Electrical question

[Nick1911]

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?

[280plus]

I believe if the frame is hot and he's grounded he gets the cheap buzz.

[280plus]

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. 

[Fly320s]

Roger, I'll hang up my cordless phone next time. 

[Leatherneck]

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

[geronimotwo]

i believe that in theory you are correct.  an isolation transformer should protect you. 

[Tallpine]

The answer might shock you 

[230RN]

"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

[KPT]

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.

[zahc]

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.

[Jim147]

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

[280plus]

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.

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.  

[280plus]

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. 

[280plus]

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.

[Mabs2]

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?

[230RN]

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
_____________

* 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.

[Nick1911]

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.

[280plus]

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?

[230RN]

^ 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

[280plus]

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. 

not today though, too HOT! 

[Nick1911]

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. 

I appreciate the vote of confidence.   

Here's another take.  In this case, the user shouldn't get shocked, but will a breaker blow?

[lupinus]

What happens to the user?

He get's enlightened?

[280plus]

Until I wire me up a transformer and take some readings to ground I'm going to say I don't know. 

[Hawkmoon]

Never mind -- redundant

[230RN]

"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