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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 12:54 pm 
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Mike Higgins wrote:
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

Back to basics:
The GFCI works--and is useful for safety--only if one of the inputs is tied to the building ground (earth). The issue I cited earlier is that this defeats the purpose of having the isolation transformer in the first place

There are many situations where you cannot make things better by adding parts. They way I operate, adding a GFCI after an ISO trans, would simply make things worse---for the simple reason that I could no longer connect my (grounded) test equipment to the DUT.

The ultimate key to safety is good work habits.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 3:00 pm 
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Pixallany,

Just because GFIs are usually used to protect from ground fault situations, that does not mean they can not be used for other protection purposes. As you pointed out, in the particular situation under consideration here, as long as the isolation transformer functions properly, there is no ground fault issue, but that doesn't mean the GFI can't be effectively used to protect from another kind of fault situation that involves the wire in the standard three wire electric cord that I have been calling the "safety ground" because 1) I don't know if there is a commonly recognised standard name for this conductor and 2) in ordinary usage this wire is connected ultimately to the electrical system ground and only carries current in fault (potentially unsafe) conditions. If my usage of the term has caused confusion, I am truly sorry, but whatever name you give to it, it still can function as I noted. You seem to be saying that because GFIs are meant for ground fault conditions, that they won't work for anything else. So much for the first paragraph.

The second paragraph is really the crux of the situation. I am arguing that UNDER CERTAIN CONDITIONS you can use isolation transformers to free your self from this restriction--UNDER CERTAIN CONDITIONS. You might argue that it is safer to just be conservative under ANY AND ALL conditions, since you might not know enough about the equipment you are troubleshooting to make such a decision, and I think this would be a valid agument, especially for tube type equipment since tube equipment almost always has voltages in excess of safe levels. However, most solid state equipment operates with low voltage levels where you could safely use the scheme I proposed. I made this proposal not as an absolute truth but as something to consider.

In regards to your third paragraph, I couldn't agree more. I think what you are implying is that if you make it a habit of always doing it the "right" way that you will automatically avoid the risk of not doing it "right", and that certainly is true in general, but that doesn't mean that in some situations that there can't be another "right", or at least "safe", way.

Regards,

Mike


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 7:51 pm 
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GFCIs are intended to protect against unwanted current that goes to GROUND CENTRAL---to wit the earth ground where power comes into the building. When you speak of the "safety ground" at a plug, it seems you are talking about something that connects to this point. The GFCI will trip if there is a fault to this ground by any path, AND if one of it's inputs also connects to GROUND CENTRAL.

Considering the above, what other application did you have in mind?

Regardless of the above, one fact remains: If you connect one of the output leads of an isolation transformer to GROUND CENTRAL, you have now defeated the whole purpose of having the isolation transformer. In this context, one very common reason for the iso trans is simply that it allows you to connect your grounded test equipment to the chassis of a transformerless DUT---eg an AA5---without major fireworks.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 9:00 pm 
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Mike Higgins wrote:
Pixallany, Just because GFIs are usually used to protect from ground fault situations, that does not mean they can not be used for other protection purposes. As you pointed out, in the particular situation under consideration here, as long as the isolation transformer functions properly, there is no ground fault issue, but that doesn't mean the GFI can't be effectively used to protect from another kind of fault situation that involves the wire in the standard three wire electric cord that I have been calling the "safety ground"
Regards, Mike


What other purpose do you have in mind? The name "ground fault circuit interrupter" fully explains what the devices do. Have you installed GFCI outlets lately? Most of the new ones that you can buy now come out of the packages in tripped condition and cannot be reset unless hot, neutral, and ground are properly connected. It would be interesting to find out how you can use them in other ways than the way they were designed to be used.

When you isolate something with a safety isolation transformer, ground is what you are isolating it from. Not hot or neutral as those are both replicated by the secondary of the transformer. Ground is what gets removed in the process. You do not want to re-introduce ground by any means except through your grounded test equipment.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Thu 08, 2018 9:03 pm 
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Chris108 wrote:
When you isolate something with a safety isolation transformer, ground is what you are isolating it from. Not hot or neutral as those are both replicated by the secondary of the transformer. Ground is what gets removed in the process. You do not want to re-introduce ground by any means except through your grounded test equipment.

Exactly!!!---well said.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Fri 09, 2018 4:31 am 
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I give up.

I still think this is a communications problem, but the solution that I was trying to suggest could also be handled perfectly well by simply letting the safety ground in the electric cord going to the DUT float, in which case you would also not have a safety problem, so it really is not worth all this turmoil. And by the way, I did recently put a bunch of GFIs in when I wired my new shop, but I didn't put any in wrong so I didn't know about the new feature you described.

Regards,

Mike


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Fri 09, 2018 4:40 am 
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Mike Higgins wrote:
I did recently put a bunch of GFIs in when I wired my new shop...
Mike,

We find it hard to take you seriously when you don't even know the proper device name.

It's a GFCI, short for "Ground Fault Circuit Interrupter", not a GFI.

- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Fri 09, 2018 5:45 am 
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Chris108 wrote:
........... Most of the new ones that you can buy now come out of the packages in tripped condition and cannot be reset unless hot, neutral, and ground are properly connected........


I hope they don't start to make all of them this way. Requiring the safety ground makes them useless in two-wire systems, one of the places they can do the most good.

I made up a short GFCI extension cord to use on the outside outlet (2-wire) at my family home (1940, 1951 built).

RRM


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Fri 09, 2018 6:24 am 
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Leigh wrote:
Mike Higgins wrote:
I did recently put a bunch of GFIs in when I wired my new shop...
Mike,

We find it hard to take you seriously when you don't even know the proper device name.

It's a GFCI, short for "Ground Fault Circuit Interrupter", not a GFI.

- Leigh

They were called GFI when they first came out and for many years before they started calling them GFCI

Dave


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Fri 09, 2018 8:04 am 
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easyrider8 wrote:
....... They were called GFI when they first came out and for many years before they started calling them GFCI. Dave


That's what I remember also. The first time I saw them described as "GFCI" I thought is was a different device. I'm guessing someone thought "GFI" might be interpreted as removing the ground fault instead of breaking the power circuit.

RRM


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Fri 09, 2018 12:42 pm 
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GFI in today's parlance refers to a circuit breaker or a stand-alone device used to protect motors or other equipment from further damage resulting from insulation breakdown. You might see them on heat pump and HVAC systems, swimming pool pumps, and things of that sort. They are usually associated with hard wired equipment, not plug-in-the-wall appliances.

Since the purpose is equipment protection rather than people protection, they are usually set to trip a lot higher than the typical GFCI outlet (eg. 50 mA typically). Therefore if you see a GFI breaker associated with a motor or large hardwired appliance, don't assume it is there to protect you [from getting a shock].

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Last edited by Chris108 on Mar Fri 09, 2018 3:49 pm, edited 1 time in total.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Fri 09, 2018 3:23 pm 
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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 10, 2018 12:04 pm 
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Mike Higgins wrote:
........the solution that I was trying to suggest could also be handled perfectly well by simply letting the safety ground in the electric cord going to the DUT float, in which case you would also not have a safety problem.......
Mike

What scenario is this referring to?

I think maybe we need some diagrams of how things are wired for the different cases.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 10, 2018 4:41 pm 
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I have just found a way to get 640x480 pictures, so based on what Barry said, I hope to be able to clear this up.

I want to make VERY clear that what I would consider as my "counter example" (to use a mathematics term) is ABSOLUTELY TRIVIAL. So you skeptics might as well save your criticism this regard. I will also try to give some insight as to why I would even consider such a "silly" circuit. I am restricting the discussion to troubleshooting some device that usually plugs into the normal 120 VAC house wiring and has 3 conductors which I have referred to as the "hot" (the black wire), the "neutral" (the white wire), and the "safety ground" (the green wire).

At some point, the question arose "What would happen if you put a GFI after an isolation transformer?" If you did, the obvious way to use it would be the following, which is analogous to the way it would be done in house wiring: Figure 1

The next question was "What could this protect you from?" Clearly, there is no concern with contact with either the earth ground or anything on the house wiring side because of the isolation transformer. But there is one possibility. Here is a possible fault condition that the GFI (or GFCI) could protect you from. This is unlikely, unless you are being very careless, but possible. Figure 2:

So why do I say this is "trivial"? Because you can get the same degree of personal protection by omitting the GFI and just letting the safety ground in the cord "float". Figure 3:

In my defense, the circuit in figure 2 would operate the way I said and could protect someone in this unlikely and somewhat contrived situation. It IS a situation in which a GFI could protect from a sort of non-ground fault. It also is a situation where you could work safely on it with just an isolation transformer.

I will end now so as to give some of you time to think up something ugly to say.

(I also have some thoughts about how an isolation transformer could be used in equipment that have more than one dc power supply referenced to the same "ground", but I think I will wait to see if I get my head handed to me about this one.)

Regards,

Mike


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 10, 2018 5:16 pm 
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Mike,

Regarding your diagrams...
NONE of them show an isolation transformer.
They ALL have one output line connected to ground.
That's NOT how an isolation transformer works.

- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 10, 2018 6:01 pm 
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First, none of the diagrams shows a connection to the main building ground. Therefore, none provide protection for faults to a damp basement floor, grounded test equipment, etc. The only protection is to the "local ground" as shown in the pictures.
I hate to sound like a broken record, but the intended safety function of a GFCI is not satisfied unless it is wired according to the manufacturers instructions---which includes the connection to the building ground. This is a major issue which could potentially kill someone, negate an insurance policy, or--as a minimum--get an electrician fired.

Second generic comment: Connection (of anything) to the "ground" terminal on a GFCI does nothing. The function of a GFCI is to sense the difference (~5mA) between hot and neutral currents.** That ground terminal goes only to the outlet ground pin, and you would NEVER connect either side of the line there. (See earlier comment about death, insurance, and electrician getting fired)

Diagrams 1, 2, and 2.5 seem to be all the same---they do provide protection against a fault to the DUT chassis, but no general protection as discussed in the first paragraph. (If the frowny face was intended to mean no protection, then that's backwards.)

Diagram 3 provides no fault protection of any kind.




**For completeness, they all provide the normal breaker current overload protection---which would also be provided by a normal breaker.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 10, 2018 6:10 pm 
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Quote:
I will end now so as to give some of you time to think up something ugly to say.

Nobody has said anything "ugly"......all of the statements here have been simply been opinions about what was presented, requests for clarification, and attempts to explain things.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 10, 2018 6:54 pm 
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Well! It didn't take long!

Leigh,

So I want to say that I made a mistake on figure 3. I meant to delete the GFI entirely, but the figure still makes the point that I wanted to make, no chance of electrocution from touching the chassis.

A 120 volt to 120 volt transformer does provide electrical isolation. If you want to call it something else, that's O.K with me. I don't think the purpose of this site is debate semantics and I see no benefit in debating GFI vs. GFCI or exactly what does or does not qualify for the name "isolation transformer".

By the way, in order to avoid confusion, I did use the "GND" term on the GFI because I tried to use the terms that are embedded in the plastic on the back of the unit (at least on the ones that Home Depot carries), otherwise there are NO grounds in the figures. I am sorry if this confused you. I intended just the opposite.

Your continual comments about building grounds puzzle me. The transformer provides complete electrical isolation from anything on the primary side of the transformer including the earth ground or the "building ground". Unless the transformer is defective, there is no electrical connection between secondary windings of what I have been calling the isolation transformer and anything else, and therefore, any fault condition from anything connected to the secondary to any ground would have no return path, and therefore, no possibility for electrocution.

Regards,

Mike


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 10, 2018 8:09 pm 
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Mike Higgins wrote:
Your continual comments about building grounds puzzle me. The transformer provides complete electrical isolation from anything on the primary side of the transformer including the earth ground or the "building ground".
Again you fail to understand.

The two leads from the iso secondary CANNOT be connected to ANYTHING other than the socket and then to the load.

In all three diagrams you have one lead connected to chassis. This creates a ground fault as soon as you connect any test equipment. All modern test equipment has chassis connected to AC ground.

- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 10, 2018 8:22 pm 
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Mike;

Let's try something. Take some statements one at a time and tell us if you agree or disagree. Assume normal building wiring with no isolation transformers.

1. A GFCI is intended to provide protection against a "fault" to ground--meaning anything remotely connecting to the building ground. They are normally specified for any potentially damp location, such as a garage, exterior plug, kitchen or bathroom.

1A To provide this protection, the neutral input lead to the GFCI must be connected to the building ground.

2. A GFCI can also be used to provide limited protection to some local reference point. In this usage, the GFCI neutral input would have to connect to that reference point.

3. A GFCI at the output of an isolation transformer cannot provide any protection unless the connections described in 1 and 2 are re-established.

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