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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Wed 02, 2021 1:35 am 
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xmo wrote:
I doubt if anyone ever got knocked on their keister while examining a '55S penny.


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-Steve W.


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Wed 02, 2021 1:46 am 
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BPlus wrote:
xmo wrote:
I doubt if anyone ever got knocked on their keister while examining a '55S penny.


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penny in fusebox.jpg


-Steve W.


Ha!

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Wed 02, 2021 2:03 am 
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anchorman wrote:
ttx450cap wrote:

so you are saying to carry my grd through my transformer variac? :shock:

my iso is grounded of course, after that no.


The safety ground pin on your variac will not "unisolate" your isolation transformer if it is connected to ground. The safety ground pin on the outlet of your isolation transformer will not harm its ability to isolate the output of the isolation transformer unless you bond one of the outputs to ground (which you should not). All metal cased equipment should always be grounded, unless you have good reason not to do so.

I personally check for continuity between the case and the pin on the cord. If the cord looks damaged, I replace it before plugging it in, regardless, and make sure that there is continuity between the round pin on the plug and the case.

The way mikeinkcmo does it is also inherently safe, IMHO. DUT isolated, and grounded as needed. you can't be shocked from touching the case of most ordinary stuff if the power inputs are isolated. but you also can't be shocked by a case that is grounded through the plug in most instances either. There's a difference between a single outlet on a repair bench, and an outlet somewhere else in a house that one would normally assume is grounded. I would personally mark the outlet as not having a ground, and also mark that it is isolated, but I understand why some folks don't feel the need to do that in a shop that only they use.


hi anchorman, it might be better to draw up a wiring diagram, the verbiage and nomenclature can get confusing.

I absolutely have no grd for the DUT, unless I add it in somehow. My variac gets its power from a 220v 3 wire step down iso transformer and there is no grd carried though.

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Last edited by ttx450cap on Jun Wed 02, 2021 3:50 am, edited 1 time in total.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Wed 02, 2021 3:39 am 
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XMO said;
Quote:
...The advice they get should be safe and and comply with electrical code...
In that case, make sure you DO NOT remove the connection between the secondary of your "isolation transformer", and ground! Good luck with that!!

Example #1.
Complies with "code"
Image

Example #2
DOES NOT comply with "code"
Image

I vote for #2, and, You vote for...?
=====

Quote:
...then build your power center with two prong outlets...
That is what I have for AC power on the bench, the connector just happens to have an extra hole in it.

Once gain...
Image

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Wed 02, 2021 12:05 pm 
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Example number 1 does not comply with code as I understand it. In example #1 he has the output of the isolation transformer bonded to ground, whereas it should only have the ground pin grounded.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Wed 02, 2021 12:09 pm 
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Thread Hijack Alert: Mike, just one question: What finish do you use on your bench? I tried polyurethane on mine and it holds up fine except that flux remover eats it. What holds up well to our common chemicals? DeOxit, Contact Cleaner, Flux Remover, Iso Alcohol .......

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Wed 02, 2021 12:11 pm 
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See bonding between isolation transformer outputs and ground removed.


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9325E087-AB0F-41FD-83CA-B273ED43F827.jpeg
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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Wed 02, 2021 12:22 pm 
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The ground pin on the output is grounded directly through to safety/earth ground. No connection between it and the output of the isolation transformer. It still isolated, but the chassis is still grounded as it should be.according to code. The only the place the current carrying conductors are grounded is at the main breaker as proscribed in the code.

If this were a transformer used as a step down or step up transformer that wasn’t meant to isolate the outputs, rather than isolate the outputs for equipment repair, you’d probably have to bond one leg of the output to ground. But given the point of the isolation transformer, it would be pointless to bond the output to ground.

Maybe Marcc and others could comment on whether the diagram I edited is in compliance with what they were saying too?

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Wed 02, 2021 3:17 pm 
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Why do you keep trying to muddy the water Mike?

Showing irrelevant diagrams is no help here. We are talking about a piece of test equipment, not something for a different application.

People who read these discussions may not have sixty years of experience. Why confuse the issue?

It's simple:

1) The current carrying conductors on the test outlet are completely isolated from utility power and from ground.
2) If the test outlet has a third prong it should be grounded.

How hard is that?

The deluxe isolation transformer schematic is correct.
The test equipment manufacturers did it right.

Advice given should be safe to follow by people with different levels of experience.

One of them could be the guy that needed two weeks of guidance to figure this out:


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Thu 03, 2021 12:27 pm 
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Quote:
...It's simple: The current carrying conductors on the test outlet are completely isolated from utility power and from ground....
They should be, but they aren't!!

I suggest you purchase an isolation transformer, and measure the resistance between the case/ground and the Neutral pin of the output receptacle.

When I purchased my TRIPP-LITE 1000VA isolation transformer some years ago, I had to remove the connections between the case Ground, and "Neutral" on the output receptacles. My drawing is per code, which I presume, Tripp-lite products surely are. They are re-establishing the connection between ground and neutral simply because, surprise surprise, the original connection, per code, at the service panel can't be carried through the transformer's windings.

Modern isolation transformers are designed to reduce/isolate the transmission of line noise, from sensitive life support electronics hence the inclusion of the Faraday shield, NOT, to establish metallic isolation of the output winding from ground.

Image
=====
As for your obsession with the ground pin, allow me once again state my reasoning as simply as I can, hoping you can grasp the concept. ANYTHING, I plug into that receptacle, regardless whats inside, regardless how its wired, regardless the connector or power cord used, will have NO REFERENCE TO GROUND. I, and I alone, will establish the ground plane on the DUT, when I connect my test equipment. Should someone feel overwhelming urge to Placard the receptacle warning the WORLD about the single AC outlet on their private test bench, be my guest!!

Here is what I want to see when I plug in the unknown DUT on the bench. Regardless of whether or not I duly assessed it's condition before applying power. Mistakes happen, I want them harmless.
Image

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Thu 03, 2021 3:32 pm 
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You win, Mike.


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Thu 03, 2021 4:38 pm 
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Hello Mike,
I also have a Tripp- Lite isolation transformer and had to do that exact thing to.
Sincerely Rich


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Thu 03, 2021 11:26 pm 
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Thank You for the support Rich. Unless one has experienced the issue it difficult for some to understand. Rules and Regs don't always reflect real life circumstances.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Fri 04, 2021 12:51 am 
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hello Mike,
You said it Some of those rules and regulations are for the birds .
I really find Tripp-Lite products well made .

Sincerely Rich


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Fri 04, 2021 3:28 pm 
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So just asking here (timidly) and NOT getting into this fight. I just need a couple practical clarifications.

I have reviewed the drawings and schematics in this thread and I thought I got it - but now with the battle between Mike and XMO, et al, I think maybe I'm doing something wrong on my bench. (GLAD to fix it - just need clear explanation, o.k? NOT interested in the fight!)

SITUATION: I have a Sencore PR57. The front panel "isolated" output receptacle for DUT on this device IS a three prong plug, as is the mains power plug. The PR57 schematic shows BOTH these safety connection pins connected to EARTH inside the PR57. This would practically seem to my little pea brain to mean that the PRIMARY Neutral and Earth co-bonding at the service panel carries through from service panel all the way to the ground pin on the output of the PR57. But the schematic also claims the AC output receptacle is "ISOLATED", so maybe I'm an idiot?

QUESTION: But what does this really mean on my bench?

Do I A.) Use a ground lift adapter plug here when connecting modern grounded chassis three prong power plugs to my Sencore PR57? Without that, I don't see how it can realy "isolate" anything if the ground pin on the output is effectively tied to the Neutral blade all the way back to the panel via the aforementioned "earth" bonding, as shown in the schematic. (Yes, all warnings for two prong stuff apply and are understood here)

Or, B.) Leave it alone - it is correct as is - practically speaking, isolation is achieved even though Earth is bonded from service panel to device via pin 3 of plug?

Or, C.) Do I open the Sencore up and make sure that the output three pin is actually a trick - i.e. a dummy earth connection?

Or, D.) Do I do what I THOUGHT Mike said and go ahead and physically disconnect that output receptacle connection to earth to truly make the DUT outlet fully "isolated" no matter what I plug into it?

(Mike, if you didn't say that, forgive me. I was only going by your drawing, which appears to suggest this?)

Or, E.) Am I an unqualified idiot who should immediately place his sixty plus year lifetime's worth of test gear plus tens of thousands of tubes, resistors, caps and vintage gear on eBay at scandalously low prices? :lol:

To be clear here - I'm not being sarcastic :-) I'm trying to ask a legit question. I realize that A and D are sort of the same thing, one being elective and one being permanent. I'm leaning toward A, but after reading the thread, I'm confused.

Thanks for any advice on the right thing to do, and in advance I do get that part of the answer is that "it depends" on the application.

Cheers,

CBG


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Fri 04, 2021 3:55 pm 
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CBG
Your Sencore PR57, BK1653, BK1655, Heathkit IP-5220 all do it the correct way with the isolated socket grounded, but leave the secondary winding floating. On here you simply get opinions, but most manufactures of these units got UL approval and understood what a service bench needed.
Bill


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Fri 04, 2021 5:04 pm 
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You make my point about confusing something thats simple.
The fact that an 'isolation' transformer intended to create clean power for computers is not satisfatory for use as test equipment without modification shouldn't be a surprise.

The test equipment manufacturers know what they are doing.
The third prong of a three prong outlet should be grounded.

Here is a simplified schematic of the PR57.

Mike's happy little guy is still happy.


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Sat 05, 2021 5:45 am 
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XMO said;
Quote:
...Mike's happy little guy is still happy...
Yes he is, IF, he checked to make sure the radio was wired right.

Image

But, swap the Green with either of the other two, and you have something like this. Yes, I know, no body likes these pesky little drawings. But it would only take one episode to really ruin your day.

Image

Of course everyone can do as they wish, but I still believe its better with "NO GROUND AROUND".

EDIT; Added cleaned up schematic drawings

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Last edited by Mikeinkcmo on Jun Sun 06, 2021 12:54 pm, edited 1 time in total.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Sat 05, 2021 6:48 am 
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Mikeinkcmo wrote:
But, swap the Green with either of the other two, and you have something like this. Yes, I know, no body likes these pesky little drawings. But it would only take one episode to really ruin your day.

Image

Of course everyone can do as they wish, but I still believe its better with "NO GROUND AROUND".


So the radio has no power going to it as the output is open. So what's the problem? You can touch any part of the radio and not get shocked.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Sat 05, 2021 7:21 am 
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Hello CoolBlueGlow,
I Also have the Sencore PR57 as explained it is isolated also most older equipment that you may be servicing have only 2 prog power cords .
Sincerely Richard


Last edited by radiorich on Jun Sat 05, 2021 5:48 pm, edited 1 time in total.

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