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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Wed 07, 2018 4:53 pm 
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This topic is mildly complex and, is now getting pretty confusing. I will try to bring some order into this topic at least as best I can figure it out.

1. Contrary to some of the comments already made, putting a GFI on the output of the isolation transformer CAN prevent a shock hazard in the specific instance that the DUT (Device Under Test), the one being worked on, is plugged into the GFI and contact is made between the hot line out of the isolation transformer and anything connected to the safety (round plug /green or uninsulated conductor) of the DUT such as the chassis or a metal case that is connected to the DUT's safety ground conductor. Otherwise, no additional benefit, but GFIs are inexpensive and fairly reliable, so why not? There is no real downside. I can't see any benefit to using a GFI isolation transformer for any of the test equipment, only the DUT.

2. For electrical personal protection, any isolation transformer should have NO electrical continuity between the safety ground of the 120 volt, line input and anything connected in any way to the output of the isolation transformer.

These fairly simple principles cover personal protection from 120 volt line voltages and possible touching earth ground considerations, but there is another type of isolation to consider. Basically this boils down to how to use the isolation transformers for the test equipment or anything else (e.g. external power supplies) used for troubleshooting the DUT.

In the incident I related in the beginning of this thread, I clearly had either read the schematic wrong and connected the scope ground to the wrong place or I had simply not connected it to what I what I meant to. The danger was not that of my being electrocuted, but protecting the equipment from my mistakes. So now, two more general rules:

1. If the DUT has no voltages (except the 120 volts AC power to the DUT through a proper isolation transformer) above, say, 50 volts, i.e. no likelyhood of voltages high enough to cause personal injury, then it is best to run each separate test device or other equipment off of separate isolation transformers. This potentially could protect the the DUT from the troubleshooter.

2. However, if the DUT has any potential of dangerous (to humans) voltages, then it safest to put the DUT on a separate isolation transformer and all the ancillary equipment on another, separate isolation transformer as someone already suggested. If these were on separate isolation transformers, it would be possible to have the ground of one device at a dangerous potential with respect to another because of a voltage source in the DUT itself. Better to risk shorting out or damaging the DUT than risk electrocution. I think someone already suggested this, but I can't remember who.

I think that the above is correct, but I have been know to make mistakes. In addition to one or two curmudgeons, there are some sharp folks who read these, and if I have made any mistakes, I'm sure we will hear about it!

Regards,

Mike


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Wed 07, 2018 8:08 pm 
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Mike Higgins wrote:
Contrary to some of the comments already made, putting a GFI on the output of the isolation transformer CAN prevent a shock hazard in the specific instance that the DUT (Device Under Test), the one being worked on, is plugged into the GFI and contact is made between the hot line out of the isolation transformer and anything connected to the safety (round plug /green or uninsulated conductor) of the DUT such as the chassis or a metal case that is connected to the DUT's safety ground conductor.

Completely untrue.

In fact, it violates the design principle of the isolation transformer by definition.

Neither output line from the iso has any circuit relationship to ground. There is no voltage differential between ground and either output line. You might measure some voltage with a DMM, but that's just capacitive coupling, not real voltage.

- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Wed 07, 2018 8:23 pm 
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Mike;

It will help if you directly address any comment that you feel is incorrect. Here is the essence of the issue--from my earlier post:
Quote:
The only way to make a GFCI work downstream of an ISO trans is to effectively recreate the configuration of the building supply, where one side of the line is grounded at the distribution point. This however defeats the purpose of having the iso trans in the first place.
I'll attempt to explain why this is true, and then ask you to address where I made a mistake.

First, the purpose of GFCI is to protect against any unwanted current flow back to "ground central"--namely the earth ground where power enters the building. A "ground fault" is any current path that eventually goes to ground central---it could be the safety ground in any outlet, a water pipe, or the damp floor in the shop, shed, basement, etc.

A GFCI works by sensing the difference in the currents in the hot and neutral conductors. If there is a difference, it means current is getting back to ground central through an "unauthorized" path. In order the sense the difference, one side of the line going into the GFCI must be referenced to ground central. Without this, there is no way to wire a GFCI to provide the intended protection.

Again--if you disagree with some statement here, let us know your basis.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Wed 07, 2018 11:25 pm 
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pixalany and Leigh,

I don't know how to append pictures to this, so I will try to describe it. If the GFI is connected as I said, and if you touch the "hot" wire from the output side of the GFI somewhere in the DUT, which is connected to the hot output side of the transformer through the GFI and the DUT power cord, and also touch the chassis, if the chassis is "grounded" i.e. connected to the ground terminal of the GFI through the DUT power cord, then current will flow out of the transformer hot terminal, through the GFI, through the black wire in the DUT cord, into the DUT, through you to the chassis of the DUT, back through the safety ground wire (green usually) in the DUT power cord, and back to the neutral side of the transformer, which is also connected to the safety ground connection of the GFI (as well as to the "line in" neutral of the GFI), thereby bypassing the neutral (white) wire in the GFI and the power cord of the DUT. Since the current that flows through you goes through the GFI hot, but is not matched by the current through the neutral back through the GFI, it will trip, hopefully protecting you. If you draw this out, I think it will become obvious.

I think that "What we have is a failure to communicate.", that famous quote. The term "ground" is ambiguous and I think that is the real problem! Call it "ground" or anything you want, but draw the circuit out. It WILL work.

Mike


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Wed 07, 2018 11:32 pm 
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You're making up a lot of rules here!

Quote:
1. Contrary to some of the comments already made, putting a GFI on the output of the isolation transformer CAN prevent a shock hazard in the specific instance that the DUT (Device Under Test), the one being worked on, is plugged into the GFI and contact is made between the hot line out of the isolation transformer and anything connected to the safety (round plug /green or uninsulated conductor) of the DUT such as the chassis or a metal case that is connected to the DUT's safety ground conductor. Otherwise, no additional benefit, but GFIs are inexpensive and fairly reliable, so why not? There is no real downside. I can't see any benefit to using a GFI isolation transformer for any of the test equipment, only the DUT.


Respectfully, the answer is no. A GFCI outlet operates by monitoring the current differential between the hot and neutral terminals. If this exceeds 5-mA for a modern outlet, the assumption is made that there is current leakage somewhere (e.g. to ground). This causes the outlet to trip. So while it is not necessary for the outlet itself to be grounded, the power source does have to be ground referenced--which it is not if you are using a safety isolation transformer. Without a ground reference there isn't any way to get a current differential between hot and neutral and the outlet will never trip. One would be better off with an ordinary outlet since it doesn't convey a false sense of security.

Quote:
2. For electrical personal protection, any isolation transformer should have NO electrical continuity between the safety ground of the 120 volt, line input and anything connected in any way to the output of the isolation transformer.


Here, the answer is, it depends. A medical isolation transformer or a line noise isolation transformer relies on having one side of the secondary grounded in order to maximize shielding. It is also required by code to protect the secondary winding insulation from puncture by static discharge. The requirement is waived for safety isolation transformers intended for applications such as ours, but even in those, the frame and chassis of the transformer are supposed to be grounded, the electrostatic shield (if so equipped) is supposed to be grounded, and if the outlet is a grounding type, the ground terminal should be grounded. This is so if anybody plugs something in expecting the ground to be there, they won't get hurt. You don't have to use the ground if you don't want it; just plug in a two-prong plug.

Quote:
1. If the DUT has no voltages (except the 120 volts AC power to the DUT through a proper isolation transformer) above, say, 50 volts, i.e. no likelyhood of voltages high enough to cause personal injury, then it is best to run each separate test device or other equipment off of separate isolation transformers. This potentially could protect the the DUT from the troubleshooter.


No sir. The purpose of ground on a test instrument (or any grounded appliance for that matter) is to blow a fuse or trip a circuit breaker in the event a short develops between the AC line and the case or front panel of the instrument. If you put something having a grounded plug on a safety isolation transformer, that purpose is defeated, for the same reason a GFCI outlet won't work. Therefore, only the hot chassis equipment you are working on needs to be connected to an isolation transformer.

If you connect a grounded lead from a piece of grounded test equipment to the AC line, one of two things are going to happen. If you hit the hot side, you will blow a fuse or trip a breaker. If you connect to the neutral side, a current will flow between neutral and ground through the test equipment, since the two are not usually at the same voltage. Small currents can cause hum and noise problems which will plague your measurements. Large currents, perhaps due to fault conditions elsewhere in the building, can be dangerous. These wayward currents are what an isolation transformer is meant to suppress. It also protects you from shock in case you get between your grounded test equipment and a "hot" chassis.

Quote:
2. However, if the DUT has any potential of dangerous (to humans) voltages, then it safest to put the DUT on a separate isolation transformer and all the ancillary equipment on another, separate isolation transformer as someone already suggested. If these were on separate isolation transformers, it would be possible to have the ground of one device at a dangerous potential with respect to another because of a voltage source in the DUT itself. Better to risk shorting out or damaging the DUT than risk electrocution. I think someone already suggested this, but I can't remember who.


No again. AC line and B+ voltages are present in radios whether they are connected to an isolation transformer or plugged into a wall. TV sets still develops B-boost, focus, and CRT anode voltages. So an isolation transformer does not do anything to protect you from voltages within a chassis. The only safety it provides is that it reduces the possibility of ground shock fault. Putting other equipment on isolation transformers will not provide additional safety, and may actually make things less safe as noted above.

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Last edited by Chris108 on Mar Wed 07, 2018 11:41 pm, edited 2 times in total.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Wed 07, 2018 11:33 pm 
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Mike Higgins wrote:
f the GFI is connected as I said, and if you touch the "hot" wire from the output side of the GFI
Mike,

There is a fundamental flaw in your description.

The output of a GFCI has NO "hot" wire, none.
That term suggests a voltage (not leakage) relative to ground.
The output of a GFCI has no relationship to ground whatsoever.

- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Wed 07, 2018 11:57 pm 
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Perhaps it is time for some definitions in this discussion. I will not pronounce any, leaving that up to those with more theoretical knowledge than myself. But it seems that if one were to define "ground" as EARTH GROUND and "that other ground as "neutral", perhaps it might make the discussion easier to understand. While there is an entirely different discussion that could be centered around whether ground and neutral are "bonded" or connected together, and where, and what effects that might have on both isolation transformers AND GFCI systems ....aieeeee makes the head spin really. But I digress ...

Now, it seems that were it can get even more confusing, is with an isolation transformer (the completely isolated type) breaking the connection between, say, the house AC line outlet, and the chassis of the DUT. You still have, in effect, two grounds..... the one that is connected to the wall outlet, and the one you are standing on. If that is a damp basement floor, and you are barefoot, you are still in for a shocking experience, no ??

If the issue is one of safety, use an iso transformer and don't stand on a concrete floor. If the issue is one of electrical noise, there are a million other factors that can come into play. And of course, you can still hook up a piece of test equipment, the ground lead for instance, to your DUT chassis, and defeat the entire purpose of an iso transformer methinks. Seems that any attempt to use a GFCI on the isolated side of the isolation transformer, is going to un-isolate it. Or will the GFCI act as a small current interrupter even on the floating side of the iso?? I'm not quite sure what is inside those GFCI critters...

Is this more or less correct ? 8)

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 12:07 am 
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Barry H Bennett wrote:
...if one were to define "ground" as EARTH GROUND and "that other ground as "neutral"

Therein lies the essence of the problem.

Both "hot" and "neutral" have meaning ONLY in a system that has one line connected to earth ground. If neither line is grounded, those terms are totally meaningless.

- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 12:13 am 
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Quote:
Both "hot" and "neutral" have meaning ONLY in a system that has one line connected to earth ground. If neither line is grounded, those terms are totally meaningless.

We are discussing a system that DOES have one line connected to earth ground. Namely the AC service in the house. I say the use of the word meaningless, is itself meaningless :-D

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 12:26 am 
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Barry H Bennett wrote:
Quote:
Both "hot" and "neutral" have meaning ONLY in a system that has one line connected to earth ground. If neither line is grounded, those terms are totally meaningless.
We are discussing a system that DOES have one line connected to earth ground.
No we're NOT.

We're discussing the OUTPUT of a true isolation transformer.
Neither output line has any relation to earth ground at all.

That's the point you fail to understand.

- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 12:50 am 
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Quote:
No we're NOT.

Forgive me, but I was discussing BOTH sides. So were a few others.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 12:53 am 
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Barry H Bennett wrote:
Quote:
No we're NOT.

Forgive me, but I was discussing BOTH sides. So were a few others.

Discussing the input side of an iso is just discussing house wiring.

There is absolutely no aspect of that subject unique to isolation transformers.

- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 1:01 am 
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Unique or not, the house wiring side of this isolation transformer IS pertinent to the discussion, since that is precisely what we are trying to isolate ourselves from.. Is your intent to simply be as condescdending as possible? I fully understand the difference between the AC line side of the iso, and the isolated side. That changes nothing in my comments. Perhaps YOU fail to understand what it was I was trying to say.

Leigh, I am rather shocked (pun intended) that you'd take this tone. I have and still do respect your knowledge of electronics and electricity, and your tone towards my comments is totally unwarranted. You very likely possess superior knowledge than I do, but that really is no justification to berate someone else for their comments.

Regardless, I think I"ll bow out of this one and just watch. There's hardly any need for a discussion on isolation transformers to become so contentious.

cheers :)

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 1:06 am 
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Quote:
Perhaps it is time for some definitions in this discussion. I will not pronounce any, leaving that up to those with more theoretical knowledge than myself. But it seems that if one were to define "ground" as EARTH GROUND and "that other ground as "neutral", perhaps it might make the discussion easier to understand. While there is an entirely different discussion that could be centered around whether ground and neutral are "bonded" or connected together, and where, and what effects that might have on both isolation transformers AND GFCI systems ....aieeeee makes the head spin really. But I digress ...


Ground and neutral are not the same thing. Neutral is a current-carrying conductor, part of the electrical system that carries current back to the distribution point to make a complete circuit. In most systems it is approximately at ground potential, but depending on load and distance may have some voltage drop. Ground in power systems means local earth ground. It is not a current carrying conductor in normal building wiring. It is provided for safety since anything connected to it will be forced to local earth potential in the event of an electrical fault or short circuit.

Ground and neutral are only permitted to be connected together at one point in a building wiring system and that is at the service entrance. The reason for this connection is to ensure that if there is an insulation breakdown from primary to secondary in the outdoor distribution transformer, the high voltage will be shorted to ground through the neutral side, blowing the distribution line fuses. This prevents high voltages from getting inside houses. No other connections between neutral and ground are permitted, because so doing would put the ground wire in parallel with neutral and it would become a current-carrying conductor.

The problem for vintage electronics is, that connection between neutral and ground also ensures that a circuit can be completed from the hot side of the line to anything else that is grounded. Thus it becomes possible to complete circuits from hot to unintended things that happen to have a connection to ground like damp concrete basement floors. A safety isolation transformer provides a source of AC power that is not referenced to ground on either side, so no circuit can be completed to a grounded object or surface. The chances of ground fault shock are thereby reduced.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 1:13 am 
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Chris108 wrote:
A safety isolation transformer provides a source of AC power that is not referenced to ground on either side, so no circuit can be completed to a grounded object or surface. The chances of ground fault shock are thereby reduced.

Exactly.

- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 1:45 am 
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Since I sometimes make a mistake and think I didn't, and even though I was sure (well almost) that I was right this time, I just went to the shop to check things out.

First, I checked the assertion that the ac power plug round ground pin would never be electrically connected to a chassis. My two year old Rigol DS1102E scope measured 0 ohms between the round pin on the power plug and the outside of the BNC connector that the test leads go into. Just for fun, I check my ancient HP 1741a scope. Same thing.

Second, I have several isolation transformers, but I have one that I made myself from a one to one, 1 amp power transformer and to which I also connected a standard, Home Depot, GFI on the output as I described. I also have a commonplace small plug in tester with the usual two yellow and one red light to indicate whether a standard AC outlet is properly wired. It also has a button that you can press cause a current path from hot to safety ground to see if the GFI is functioning properly, and the GFI tripped exactly as it should have.

If someone can tell me how to post the pictures to this site, I will go out tomorrow and take enough pictures to confirm what I just said.

Mike


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 1:56 am 
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Take a picture, save it on your computer. Take it in lo-resolution or use any photo or paint or similar program to convert it to a 640X480 or so photo. Might be able to go larger but that should suffice. There is a link somewhere on "how to post photos to the forum. (256K is the max file size)

But anyway, at the bottom of the window where you are typing your messages, there's a red "upload attachment" red bar. hit the "browse" button below and to the left of that, and then go browse for your de-rezzed photo file. Click on it, then hit the "add the file" button to the right of the browse button below. If you want to include the photo in the body of the text as below, click the "place inline" button to the right of your now included file just below this window. Hope that gets it for ya.
Attachment:
BACON BISC EGGSs.jpg
BACON BISC EGGSs.jpg [ 178.68 KiB | Viewed 566 times ]

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 2:12 am 
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Mike Higgins wrote:
S
Second, I have several isolation transformers, but I have one that I made myself from a one to one, 1 amp power transformer and to which I also connected a standard, Home Depot, GFI on the output as I described. I also have a commonplace small plug in tester with the usual two yellow and one red light to indicate whether a standard AC outlet is properly wired. It also has a button that you can press cause a current path from hot to safety ground to see if the GFI is functioning properly, and the GFI tripped exactly as it should have.
Mike

That works if the "ground pin" is connected to one one of the inputs to the GFCI----as you discussed earlier. But---the ground fault to worry about is the one that goes to GROUND---AKA Earth. You have no protection unless one of the GFCI inputs is connected the GROUND. Once you do that, you have defeated the purpose of the iso transformer.

For any scenario, draw the current path.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 4:00 am 
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V
Pixallany,

That's all I've been trying to say from the very beginning. It doesn't have to do with grounding per se. The point that I've been trying to make is that putting the GFI on the other side of the isolation transformer can make things slightly safer in terms of slightly reduced electrocution risk. That's all. It protects you from a situation that could kill you. It's not very likely that that situation would arise, but it could, and if it did, you would be protected.

Regards,

Mike


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 4:07 am 
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pixellany wrote:
the ground fault to worry about is the one that goes to GROUND
...
For any scenario, draw the current path.

As you're so fond of saying... follow the current.

In a vintage iso, neither of the two output lines is connected to earth ground.
The current through the two lines must always be equal because there's no other path.

- Leigh

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