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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sat 29, 2021 5:38 am 
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Location: Auburn, AL
Mikeinkcmo wrote:
Anchorman asked;
Quote:
...Mike, why not have the ground pin on the killawatt grounded too, to the same ground as your test equipment, so that the device under test is always properly grounded if it has a three wire cord?...
Not being an alarmist, or screaming "the sky is falling"....But. Do you know FOR A FACT, that anything you drag home from who knows where, having a 3 wire cord, actually has the ground connected to ground? Will you, for the next 20 or 30+ years, check each and every 3 wire corded piece of whatever, to verify that? I think not, and, depending on circumstances of course, it'll only take once, for the big "LIGHTS OUT".


I’m not in the habit of plugging in unknown electrical devices without going over them first. But my question was, why have the ground disconnected? If it’s connected, and you are grounding your test equipment to the chassis, I wouldn’t imagine there would be any risk. And if you end up with a problem because of a ground loop when servicing, the DUT, you can easily plug in with an adapter that lifts the ground at the cord.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sat 29, 2021 5:55 am 
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OH, boy. This food fight again.

An isolation transformer is a good idea, and there should be no connection from the input side to the output side. Doing so defeats the purpose.

As said, you can use two power transformers with their secondaries tied together. A side benefit of this setup is the output will be a bit lower voltage than the input, which is better for the radio. They were generally designed for 110-117 volts, but today it's not uncommon for the voltage to be 120-125 volts.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sat 29, 2021 11:12 am 
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anchorman said;
Quote:
...I’m not in the habit of plugging in unknown...
Didn't ask about your habits. I asked if you could unconditionally guarantee you will never make a mistake that might cost you your life...

ttx450cap said;
Quote:
...I always keep a handful (of these) around...
If using a ground lifter/adapter, after they find you laying there, cold on the floor works for you, go for it.

No ground at the power receptacle means unconditionally, there's no worries. I like that.

=====

Michael said;
Quote:
...OH, boy. This food fight again....
Yep, I agree, but I've said my piece so I'm bowing out. I'll let the experts take it from here.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sat 29, 2021 1:38 pm 
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I would go back to my point. The only reason that I use the isolation transformer is for start-ups and the rare AC/DC & "Hot chassis sets". Most of the restored /in-work sets with transformers are grounded chassis and those with shielded transformers, need to be grounded. A hot chassis set will not have provision for grounding, it often relies on Neutral to be on the chassis, but with the NEMA plug tops, that cannot be guaranteed. Consider with a wired three pin socket with ground: With a two pin plug top & two wire cable it cannot ground via the socket.

Where the problems start is with coupling instrumentation & that's where the transformer set is at an advantage. Also note that much of the modern test equipment is not able to handle Tube radio & Tube instrument voltages.

There are no legal domestic GPO wall sockets in Australia & NZ with two pins: All have 3 and its a protected trade. Regulations govern the way its wired and a two pin plug top cannot be reversed, as the pins are inclined.

Coupling instruments to Hot Chasses is a subject on its own.

Marcc


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sat 29, 2021 3:53 pm 
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This subject should not be all that complicated for people to understand.

An isolation transformer is simply a way to allow you to safely connect ground referenced test instruments to 'hot chassis' equipment or some equipment with switching power supplies.

It also somewhat reduces your chance of shock while working on that equipment but isn't a cure for careless.

You can still get yourself across both sides of the isolated AC or across B+ and B- with predictable unpleasant results.


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sat 29, 2021 4:12 pm 
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Location: Melbourne, Florida
xmo wrote:
This subject should not be all that complicated for people to understand. ......


And yet on and on it goes ............

RRM


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sat 29, 2021 7:06 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Auburn, AL
Mikeinkcmo wrote:
anchorman said;
Quote:
...I’m not in the habit of plugging in unknown...
Didn't ask about your habits. I asked if you could unconditionally guarantee you will never make a mistake that might cost you your life...

ttx450cap said;
Quote:
...I always keep a handful (of these) around...
If using a ground lifter/adapter, after they find you laying there, cold on the floor works for you, go for it.

No ground at the power receptacle means unconditionally, there's no worries. I like that.

=====

Michael said;
Quote:
...OH, boy. This food fight again....
Yep, I agree, but I've said my piece so I'm bowing out. I'll let the experts take it from here.


Still curious what the advantage is of *not* connecting the ground pin on that outlet from your isolation transformer… how does that mean unconditionally no worries? If the ground pin is grounded, and you connect a ground from your test equipment to it, where is the problem? How about educating us, instead of telling us we’re carelss and stupid and going to die someday?

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sat 29, 2021 7:08 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Auburn, AL
And I’m not talking about bonding it to the neutral (large blade - obviously there is no hot or neutral at the isolation transformer output) from the isolation transformer, rather connecting it to the safety/earth ground wire. Binding the ground and the neutral aside from at the service entrance is an obvious no-no, and I wouldn’t bond the ground to either output of an isolation transformer either.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sat 29, 2021 9:12 pm 
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Joined: Oct Thu 29, 2020 10:17 pm
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Location: Centennial, Colorado
Seems like this topic has been beaten to death, but still lives on.

Why not look at what companies like BK, Sencore and Heathkit that manufacture or manufactured the combination variable isolated AC supplies did. For them getting it wrong was a liability unlike just an series of opinions some of which could be risky. BK, Sencore and others, which are still in production also have to have UL approval.

Here are schematics for the BK 1655 and the Heathkit IP-5220.


Attachments:
BK Precision 1655(1).jpg
BK Precision 1655(1).jpg [ 361.68 KiB | Viewed 1324 times ]
Heathkit Ip-5220(1).jpg
Heathkit Ip-5220(1).jpg [ 371.12 KiB | Viewed 1324 times ]
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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sat 29, 2021 11:16 pm 
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Joined: Jan Sat 16, 2016 4:15 pm
Posts: 358
Bill wrote: "...Why not look at what companies like BK, Sencore and Heathkit that manufacture or manufactured the combination variable isolated AC supplies did. ..."
-------------------------------------------------------------

Why not indeed?

Unfortunately, you have to make the assumption that people can read a schematic diagram and that they understand domestic AC electrical service, circuits, and code.

An unsupported assumption. It suggests that perhaps there should be fewer radio collectors and more coin collectors.

I doubt if anyone ever got knocked on their keister while examining a '55S penny.

Here's a partial of the Sencore PR570. You can see they put the variable transformer ahead of the isolation transformer. Either order is acceptable.


Attachments:
PR570 partial.JPG
PR570 partial.JPG [ 66.39 KiB | Viewed 1310 times ]
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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sat 29, 2021 11:59 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 2808
Actually I think the variac should come first. That way it has full flux density and low output impedance.


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Sun 30, 2021 12:19 am 
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Joined: Jan Sat 16, 2016 4:15 pm
Posts: 358
Bob wrote: "Actually I think the variac should come first..."
----------------------------------------------------

Heathkit and Sencore agree with you!


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: May Mon 31, 2021 2:04 am 
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Mikeinkcmo wrote:
anchorman said;
Quote:
...I’m not in the habit of plugging in unknown...
Didn't ask about your habits. I asked if you could unconditionally guarantee you will never make a mistake that might cost you your life...

ttx450cap said;
Quote:
...I always keep a handful (of these) around...
If using a ground lifter/adapter, after they find you laying there, cold on the floor works for you, go for it.

No ground at the power receptacle means unconditionally, there's no worries. I like that.

=====

Michael said;
Quote:
...OH, boy. This food fight again....
Yep, I agree, but I've said my piece so I'm bowing out. I'll let the experts take it from here.


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

my iso is grounded of course, after that no.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Tue 01, 2021 12:08 pm 
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anchorman asked, yet again;
Quote:
...Still curious what the advantage is of *not* connecting the ground pin on that outlet...
As simply as I can construct an answer, it means that ANYTHING I plug into that receptacle, regardless whats inside, or how its wired, or the connector and power cord used, has NO reference to ground. I, and I alone, will establish the ground plane, when I connect my test equipment.

This image I copied from, I think from one of Peter Balazsy's posts some years ago, illustrates 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. Please note the smile, mistakes happen, I want them harmless.
Image

The rack/test equipment is grounded, the bench is all wood, and the floor mat (barely visible center bottom) is rubber,
Image

....the chair is plastic.
Image

And the AC power on the bench is completely isolated from ground.
Image

The same basic configuration I learned 60+ years ago on my first summer job in a radio and TV repair shop.

If you want a different answer, ask a different person.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Tue 01, 2021 3:48 pm 
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Joined: Jan Sat 16, 2016 4:15 pm
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"If you want a different answer, ask a different person."

You volunteered.

People ask for advice. The advice they get should be safe and comply with electrical code.

The ground prong of a three prong outlet should be grounded. It can't get any simpler than that.

Any appliance with a three prong cord is supposed to be grounded.

If you know what you are doing and need to float a three prong corded appliance - use an adapter.

If the presence of an outlet with a grounded third prong bothers you - then build your power center with two prong outlets.


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Tue 01, 2021 4:25 pm 
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Location: Long Island NY
Not to throw more oil on the fire, just wanted to point out that no Heathkit nor any other kit products were UL listed. The reason is simple, in order to obtain UL listing for anything, the factory has to be inspected and monitored to ensure they don't switch parts around or change the way the product is assembled after the listing is obtained. Since kits are assembled by customers, there is no way to enforce those rules, so such products were ineligible for listing. Products from Eico, Heath, Knight, and others that were sold factory assembled were a different story since those could be UL listed if desired. But in Heathkit's case, the kits were designed to UL standards and recognized components were used where necessary. If the customer followed the assembly manual to the letter, they'd end up with a safe product that met all applicable standards and codes when it was shipped.

You'll note that the Sencore "Powerite" includes a relay in its test outlet ground. This is so the ground leakage current can be measured. Actually an extremely useful thing to be able to do when testing products that have grounded plugs (or even without them), as long as it is understood that the ground circuit is passing through a measuring device and is not direct. An increase in leakage current to ground above what is normal for the appliance can be an indication of moisture in transformer or motor windings, or other insulation defects.

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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Tue 01, 2021 4:45 pm 
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Different countries, different regulations and if the Europeans have been there its likely 230 -240V domestic mains. Regulations here prevent everything mains in buildings being wired by anyone other than a registered electrical contractor. As was noted above, It is illegal to wire domestic & other sockets designed to take an earth wire, without one: As before no two wire sockets here banned. Tying a knot in the cable as a retainer is also banned.

You can have a three wire plug top, on "double insulated" cable where there are two wires, if that's how it designed to be. Most stuff like that has a moulded two pin plug from the factory, but things happen. As I noted before, if there are only two wires, A & N connected to the plug top, be it two pin or three. Its not going to ground.

One of my hates are two wire cables on transformer devices with a metal case and a cap of any description going from line to the metal case. That gets modified real quick.

If you choose to understand electricity and procedures, that will help you live longer.

Marc


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Tue 01, 2021 5:12 pm 
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Joined: Jan Sat 16, 2016 4:15 pm
Posts: 358
"If you choose to understand electricity and procedures, that will help you live longer."
----------------------------------
Absolutely. But not everyone does understand electricity - and it takes some people a while to learn the basics.

There have been discussions on ARF where it has taken three pages of questions and answers to talk someone through getting the filaments on an AA5 to light - and that's with a schematic.

With that diversity of knowledge and given that any discussion may be read by others months or years later - it makes sense to advocate for safety above all else.


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Tue 01, 2021 7:54 pm 
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Marcc wrote:
...One of my hates are two wire cables on transformer devices with a metal case and a cap of any description going from line to the metal case. That gets modified real quick.
I'm in totally agreement with this, having experienced the dogged insistance by "engineers" on another forum that the original ground configuration in a vintage radio (or whatever) be maintained as long as the user only receives a MILD shock when touching the chassis :shock: .


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 Post subject: Re: Do I need an isolation transformer with my variac?
PostPosted: Jun Tue 01, 2021 11:21 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Auburn, AL
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.

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