless the outlet's neutral is grounded no leakage is possible so the device serves no purpose, there can never be a difference in current unless the supply its attached to is a grounded supply.

 

Lots of back and forth here. So here's my answer to HOW an isolation transformer works. As to "When" - That's been covered quite well.

For clarification: Ground is referring to EARTH GROUND.

Below:
Primary side:
Touch Line and Neutral - current flows - you get a shock.
Touch Line and Ground - current flows - you get a shock.
Touch an OPEN Neutral and Ground - current can flow - you can get a shock.
Secondary side:
Touch either leg of the secondary and ground - No current flows - No Shock!

Whether an appliance is grounded or not - if it's isolated from the mains power (from the pole) there is no shock potential for you or a blown Circuit Under Test. Even if you connect a scope and touch the wrong thing - my understanding is there's no blown scope or circuit.

 

Lots of back and forth here. So here's my answer to HOW an isolation transformer works. As to "When" - That's been covered quite well.

For clarification: Ground is referring to EARTH GROUND.

Below:
Primary side:
Touch Line and Neutral - current flows - you get a shock.
Touch Line and Ground - current flows - you get a shock.
Touch an OPEN Neutral and Ground - current can flow - you can get a shock.
Secondary side:
Touch either leg of the secondary and ground - No current flows - No Shock!

Whether an appliance is grounded or not - if it's isolated from the mains power (from the pole) there is no shock potential for you or a blown Circuit Under Test. Even if you connect a scope and touch the wrong thing - my understanding is there's no blown scope or circuit.
View attachment 231562

Other than "Touch an OPEN Neutral and Ground - current can flow - you can get a shock" I agree, this is exactly how I perceive it. I think in the Tektronix warning they should have pointed out that their UUT was assumed to be connect to the mains (not isolated) and only the scope was isolated, that would make their explanation consistent.

You'll have to explain what an open neutral is, given that the neutral conductor is physically tied to ground at the customers house I cannot see how there can ever be a voltage between neutral and ground.

And also actually "Touch either leg of the secondary and ground - No current flows - No Shock!" is only true if the device attached to the secondary has no connection to ground, if it does you're in trouble. I guess this depends on whether the device internally connects "neutral" to ground though, I guess that is never the case - so you may be correct here.

 

An "Open" neutral can be a loose wire nut, loose screw terminal, broken wire anywhere before it gets back to the panel. Touch before the open circuit and you can get a shock. The illustration below is of a lightbulb that SHOULD be on. But because of a bad (open) neutral there is no return path, no current flow. Touch the lightbulb side of the circuit and current will flow through you. But the filament will act as a resistor you will still get quite a good shocking. Potentially terminal. Hence; you CAN get a shock. But it depends on other factors; where you touch, what the load is, etc.

 

An "Open" neutral can be a loose wire nut, loose screw terminal, broken wire anywhere before it gets back to the panel. Touch before the open circuit and you can get a shock. The illustration below is of a lightbulb that SHOULD be on. But because of a bad (open) neutral there is no return path, no current flow. Touch the lightbulb and current will flow but the filament will act as a resistor and you will get quite a good shocking. Potentially terminal. Hence; you CAN get a shock. But it depends on other factors; where you touch, what the load is, etc.
View attachment 231568

OK that makes sense, always a risk in house wiring too unless one is careful.

 

Did some clarification editing in my statement. But it seems you got the point.