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 Post subject: Re: Variac Problems
PostPosted: Dec Fri 01, 2017 7:37 pm 
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criageek wrote:
Unless that point was raised in the case where something might go wrong with the variac and short a connection to the cabinet. If that is a real concern I'll try to figure out a way to isolate it from the cabinet.


I had gone back and forth on this point, only because originally I was thinking in terms of building a device, not a piece of test equipment. It is correct that you want the cabinet grounded in case you have a hot wire come loose in the cabinet, however as stated before, if you have another piece of test equipment connected, now the chassis of the device under test is tied to the cabinet of your variac. If you go to adjust the voltage going in, you could tie yourself electrically to the chassis. What I would do is get a sheet of ABS plastic, which you can find on Amazon, and make a panel from that. You can even overlay that on top of the existing aluminum, but between that and the plastic knob of the variac itself, you would eliminated that problem. The only thing then that would tie you back to the chassis would be if you touched set screw in the variac knob, so keep that in mind when you work.

On another note, I would also add an ammeter. I have an isolation transformer/variac combo like what you are building, but for a few reasons, I'm building a better one, and this time including an ammeter. Here's the joy I'm running into on that. Both my transformer and variac are rated to 2.5 amps, which I want to take full advantage of. You can find 3 amp panel meters, but not any matching voltmeters. There are plenty of voltmeters that have a 1 amp matching ammeter. In my case, I'm redoing the shunt resistor (I'll update my post on that one when I get it done), which is one option. The other option is to limit yourself to 1 amp if that's what you get.

The reason for the ammeter with the variac is that if you have a short or fault, you can see the current rise while the voltage is still very low and both are low enough not to cause major damage.

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 Post subject: Re: Variac Problems
PostPosted: Dec Sat 02, 2017 1:08 pm 
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Thanks again Michael - I agree there should be an ammeter and when I rebuild this I will include it. This unit is one I acquired many, many years ago and I'm just trying to make it work safely. For the time being I built this to monitor current:

viewtopic.php?f=8&t=322407

It has matching volt and current (3A) meters, but they are cheap and the ammeter is not particularly accurate. That's one reason I added the digital meter. The analog meter gives me a general idea and a visual indication of spikes, and the digital meter gives me a reasonably accurate absolute reading.

Rich


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 Post subject: Re: Variac Problems
PostPosted: Dec Sat 02, 2017 4:53 pm 
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TPAairman wrote:
If you go to adjust the voltage going in, you could tie yourself electrically to the chassis.

In my opinion, the only function of an isolation transformer is to prevent damage to your test equipment. Electrical safety depends on you always exercising caution, not on some piece of equipment.

Even with an isolation transformer mounted in a double-insulated case with no ground connection to the case and no ground connection to the equipment under test, you can still kill yourself.

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 Post subject: Re: Variac Problems
PostPosted: Dec Sat 02, 2017 11:03 pm 
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criageek wrote:
I do appreciate the fact that this is being discussed...everyone is making some great points and I look forward to the dust settling and learning the proper way to hook this stuff up. For the time being I've gone with this setup:
Attachment:
variac_schematic.png

I
Rich

This has certainly turned into a longer thread than I thought it would be.
Rich, I think this last diagram is fine.
It is standard practice in electrical work to assume that a person could be directly in contact with the earth ground. You will find that the breaker box case, the casing of you HVAC unit, metal outlet boxes, kitchen oven and ranges cases, electric washers and dryers, etc. are all connected to earth ground. The idea being that if anything hot comes loose inside and contacts the case, a breaker or GFCI will open and minimize the danger.
Since the secondary of a isolation transformer has no connection with ground there is no way that a complete circuit through ground can occur that would result in electrocution.
The isolation transformer isn't intended to protect other equipment.
That being said, if a person isn't connected to ground, actually any conductive object not connected to ground, does acquire a small voltage (but very low current) with respect to ground because if capacitive induction and leakage. A meter with high impedance or a oscilloscope can be connected between earth ground and a object insulated from ground will register the voltage. I think some of your measurements actually were effected by this. A common test of audio circuits is to touch the terminals of the volume control, the hum produce is because of this phantom voltage.

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"Excellent!" I cried. "Elementary," said he. - Sherlock Holmes


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 Post subject: Re: Variac Problems
PostPosted: Dec Sat 02, 2017 11:36 pm 
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I meant to say "only function of an isolation transformer is to prevent damage to your test equipment when working on circuits directly connected to line voltage, e.g. AA5 radio or switching power supply".

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 Post subject: Re: Variac Problems
PostPosted: Dec Mon 04, 2017 3:39 am 
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TPAairman wrote:
We're still not seeing eye to eye on this one.

No, but we're trying.

I think part of the problem is that you use the word "chassis" and I use the words "cabinet" or "case". I visualize the chassis as being a separate unit inside of, and insulated from, the cabinet. The third wire on the power cord grounds the cabinet, not the chassis. If the power cord does ground the chassis, then both sides of the AC line should be insulated from it.

TPAairman wrote:
As far as your Case 3 example, actually you are correct on that one - when you said same as Case 2, that threw me off. However, we've both actually already agreed on that one - that if you connect a piece of test equipment to the device under test, with the ground lead connected to the chassis, then by touching the case of the test equipment, and a hot point in the device under test, you will get shocked.

Yes, if one side of the AC line coming into the device under test is connected to the chassis. This was common in the old metal case portable TVs; one side of the AC line was connected to the chassis which was insulated from the cabinet. But these sets didn't have 3 wire cords and you probably took the chassis out of the cabinet to work on it anyway.

TPAairman wrote:
Here's why I still disagree with you and contend that the ground wire should not be connected to the outlet. When we connect the piece of test equipment, we've established that the case of that test equipment is tied to the chassis. I think we're also in agreement in that it also means the chassis is tied to the wall ground if the test equipment has a 3 prong cord. That means that every grounded item on my workbench is electrically tied to that chassis. My refrigerator in the kitchen, on the other side of my house is now electrically tied to that chassis. So I don't have to just touch the case of my test equipment that's actually connected. I could touch the conduit between my outlets over the workbench and I'm electrically connected to that chassis.

Yes, we agree on this point.

TPAairman wrote:
Where we differ is, If I disconnect the test equipment, the chassis is no longer grounded through the test equipment, but it would still be grounded the exact same way if we connect the ground wire from the plug of the transformer to the outlet of the transformer.

No it wouldn't. The ground wire from the power cord is connected to the cabinet, not the chassis.

TPAairman wrote:
So that means if I have my amp hooked to the transformer, with the ground wire connected in the transformer, I now face the EXACT same risk as when my test equipment is connected. The whole point of the isolation transformer is to eliminate that part of the risk - to make it so that if I do become grounded and I touch a hot lead in the device under test, there cannot be a complete circuit. But adding that ground wire now completes the circuit and eliminates the protection that the transformer is supposed to provide, because I can now touch the conduit on my bench outlets, and a hot point in that amp and get shocked. If we remove the ground wire in the transformer, that can't happen

I think we're getting close to agreeing on this. Since the ground wire is not connected to the chassis in the device under test, it is isolated and this particular shock risk is eliminated.

TPAairman wrote:
And I will say this - Yes I know we are supposed to work with one hand in a pocket, etc, etc, but the reality is, we've all had a stupid moment at one time or another working with what we work with.

And sometimes one hand just isn't enough.

TPAairman wrote:
Here's a couple good visuals to explain it:

On this first one, scroll to 4:55, and he addresses this.
https://www.youtube.com/watch?v=3rvEdHyAd2k

Note that the visual text says: "It's safer that the device your (sic) working on is plugged into the isolation transformer, not the oscilloscope." This is indeed the correct way to do it.

As far as what he says at 4:55, he is talking about the chassis which isn't connected to the cabinet which is grounded by the third wire. At 5:38 he talks about using an adapter to isolate the ground. Since the chassis isn't grounded, that isn't necessary.

TPAairman wrote:
This is from Mr. Carlson's Lab, and in this one, He's showing the same danger in a different way.
https://www.youtube.com/watch?v=XBsQ3sZ45Fk

In this video he is talking about connecting the input of an oscilloscope directly to the AC line and pointing out the problem with doing this. However, his solution is completely wrong. By isolating the ground of the oscilloscope by disconnecting the ground pin on it's power cord he shows that connecting the ground lead of the scope probe to the hot side of the AC line doesn't cause a short circuit. That is true. But what he doesn't say is that by doing this, he is connecting the case of the scope to the hot side of the AC line. If he touched the scope and any grounded object, he would get shocked. Failing to mention this potentially deadly hazard is completely irresponsible on his part.

So, does this mean that you can't look at the AC line with an oscilloscope? Of course not; you just have to do it safely. There are a number of ways to do that. Here are three simple ones.

Method 1. Use a transformer. Any transformer that has a primary that will accept line voltage and has an isolated secondary that supplies a voltage within the ratings of the scope and probe can be used. Connect the secondary to the scope; connect the primary to the line. The scope will now show what is on the AC line adjusted by the turns ratio of the transformer. The limitations are the bandwidth of the transformer and any distortion introduced by the transformer. The distortion can be reduced by using a transformer with a higher primary voltage rating, e.g. use a 240 volt transformer on the 120 volt line. This eliminates distortion caused by approaching core saturation and reduces distortion from other causes.

Method 2. Remove the ground lead from the scope probe or fasten it to the cable with a rubber band; any method to keep it out of trouble. Connect the input of the probe to the AC line. The scope will display what is on the AC line. The scope is grounded through the third wire in the power cord so the probe ground is not needed. The limitation of this method is that the scope will also show any noise that appears between the third wire ground and the neutral line that you are testing.

Method 3. Use a two channel scope and two probes. Remove or restrain the probe grounds as in the above. Connect the channel A probe to the hot side of the AC line and adjust the scope for the desired display. Then set the channel B range switch to the same volts per division as channel A. Connect the channel B probe to the neutral side of the AC line. Set the scope's vertical mode switch to A-B. The scope will now subtract the stray noise to the best of it's ability. The limitation is how well matched the scope channels and probes are and how good the common mode rejection of the scope is. Note that some scopes don't have an A-B setting but have A+B. Use that and set the channel B inverted switch to give the same effect.

TPAairman wrote:
The current would not flow through the ground, then back through the test equipment. The path from the test equipment ground lead to it's own power cord wire ground is a parallel path to the ground wire in the outlet of the transformer. They both connect between the chassis and the wall outlet ground.

With a fault in the device under test, current would flow out of the transformer, to the device, through it's fault, and back to the other secondary wire of the transformer. And that would happen whether you have any ground connection or not since the ground is not an alternate path back to one side of the transformer secondary.

This still assumes that the third wire ground is connected to the chassis of the device under test and therefore to it's AC line connection. That isn't what case 1 is about; the chassis is insulated from the cabinet (which is grounded). The assumed fault is from the AC line to the cabinet. The third wire of the power cord should not be connected to any point that is also connected to the AC line.

For other people who might read this, a point that has come up in other threads and has been touched on here is the mistaken belief that using an isolation transformer somehow makes it safe to work on something no matter what. That is totally not true. There are many ways to kill your self with electricity. An isolation transformer only eliminates one of those ways and only under certain conditions. All safety precautions should be observed at all times.

An isolation transformer was originally intended to reduce shock hazards. But that was back when test equipment had two wire cords. Now that they have three wire cords, the situation has changed and the isolation transformer sometimes ends up protecting the test equipment rather than the tech.

The conclusion: the third wire ground on the isolation transformer should be grounded. It provides some safety and there is no reason to not connect it.

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 Post subject: Re: Variac Problems
PostPosted: Dec Mon 04, 2017 5:39 am 
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Jim Mueller wrote:
I think part of the problem is that you use the word "chassis" and I use the words "cabinet" or "case". I visualize the chassis as being a separate unit inside of, and insulated from, the cabinet. The third wire on the power cord grounds the cabinet, not the chassis. If the power cord does ground the chassis, then both sides of the AC line should be insulated from it.


I would say this is a major part of the disagreement. I would not think of the cabinet and chassis as being insulated from each other. I'll use my Oscilloscope as an example. It's a GW Instek, which I'm not sure how old it is right off the bat, but I'll say 101-5 years old tops. So certainly modern standards. That chassis of it obviously has circuit boards in it, but it is a metal frame mounted to the case. I'm not sure where the ground wire from the power cord is exactly tied in, but I do know that the case is connected to the ground pin, as well as the ground sides of the BNC input jacks. But I do know that the chassis is screwed right to the case, and all is grounded right back to the power cord.

On a different note, if you look at an amplifier, whether a guitar amp or a stereo amp, the power cord normally goes right into the chassis, and the ground wire screws or is soldered right to the chassis itself. If it then has a metal case, that case then becomes grounded. But regardless of the device, the ground wire would not be connected to the AC line at any point in the device.

I do have one question, and this may shed light on how we are looking at this differently. In this whole discussion, what are you thinking of as far as devices under test? The reason I ask is, when I think of anything i would work on that would be grounded, it's basically amplifiers. Tube radios don't normally have a grounded cord, so those are out. I might be wrong, but I'd think a Ham radio would have a grounded case and chassis. So I can't really think of anything that you would put on the bench that would have a grounded case, but an insulated chassis.

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 Post subject: Re: Variac Problems
PostPosted: Dec Mon 04, 2017 6:11 am 
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stevebyan wrote:
TPAairman wrote:
If you go to adjust the voltage going in, you could tie yourself electrically to the chassis.

In my opinion, the only function of an isolation transformer is to prevent damage to your test equipment. Electrical safety depends on you always exercising caution, not on some piece of equipment.

Even with an isolation transformer mounted in a double-insulated case with no ground connection to the case and no ground connection to the equipment under test, you can still kill yourself.


You're right that safety does begin with you. However, the ISO does still offer a level of protection to yourself in addition to the test equipment. We've already established that when a piece of test equipment is connected, it then ties all of your other equipment via it's negative lead, to the chassis of the device under test. So yes, if you touch a grounded piece of equipment while in contact with something hot in the chassis, then you will get fried. The protection comes in when, for example you use something like a multi meter that is not plugged int the wall. Or maybe making an adjustment to something.

Or take an AA5 set. There's no ground wire in the cord, but the plug is also not polarized. So you plug it in, and a capacitor is leaky. If you plug it in one way, you don't have a problem. If you turn the plug the other way, then it's possible that through that leaky cap, that the chassis could now be hot. If it were a dead short, then the fuse or breaker comes into play. But maybe the capa isn't quite shorted. If for any reason you are grounded, and plugged that radio right to the wall, then touching the chassis might not be so pleasant. But through the ISO, that won't happen.

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 Post subject: Re: Variac Problems
PostPosted: Dec Mon 04, 2017 7:07 am 
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Here's the point demonstrated -

https://www.youtube.com/watch?v=nMHp_DFSeg0

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 Post subject: Re: Variac Problems
PostPosted: Dec Mon 04, 2017 3:40 pm 
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I want to make sure everyone understands my particular situation...this comes to mind because of the discussion about 'case' vs 'cabinet'. In my situation, I have an all aluminum box that contains a variac (it's just a variac, with connections on the side...no outlet or power cord of it's own), isolation transformer (it's just an isolation transformer, with 2 primary wires and 2 secondary wires...no outlet or power cord of it's own), switch, outlet, and voltmeter. All are mounted directly to the aluminum box, therefore their 'cases' are connected that way. I have the aluminum box connected to the green ground wire on the ac line cord. I measure no continuity from any lead of any device to the aluminum box. The outlet is a 2 prong polarized outlet, so no ground connection.

I've redrawn my schematic to try to illustrate this better, with dashed lines around the variac and isolation transformer depicting their cases tied to the aluminum box.
Attachment:
variac_schematic_002.png
variac_schematic_002.png [ 51.14 KiB | Viewed 61 times ]

Thanks again guys!

Rich


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 Post subject: Re: Variac Problems
PostPosted: Dec Mon 04, 2017 9:06 pm 
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That will indeed work.

Personally, I think it is better to keep the case/cabinet of your isolation transformer tied to earth ground as you have done. I think the safety advantage of having a metal case tied to earth ground outweighs the slight disadvantages that are noted in the above discussion. There should not be any need to adjust the variac while you're probing inside the device under test. It's more important, IMHO, to understand what is grounded, and what voltages are ground referenced, regardless of the exact setup you use.

Plugging the setup into a GFCI is still recommended to provide additional margin of safety.


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 Post subject: Re: Variac Problems
PostPosted: Dec Mon 04, 2017 10:13 pm 
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The fewer pieces of equipment tied to ground on the work bench the better off you are. Eliminate all references to ground and it will be much safer.

Dave


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