Armed Polite Society
Main Forums => The Roundtable => Topic started by: Nick1911 on July 23, 2010, 06:18:33 PM
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What happens to the user?
I think the answer is that he doesn't notice anything, because he's not part of the circuit. That for him to get shocked after the isolation transformer, he would need to get between the two hot conductors.
But, if this were the case, wouldn't it be safer to have an electrical system where nothing is grounded?
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I believe if the frame is hot and he's grounded he gets the cheap buzz.
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Electrical side track. Went to a house today that got hit by lightning. The Romex that got the initial hit blew apart like a plugged gun barrel in three different spots. I thought the electricians had stripped it back (6") and spread all the wires out like a flower petal but they said it was the lightning. Never seen anything like it before. It hit the house fan in the peak of the attic. Took out the A/H in the attic, took out the attic fan romex as described, took out 7 TVs and 3 computers and the guy said he was in the basement and watched the bolt stream across the floor right next to him. So when they say don't talk on the phone and don't take a shower during lighning storms, I'd listen to them. :O
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Roger, I'll hang up my cordless phone next time. >:D
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But, if this were the case, wouldn't it be safer to have an electrical system where nothing is grounded?
Recognizing that grounding of systems is one of the most complex and discussed issues in practical electricianship, I would say: ground everything to a common ground to be safest.
TC
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i believe that in theory you are correct. an isolation transformer should protect you.
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The answer might shock you ;)
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"I would say: ground everything to a common ground to be safest."
But obviously, in this case, there was some need to have the equipment totally isolated from ground -which is why there was an isolation xfrmr and no ground on the secondary in the first place.
IF there are no other shorts anywhere except in the motor itself, the person should feel nothing.
IF the person felt something, then the diagram is in error or "the water" got in somewhere else in the circuit and perhaps partially grounded the secondary side of the circuit.
I think the OP was just making the point that the person was well-grounded by the remarks about standing in a puddle of water, but extrapolating from that, if there was enough water around so that there was an electrical leakage between the primary and secondary of the isolation xfrmr, the person might in fact feel a buzz --or maybe more.
And to be nitpicky about it, there might be enough capacitance between the secondary circuit and ground that the person might feel a buzz through that capacitance anyhow. This, depending on how far "up" the motor winding the short to the frame occurred.
Terry, 230RN
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Electrical side track. Went to a house today that got hit by lightning.
Further side track. I was working as an electrician, coworker and I were cutting in switch boxes in a storm. I saw lightning strike several hundred yards away from the house in a field. My coworker starts cussing. He was tying the grounds together an apparently got lit up. He said he felt it go through his feet. I don't know why the romex would conduct like that when it wasn't a direct hit. Not having any ground rods or anything hooked up at the panel yet apparently made him the path of least resistance though.
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The isolation transformer will certainly not save you under all circumstances, in general. Just know that. I would expect there to be some coupling to ground; I would count on it.
But, if this were the case, wouldn't it be safer to have an electrical system where nothing is grounded?
A valid theory; it seems to me that the main motivation behind grounding equipment chassises is to force a breaker or fuse to blow if the chassis comes in contact with a 'hot' wire. Fuses/breakers are part of the whole package.
I am not an electrician, nor do I play one on TV, nor did I stay at a Holiday Inn Express last night.
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I'm going to hold your beer and the camera while you test your theory. I don't mess with isolation transformers enough but it looks good on paper.
I'm going to give zahc the win on the second part of your question. Because without a ground to trip, a common 20 amp breaker can do a lot of damage to something. On a 120 volt circuit you could make a 2400 watt heater out of something.
Yes I have watched a power cord melt all the insulation off as it became a heating element.
jim
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Here's what you do, providing you can set up everything but the short to the frame (somehow I think you can ;) ) leave yourself an uninsulated place to take a reading to ground in the secondary circuit and see what you get. Probably a better method than standing in a puddle and grabbing on. Although I would hold your beer as well. =D
Interesting question. Another one I have is ok, if the motor frame is not grounded will the short to the frame even matter? Will the motor just keep running normally?
One other thing, is the old 110/24 v A/C control transformer is considered an isolation transformer? If so I know for a fact if I ground the hot leg it will blow the circuit. I've learned this the hard way. More than twice. That's why I prefer replacing them with ones that have a built in circuit breaker on the 24 side. The other ones come with a built in one time fuse and if it so much as sniffs a short they blow. One little touch is all it takes. It never happens at a good time either. =|
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I don't know why the romex would conduct like that when it wasn't a direct hit
EMF induction in the romex? or maybe static charge?
I don't know if anybody has welcomed you yet, so welcome! Just FYI, nick is our resident Tesla type mad scientist. He used to worry me a tad but is apparently pretty good at it. :lol:
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Something else I just noticed about the diagram. You are showing a direct short to ground on the primary side? I think you should draw a box to represent the transformer frame and attach the ground to it? I'm assuming when you say "125V mains" yopu mean 2 - 125V legs equalling 250V?
hell, after more thought. Just power up a transformer and see if you can get a reading to ground from either secondary leg. I'm thinking you can.
After even more thought, I'm going to guess a motor with a short to an ungrounded frame WILL still run unless the winding circuit is burnt out completely. As long as juice can flow through the windings, it will run. I've seen where motors hooked up directly to power will short to a grounded frame and continue to run though usually backwards and/or at a lower speed. If the frame on those motors was NOT grounded, the first poor schmuck who had one hand grounded and the other touch the motor frame will most definitely get lit up. Always ground stuff! You eliminate one of the safety features provided by the breaker/fuse if you don't.
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Well, I guess it depends a lot on what's under the puddle. Dirt? Concrete? Rubber?
The motor isn't grounded (wtf?), so I assume that if he was grounded he'd be shocked.
<--not an electrician. :(
Also, what's the point of a 1:1 transformer? Do they even exist?
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Once more, there is some valid reason (and there are many) why the secondary side of the circuit was not grounded. That's why an isolation transformer is used in the first place. Yes, Mabs, they sell them for exactly these reasons.
The ground in the primary side is represented as a "ground" to the primary side, when all it is, is the normal mains (line) ground.
Some of you please rethink your positions in that light.
The "puddle of water" is a device used to merely indicate that the person is well-grounded. Whether it's on concrete or quicksand does not matter. A diagrammed buss strap from the person directly to ground would serve just as well to illustrate this.
Measurements with modern high-impedance (> 10 megohms per volt) meters might well be meaningless, since they pick up readings even if two stray electrons are induced to move (deliberate exaggeration to make a point). It is possible to pick up substantial voltage (even AC) readings on these meters from any isolated circuit to ground. Low impedance (such as with a D'Arsonval movement) meters often give more "realistic" measurements then electronic meters in these situations*.
Assuming there is no stray capacitance to ground on the secondary side (a near-impossible assumption anyway), and assuming there is no other leakage either from the primary to the secondary side, or from the secondary to ground (which amounts to the same thing), the person will not feel a thing. If he does, then either the diagram is in error or the assumptions made are wrong. There is a faint (very faint) possibility that some bizarre effects of the motor's counter-EMF --or a make-and-break short from the winding to the frame --might result in the person's feeling something, but I think we may discount that as outside of the question's parameters.
See my reply #7 above.
Terry, 230RN
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* I was amazed to discover once that I got about 70V AC off my ham antenna with it totally isolated from anything --with an electronic meter. It was just picking up a couple of stray fields from somewhere, probably the supply line to the house. It even showed a substantial DC reading, probably from static buildup as the breeze brushed it. Sometimes a $6.95 1000 ohms-per-volt meter will give you more realistic readings in circumstances like that.
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125vac mains represents a hot and neutral, with 125vac of potential between them. The grounded leg shows that the neutral side of the mains is, in fact, grounded.
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Ah, thanks, thought it was a hot leg.
So, apparently, a 110/24 control transformer is NOT an isolation transformer? What would be the distinction?
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^ Any transformer except an autotransformer is an isolation transformer in the sense that there is no direct, deliberate, hard-wired connection anywhere between primary and secondary as it comes from the factory.
The designation "isolation transformer" implies that the secondary voltage is the same as the primary voltage (hence the "1:1" designation) and its purpose in life is primarily for isolation, and not to boost or reduce the input voltage. A lot of times, even though the secondary may be of a different voltage, an unstated additional purpose of the transformer may be for isolation anyway.
Nick1911's statement, "The grounded leg shows that the neutral side of the mains is, in fact, grounded" is more technically correct than my "The ground in the primary side is represented as a 'ground' to the primary side, when all it is, is the normal mains (line) ground" in reply 15 above, but I didn't want to get into that much detail.
The OP is really an interesting question, and I wondered if it appeared as an exam question somewhere. You know, "Discuss how crispy the stick figure will get if..."
Terry, 230RN
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Ok, then why can I get a reading from the secondary side to ground on any old 110/24 transformer? I'm going to have to set one up myself and check this out again. Just for kix. =D
not today though, too HOT! :P
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The OP is really an interesting question, and I wondered if it appeared as an exam question somewhere. You know, "Discuss how crispy the stick figure will get if..."
Terry, 230RN
Actually, it's a question I was hoping to answer in relation to my 460v system.
EMF induction in the romex? or maybe static charge?
I don't know if anybody has welcomed you yet, so welcome! Just FYI, nick is our resident Tesla type mad scientist. He used to worry me a tad but is apparently pretty good at it. :lol:
I appreciate the vote of confidence. ;)
Here's another take. In this case, the user shouldn't get shocked, but will a breaker blow?
(https://armedpolitesociety.com/proxy.php?request=http%3A%2F%2Fimg.photobucket.com%2Falbums%2Fv282%2Fnick1911%2Fd80425bd.jpg&hash=465561f573608f0706f781a7ce57b46b5ff19c1d)
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What happens to the user?
He get's enlightened?
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Until I wire me up a transformer and take some readings to ground I'm going to say I don't know. =D
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Never mind -- redundant
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"Ok, then why can I get a reading from the secondary side to ground on any old 110/24 transformer? I'm going to have to set one up myself and check this out again. Just for kix."
Once again, it depends on the sensitivity (input impedance) of the meter.
The voltage appearing on the two sides of the secondary of the transformer will "appear" to ground as the line voltage swings back and forth with a sensitive meter (most electronic meters nowadays are almost "standardized" at 10 megohms or more.)
But you could not draw much more than microamp-level current from either side of that transformer secondary to ground. It's just a question, so to speak, of a few electrons stacking up on one lead of the output of the transformer and then, as the input power cycle goes the other way, stacking up on the other side of the output. So to speak --I don't want to get into the capacitance between the output leads of that transformer. Again, with a sensitive meter, you will see this.
Using an old-fashioned not-so-sensitive meter which "loads" the output slightly, you will not see this --usually. I have an old 10,000 ohms per volt D'Arsonval meter I sometimes use to determine whether or not I am getting a spurious reading on my electronic meters because of their very high sensitivity.
Optional reading:
Remember, a D'Arsonval old-fashioned meter has an actual metal-to-metal connection between its leads in the form of the coil of the meter movement --regardless of how big the multiplying resistor is. This actual connection will allow electrons to drain from one lead of the meter to the other, thereby mostly eliminating false readings due to capacitance of the circuit under test.
On the other hand, most electronic meters have a Field-Effect Transistor (FET) in their input circuit ("so to speak"), which operates electrostatically, and there is (almost) no way electrons can drain from the gate, so they can "stack up" on the gate of the FET and can give these "false" readings. "In a manner of speaking," to be simple about it.
Terry, 230RN
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Soon as it cools off I'm out in the shed checking this all out. =D
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I'm wondering if 280 has an old analog milliammeter setting around somewhere. Just to see what it would show. I need to go dig around in my old furnace parts box. This could be a learning experience for us.
And Nick if you just drop a wrench you could answer this pretty quick. ;)
jim
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^ Yeah, I was going to ask him to report the sensitivity of the meter(s) he used, but I decided to let it go for now.
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I gots flukes. True rms, which means nothing to me because I forgot the whole story on that. All I know is it's supposed to be gooder.
I have a couple ideas. One is to simply try and pull in a 24v relay by connecting one leg to the transfomer and the other to ground. I may go fishing around for the parts today. I have everything.
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Terry sticks his neck out with another prediction:
With a sensitive meter such as Flukes are, the most you would read is 12 Volts, I would bet, going from either lead of the output of your 24 volt xfrmr to ground. (1.414 times that (16.97V) if you're measuring peak-to-peak, which IIRC, most Fluke RMS meters will do if you want them to.)
Reason? The secondary of the transformer is sitting in, so to speak, "free space," and while the voltage between the output leads would be 24 volts, you would be measuring each individual lead to a virtual center tap on the transformer, i.e., ground.
A relay will not operate on the very low currents available from either leg of the xfrmr to ground, as I said, in the microampere range. It would sink the low currents available so that almost no voltage could build up, just as a low-impedance meter would sink that current and give almost no voltage reading.
I notice the only ones bumping this thread are myself, 280plus, and Jim147, so I'm willing to either quit this or take it to PMs.
Terry "If It Works, It's a Fluke," 230RN
Root-Mean-Square measurements:
http://www.practicalphysics.org/go/Guidance_107.html
The True RMS meter actually takes rapid samples of the varying voltage and actually does this calculation within the meter's own software and comes up with a true RMS value regardless of the shape of the input waveform.
The non-True RMS meter is just simply calibrated to "sorta" come up with an RMS voltage assuming that the input waveform is sinusoidal. Which it might not be. Other, old-fashioned RMS meters actually measured the heating value of the current by measuring the temperature change of a calibrated heating element and working from that.
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Nah, there's always a few readers. I see what you are saying though. I'll keep you posted. ;)
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Oh, missed the second part of your post. Thanks for the down and dirty on rms. I remember now. ;)
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I'm still reading. I'm enjoying the discussion and trying to learn from it.
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I'm still reading. I'm enjoying the discussion and trying to learn from it.
It's hard for me to enjoy being reminded how little I know and how small are the chances that I might actually ever be able to understand any of what this thread is discussing.
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I found one of my analog ampclamp multimeters. It's a little too humid for me to want to be out playing with a transformer right now. I have one energized here the house but I don't think anyone would be happy if I took the furnace apart right now. It will go as low as 3A on the clamp or 150 mA in series. But it might not live long if it's used for any line-to-line voltage on Nicks setup.
Now what kind of readings can we get if we put a load between the short and the ground?
Voltage would drop to zero? I think.
jim
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I do believe that's all correct. Terry's take on 12.5V to ground is probably correct. I feel like I've sen that before. Question is amps to ground which I think I'll try with a direct shoet while readingamps. I haven't gotten to it yet but I will. probably tomorrow, it's supposed to be more tolerable outside.
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^ Watch it. Probably OK with a 24VAC transformer, but anything higher could get dangerous.
Reason? If you short one end of the output coil of the xfrmr to ground, it is no longer an isolation transformer and the full output voltage will appear on the other (ungrounded) lead of the secondary, with the capability of delivering real current, up to the design output current of the transformer, and not just the microamps I was talking about before. (Where any "ground" was through the low capacitance of the the whole setup to ground.)
Reason for that? One side of the primary circuit is inherently grounded through the mains ground, so shorting one side of the output side to ground establishes a direct connection to one side of the primary.
And you end up with an autotransformer. --Essentially, a single coil transformer with the tap on it grounded. If you then measured the voltage between the ungrounded lead of the primary and the ungrounded lead of the secondary, what would would appear is the input voltage PLUS whatever voltage the secondary is supposed to deliver. In the case of the 24 V transformer, 110 Volts plus 24 Volts, for a total of 134 Volts.
If the isolation transformer were a 1:1 (110VAC to 110VAC), you would get 220VAC between those ungrounded leads, with the full delivery amperage of the transformer behind it.
I wish to hell my scanner were working. This would all be a lot more clear with diagrams. I may take a few minutes out of my precious, long-overdue, and well-deserved breaks today to throw together a sketch of this, scan it with the company scanner, and post it.
Terry, 230RN
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Double post due to my screwing around trying to get a pdf file to show directly.
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Here ya go...
(https://armedpolitesociety.com/proxy.php?request=http%3A%2F%2Fwww.loesch.org%2F%7Earviel%2Fxfrmrs.JPG&hash=e5bedeb19c806f4dc1d74c0837ac13dc9e58afa3)
CASE I
Illustrates the situation posted by the OP, but without the motor. Due to any small stray capacitances from the isolation transformer to ground, a small but indeteminate voltage will be sensed by a grounded sensitive (usually electronic) meter at either point C or D. A less sensitive meter (such as a D'Arsonval moving-coil meter) will probably show nothing, since the few electrons available due to the coupling to ground through the stray capacitance will be drained off by the coil of the "less sensitive" meter without visibly actuating the coil-and-pointer assembly.
What happens with the addition of the shorted motor depends on how "high up" on the motor's coil the short postulated by the OP is. If the short is high up, the short will draw a lot of current through the secondary, which will be reflected back through the transformer, probably causing the fuses (or breakers) in the mains to blow.
However, if the OP's diagram reflects the true state of affairs, not much (if anything) will be felt by the grounded stick figure --depending on how large the stray capacitance is. Typically, stray capacitances such as this are very tiny, and depend, as with all capacitances, on the insulation between the "plates" of the capacitor, the distance between the "plates" of the capacitor, and the area of the smaller "plate."
CASE II
Shows the situation if 280plus "shorts" one of the transformer's output leads to ground in an attempt to measure the current to ground. In essense, an autotransformer will be created.
CASE III
The CASE II situation is redrawn here to illustrate more clearly the autotransformer configuration. (I forgot to add the iron core, but I don't want to re-scan the drawing.) The dotted line is merely to illustrate that there is a solid connection (through ground) from B to D.
For our British readers, substitute the word "earth" for the word "ground." For our American readers, substitute the word "line" for "mains."
Donations accepted at the door. Go in peace. <cue choir>
Terry, 230RN
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I've encountered plenty of 24 volt control circuits that would read zero volts to ground at both legs of the secondary. Of course in every case the transformer did not have one leg grounded as you find in most commercial HVAC units. The control circuit was truly isolated.
Having said that I would never trust a transformer and ground myself like the OP suggests. A failure in the transformer windings could be catastrophic to your health.
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We used to use isolation xformers back in school in the labs. It was required procedure whenever working directly with mains AC, safer for both the user and for the mains. Had racks of the things you could borrow.
In theory, the secondary side of the isolation xformer is floating and can't cause any potential to ground. Touching a single point of that side shouldn't cause any shock problems, it'll just make that side float so that the point you touch is at 0V compared to ground. At most you'll get some parasitic leakage current, probably not even measurable.
In practice, it's still dangerous. The real world isn't so neat and tidy as a schematic. When working with real parts and real wires on a real bench, you never know for sure that the transformer secondary is truly isolated from ground. If any part of that system is ground referenced, even indirectly, any other part of it can shock you.
And you still have 120V of potential between the high side and the low side of the secondary, so you can still fry yourself that way.
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I'm lost on this auto transformer thing. Because I do see transformers with grounded secondarys and that may be the part that is confusing me on the voltag to ground. Nut anyhoo, So you're saying if I were to grab A snd C I would be delivered 134V? Question is, why am I grabbing A? If I know it's hot, I ainta gonna be grabbing for it. I hope. :O (We need a smilie getting zapped by a lightning bolt. =D )
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:mad: now you've done it, this has to be the grabbing a hot lead icon. =D
(https://armedpolitesociety.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fen%2Fthumb%2F0%2F01%2FDEAS012.jpg%2F200px-DEAS012.jpg&hash=d67b446fa9f67f01eb17ec917f04bfaff7ba1dac) (http://en.wikipedia.org/wiki/File:DEAS012.jpg)
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I guess I should get off my dead ass and go set this up. It's been a long summer for the A/C guy(s) so I'm a bit uh, lacking in motivation right now. Tomorrow suddenly became totally freed up so I may feel more ambitious in the AM. I do believe I'll put that first thing on the schedule for tomorrow.
1. Screw around in shed for 30 minutes.
2. Climb on and off of a 20' flat roof several times, run out at least 130" of garden hose, take apart a rooftop A/C, clean the crap out of it, reverse process.
Who wants to come? =D
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:mad: now you've done it, this has to be the grabbing a hot lead icon. =D
(https://armedpolitesociety.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fen%2Fthumb%2F0%2F01%2FDEAS012.jpg%2F200px-DEAS012.jpg&hash=d67b446fa9f67f01eb17ec917f04bfaff7ba1dac) (http://en.wikipedia.org/wiki/File:DEAS012.jpg)
Bingo! Ok, now make it really really small.
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From:
http://en.wikipedia.org/wiki/Autotransformer
Autotransformer --a single-coil transformer where taps are provided to allow different voltages to be tapped, which may be either higher or lower voltage, depending on where the taps are:
(https://armedpolitesociety.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2Fc%2Fc5%2FTapped_autotransformer.svg%2F300px-Tapped_autotransformer.svg.png&hash=bc7e65d5faa2d461d2844b40141ee32ef52b7101)
My diagram in CASE III above, reply # 38, is sort of up-side down because of the layout of the previous diagrams, but the concept should be obvious.
Variac variable transformer -an autotransformer where, instead of taps, there is a carbon brush which can be slid along the length of the coil to obtain varying voltages, and the coil is formed into a circle on a big toroidal core:
(https://armedpolitesociety.com/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F0%2F01%2FVariable_Transformer_01.jpg%2F240px-Variable_Transformer_01.jpg&hash=64c697dc0393cab0dd9dd2ec6ee923c70ca78d32)
Damned handy devices.
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Ohhh, I call that a "multi tap" transformer. =)
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^ And the Variac is an "infinite-tap" transformer. :) (Well, at least in terms of one tap per turn.)
That "Don't Grab Hot Side Leads" icon becomes unscary when reduced to emoticon size. I dood a .20 by .20 reduction on it to 600 bytes and it just looks like a colored blob.
Say, why don't we just color-code the transformer leads? Like maybe black for the input side, red for high voltage output, yellow for 5 Volt output, red-yellow for HV center tap... and like that?
(https://armedpolitesociety.com/proxy.php?request=http%3A%2F%2Fwww.radioremembered.org%2Fimages%2Fxfmr4.gif&hash=0895cd34b552c2700fe15b31aa1ac51aae291df5)
And then we could all promise not to stand in puddles or use more than one hand whilst mucking about with "real" electricity (as opposed to all the 5-volt TTL-level voltages common nowadays --which isn't "real" electricity.) :)
Terry, 230RN
http://www.radioremembered.org/xfmr.htm