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 Post subject: How should an isolation transformer be grounded?
PostPosted: Mar Sat 03, 2018 3:16 pm 
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I can speak from personal and recent experience in regards to this topic. Last summer, while working on a Tek 576, using a modern o'scope where the signal ground is connected to the power line safety ground, as soon as I turned on the 576 power, the smoke started. The 576 was transformer powered, and I assumed that would provide sufficient isolation. I still don't know quite why the smoke, but putting the scope on a truly isolated isolation transform kept the problem from re-occuring.

In my opinion, a truly isolated isolation transformer is MORE safe than the alternative, but it is easy to have your cake and eat it too in this case. You put a GFI (Ground Fault Interrupter) on the isolated output winding of the isolation transform, connect the ground post of the GFI to the neutral of the output winding of the isolation transformer, and you have the added safety of the GFI with respect to possible personal shock hazards!

This would even work with a variable output isolation transformer arrangement if you put the GFI just AFTER the isolation transformer but before the Variac so that the GFI internal circuitry would still receive the 120 volts it needs to operate.

Regards,

Mike


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 03, 2018 3:44 pm 
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First, where did the smoke come from-----the scope or the DUT**?----more generally, what was the scope connected to?

The most common issue is connecting the scope ground to something like an AA-5 with a live chassis---thus connecting one side of the line to the building ground.

If you set up something with an isolation transformer, then installing an GFCI after the transformer does nothing for you.

A common setup would be to have all your test equipment on one or more GFCI-protected circuits, and then connect the DUT through an isolation transformer. Now you can ground the DUT through the chassis ground of test equipment and have no drama.....


**DUT = Device Under Test

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 03, 2018 4:13 pm 
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The main question is exactly what isolation transformer you were using.

Modern isos do NOT isolate the power line output from the wall outlet wires.
These devices are designed to attenuate noise, not to isolate the power lines.

Check your transformer with an ohmmeter set on its highest range.
Test from one blade of the input plug to each side of the output socket.
Repeat the test from the other blade of the input plug to each side of the output.

All four readings should show an open circuit (maximum resistance).

If that's true, your iso is doing what we need.
If not, it may be possible to modify it by removing an internal jumper.

To the original question...
It's OK to have a grounded input line (round ground pin to chassis).
It's OK to have a grounded output (round pin). The DUT will only use that if designed to do so.
Neither of the output AC lines (blades) of the iso will be referenced to ground, so no issue.

- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 03, 2018 5:40 pm 
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pixellany,

I am truly surprised at your response as I thought that this would be non-controversial.

In regards to the 576, I never bothered to follow up on it. I just replaced the obviously burned up resistor (in the 576), put the scope on an isolated safety ground isolation transformer, and went on troubleshooting the 576. I isolated the scope rather than the 576 partly because the isolation transformer I was using was only good for 1 amp and the scope pulled much less current than the 576.

I have some extra AC outlets on my workbench that come from a 20 amp isolation transformer, but I haven't used them in years because if you plug everything into one isolated AC power source, there are still scenarios where you can get into trouble. They are less likely, but there is a principle in Physics that says that anything not excluded by the laws of Physics will occur with a non-zero probability, i.e if it can happen, sooner or later, it will. You WOULD be protected from any scenario that involved contact between a "live" wire and something like a water pipe or anything connected to the egc (equipment grounding conductor) or to an earth ground, but there are still less likely faults that could occur if you connect everything onto one isolation transformer.

For example, any scenario between a "live" wire from the isolated side of the isolation transformer and a metal chassis connected to the safety ground connected to the safety ground of a receptacle on the isolated side of the isolation transformer. Putting a GFI on the output of an isolated isolation transformer would protect you from this kind of fault.

It is a matter of personal preference, but I now prefer to put any AC powered test equipment on individual separate isolated isolation transformers. That way I don't even have to think about it. Putting all the test equipment on one isolation transformer and the equipment being worked on into a separate isolation transformer should work too.

I recognize that there are other ways of doing things, but that works for me.


Leigh,

The incident with the 576 was because I got lazy and DIDN'T use an isolation transformer. That is the real lesson to be learned. However, if I had plugged both the DUT and the scope into the same isolation transformer, I would have been protected from accidental contact with a water pipe, for example, but I'm pretty sure that the resistor would still have smoked.

Regards to both of you,

Mike


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 03, 2018 7:27 pm 
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Mike Higgins wrote:
However, if I had plugged both the DUT and the scope into the same isolation transformer...
Hi Mike,

The DUT is the ONLY thing that plugs into the iso, always, no exceptions.

- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 03, 2018 8:21 pm 
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I have been thinking about this, and there is another isolation issue that needs to be explicitly addressed. Most of the above discussions related to safety issues and isolation from 120 VAC grounding problems.

However, there is another grounding issue that is important too, namely grounding issues between test equipment and the equipment being serviced or worked on and between other pieces of test equipment. This is where the separation/isolation of each piece of test equipment from other test equipment and the power supplies of the equipment being tested can be significant. Frankly, I think that this was the actual nature of the 576 problem that I had. I suspect that whatever point that I put the ground clip of the scope probe was actually sitting at some voltage above the 576 ground/AC power line ground and effectively shorted out one of the power supplies in the 576, causing enough current to flow through the resistor to burn it up. That is really the chief advantage of using separate isolation transformers for each piece of test equipment being used, quite separately from any personal safely issues. This allows you to put the ground lead of the test equipment wherever is most convenient, without having to worry about any untoward consequences

Regards,

Mike


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 03, 2018 9:19 pm 
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This subject is brought up at least once a year.

Your last question is actually quite simple. The ONLY AC POWER SOURCE on your bench is from your isolation transformer, and THAT is where you plug in what you're working on.

This is mine, and there is NO CONNECTION to the ground pin.
Image

Everything else on your test bench should be connected to ground through their respective power cords. Good quality plug strips are a big help in that regard.

I have found it convenient to have all the power to the test or radio bench and the associated lighting, controlled by a single "master" switch. When I'm done, flip one switch and walk away.
Image

Isolation transformers;
The vast majority of what are sold today as "Isolation Transformers", have a copper Faraday shield to reduce noise feed through, thus, are actually "NOISE ATTENUATORS" not isolation transformers. Shown as "NEW", below.


Image

The purpose of an isolation transformer is to COMPLETELY ISOLATE the AC coming out of it, from GROUND and the AC going in. Nothing more, nothing less. Shown as "MODIFIED" above. If there is a Faraday shield, so much the better.

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Last edited by Mikeinkcmo on Mar Sat 03, 2018 10:36 pm, edited 3 times in total.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 03, 2018 9:47 pm 
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A quick reality check on just one point:
If you connect a (real) isolation transformer with a GFCI in series with the output---and make no other connection to the GFCI input , there is no way to trip the GFCI.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sat 03, 2018 11:41 pm 
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Hi Mike,

While your diagram is correct as drawn, the problem is not obvious.

You need to enclose the upper diagram (with the grounded output line) in a large red outline, and have a large red X drawn through it, making it obvious that the circuit shown is a potentially fatal problem.
Mikeinkcmo wrote:
Image


- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sun 04, 2018 12:17 am 
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Wow, this topic is a lot more contentious that I ever would have thought.

However, I still think separate completely isolated transformers are the best way to go. By this I mean ones that have absolutely NO electrical connection between the 120 VAC standard input line ground and the output windings of the transformer or the ground connection of the output socket. This should provide absolute electrical isolation between the AC line input and anything connected to the output of the isolation transformer.

Pixallany,

The way to connect the GFI is to put the GFI ground to whichever end of the output windings you are going to use as the neutral conductor and put that into the line in neutral of the GFI. The other end of the transformer output winding goes to the line in hot connection of the GFI. This makes the GFI work exactly the same as it would in a normal house wiring application.

Regards to all,

Mike


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sun 04, 2018 3:47 am 
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This is a contentious subject because there are lots of opinions and old wives' tales floating around the internet.

If you were working on a 576 and you connected another scope to it and got smoke, it suggests that you connected the ground clip of the second scope to a heat sink or some other metal part inside that is not at ground potential. Not every metal part inside a unit like that is grounded. Either that, or there was some serious internal problem with the 576. You didn't say what resistor burned up so we can't tell if it was related in any way to grounding the 576 through the second scope, or just a coincidence. The Tek 576 is a reasonably modern (1969-1990) curve tracer. It originally came with a grounded three-prong cord and it has a power transformer, so if it had been plugged into a properly grounded outlet, it should not have made any difference if you connected another scope ground clip to a grounded part of its chassis.

Putting everything on separate isolation transformers until the smoke stops doesn't fix anything. It is only masking other problems that are going on. Blown fuses, or tripping breakers or GFCI outlets are trying to tell you that you've got problems you might not have been aware of. Heed their warnings and be thankful for them.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sun 04, 2018 4:59 am 
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Mike Higgins wrote:
Pixallany,
The way to connect the GFI is to put the GFI ground to whichever end of the output windings you are going to use as the neutral conductor and put that into the line in neutral of the GFI. The other end of the transformer output winding goes to the line in hot connection of the GFI. This makes the GFI work exactly the same as it would in a normal house wiring application.
Mike

edit---need to fix some errors...

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Last edited by pixellany on Mar Sun 04, 2018 5:35 am, edited 1 time in total.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sun 04, 2018 5:32 am 
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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sun 04, 2018 5:50 am 
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Greetings to the Forum:

The original post subject line was:
Quote:
How should an isolation transformer be grounded?


I think you should take away its car keys and its credit cards.

Regards,

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sun 04, 2018 1:16 pm 
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Odd question relating to this topic ... what if one were to have purchased, all at the same time, a box of 4 older isolation transformers - which I did, rather beefy ones at that .. (totally isolated, I keep them in separate rooms... also well grounded; they have had the very highest level of education available) ..... all kidding aside... real iso transformers with no common between line and load .....

Would putting your DUT on one, and your test equipment on another, or several others, be advisable from the point of view of the equipment and smoke? I can see a possible safety hazard here, but from the standpoint of not smoking equipment, would this approach have merit?

Seat-of-the-pants engineering would say that with the AC line in the building completely isolated from ground, to both your test gear and your DUT, your main shock hazard would be putting a hand across the actual line. Or do your two floating grounds actually present more of a shock hazard? I may try this out as time permits and report back. If I'm still alive ... 8) take some measurements etc

.........just stirring the pot.......

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sun 04, 2018 1:46 pm 
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Some pots here get stirred......and some boil over..:)

I would apply one of the KISS** principles: You can't always improve systems by adding parts.

An ideal safety situation is when the chassis of the DUT is at "ground" potential--eg the same potential as the damp floor in your basement shop. With an AC-DC set, an isolation transformer allow as you to do that. (The ground might be through a piece of test equipment----see earlier comments about grounding everything except the DUT, which is alone on the isolation transformer, and gets grounded by one path only--through the test equipment)

The KISS principle also says that a GFCI after an ISO trans. does nothing.

**Keep It Simple, Stupid

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Last edited by pixellany on Mar Sun 04, 2018 1:54 pm, edited 1 time in total.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sun 04, 2018 1:52 pm 
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pixellany wrote:
Mike Higgins wrote:
Pixallany,
The way to connect the GFI is to put the GFI ground to whichever end of the output windings you are going to use as the neutral conductor and put that into the line in neutral of the GFI. The other end of the transformer output winding goes to the line in hot connection of the GFI. This makes the GFI work exactly the same as it would in a normal house wiring application.
Mike

edit---need to fix some errors...

OK-chemical balance restored, and brain shows signs of activity.

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.

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sun 04, 2018 4:18 pm 
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How about showing a protective fuse in the circuit, in any case? OR a circuit breaker.


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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sun 04, 2018 4:33 pm 
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wazz wrote:
How about showing a protective fuse in the circuit, in any case? OR a circuit breaker.

Overcurrent protection is always desirable.

Even multiple/redundant fuses/breakers are sometimes useful.
These are wired in series, NOT in parallel.

- Leigh

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 Post subject: Re: How should an isolation transformer be grounded?
PostPosted: Mar Sun 04, 2018 4:49 pm 
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I'm now back to attempting simple, direct answers to the original question...
Quote:
How should an isolation transformer be grounded?
It shouldn't.

The purpose of an isolation transformer is to REMOVE the connection to "ground", so you can "ground" the DUT to some other "ground". If you are now confused, I have succeeded......:)

"GROUND" is a notoriously ambiguous term that should be removed from the lexicon. What counts with any power distribution is the wiring of supply and return. In the typical home, these are called "hot" and "neutral". Common practice--at least in the US--is that the neutral is connected to a metal rod stuck into the dirt. To the electrician, only that point is "ground".

If you are in your damp basement, what you need to worry about is whether there is a pathway from hot to neutral that goes through some part of your body. This could involve "ground", but it doesn't have to.

Follow the current.......

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