Another Generator Observation

The recent post about generators and neutral bonding to ground reminded me of a thread I read on another forum a couple of years ago, it was very interesting and caused a lot of controversy.

The writer explained a demonstration he had seen during a basic course on electricity at a local community college.

The instructor took the class outside to a grassy area where he produced a car battery, a copper rod a couple of feet long, two single strands of (single conductor) wire, one three feet long, the other about 20 feet long and a car headlamp.

He pushed the copper rod into the ground and with the three foot single strand of wire, connected the copper ground rod to the negative terminal of the battery.

He then connected one end of the longer wire to the positive terminal of the battery and the other end of the same wire to one of the two terminals of the headlamp bulb.

Then he touched the second terminal of the bulb to one of those pop-up lawn sprinkler heads that (he told them later) was screwed onto a metal pipe that came up out of the ground.

Of course, you are way ahead of me, to the astonishment of the class, the lamp lit.

He explained that the earth, and any conductor attached to it, was indeed the second conductor along with the one wire leading to the lamp.

In reality, the lamp would light if it was touched to any metal item that was "grounded" and he found other grounded items and proved it to the class.

Of course, he then disconnected the wire going from the battery to the ground rod and repeated the tests, and the lamp refused to light.

He explained that when the battery was "Grounded" that any other grounded item in the vicinity appeared "Hot" to the light bulb. (That is an interesting observation right there, that to the bulb, the earth appeared "hot".)

He stated that with the battery "not grounded" that the only way to light the lamp was to run a second wire to the bulb.

Eventually, he got around to stating that operating an electric drill (metal cased drill) while standing barefoot in a water puddle was perfectly safe if the drill was being powered by an ungrounded portable generator.

He explained that with the generator "Not" grounded, in order to get shocked you would have to actually come into contact with both wires coming from the generator.

However, he said that if you grounded the generator, you would have to come into contact with only the hot wire coming from the generator and "ANY" other conductor or surface that was grounded, which made that situation much more dangerous.

Just like an old razor outlet. Isolation transformer so you can't short to ground.

At electrical trade shows I used to attend, they would have a 120 VAC isolation transformer set up and a tub of water and have a light and the demonstrator would stuck his hand on a conductor and into the tub of water etc etc and, of course, never get shocked.

The isolation transformer simply takes the grounded 120 VAC and isolates it off any "ground" with the voltage being present ONLY between the two transformer output leads, ITS NOT ROCKET SCIENCE, its simply 120 VAC between transformer output leads with no connection to any earth ground or chassis ground or anything else. Therefore the ONLY way to get shocked is to get across the 2 transformer output leads as that's the ONLY place the voltage potential is present NOT from either lead to a piece of metal or mother earth etc.

Mother earth can be a conductor and its conductance is a function of the moisture content and iron and other minerals present.

This is pretty basic to utility folks and engineers and techs but when Billy Bob saw that dude touch hot conductors and stick his hand in a tub of water he sure was impressed lol

John T Coming to you live on the road from Stoddard Wisconsin.......

John, you sound like you have studied electricity as much or more than I have, it sounds like you understand all the concepts quite well.

I'm sure you can appreciate this statement: I was telling a friend a couple of weeks ago that I had two isolation transformers near enough to me while I was sitting here typing, that I could reach over and touch them.

(I actually own 5 isolation transformers, a couple rated at 240 volts, 5kwa and they weigh about as much as I do.)

I was explaining the concept of how safe an isolation transformer was, that the safety comes from the name of the transformer implying that the output is isolated from the power lines and grounds.

I then compared the output of an isolation transformer to an ungrounded portable generator.

You would have thought I insulted his mother.

I even went so far as to say that when I used a small portable generator while tailgating at an event that I only used a two conductor extension cord.

That was when the insults really started.

He said he was familiar with isolation transformers and that they were extremely dangerous because when he grounded the neutral of the output of his, that he still kept getting shocked through his power tools.

He asked me if I run the output of my generator through an isolation transformer.

To borrow a worn out phrase, dang kids anyway.

I can see how an isolation transformer would have the same output configuration as a generator.

I've run into 120vac control circuits that would test some random voltage to ground, because they were on an isolation transformer. I never touched the circuit to see if it would be strong enough to feel a shock, just wasn't curious enough to find out! LOL

But I was thinking, in a perfect world that type circuit would be safe without a ground. But what if there were a short to case in the generator, the wiring, or an appliance?

Wouldn't that make for a potential shock hazard? I realize it wouldn't be practical to ground a portable generator, and I don't recall hearing of anyone getting electrocuted by one, but still I would think some common sense should be exercised.

I would assume the case of an isolation transformer would be grounded on the primary side, so a short on the secondary would simply ground through the case, defeating the purpose of the isolation circuit.

Not that I'm in disagreement with anyone, just thinking out loud... LOL

It was fairly common in the 30's through 60's to power industrial plants with a NON Grounded Floating Delta 240 volt three phase three wire system. Even if one leg got shorted to a case or frame or mother earth etc. the machines kept right on working. Of course, if a second leg got shorted similar there were big time sparks and arcs. At our Naval industrial facility I think I was the first engineer to CORNER GROUND a 240 volt three phase delta because I preferred a grounded system. So one day a tech came to me all excited saying how there was a problem BECAUSE ONE OF THE LEGS WAS DEAD, it measured 0 volts to ground YES YOURE RIGHT I EXPLAINED LOL

The grids primary and secondary benefit from tying Neutrals to one common low voltage reference, that being good old mother earth.

Usually an isolation transformer has only the 2 output leads and if it had a center tap it still wouldnt require any sort of grounding to a case or earth etc.

As far as small portable gensets, their Neutral is usually bonded to the case/frame as is the third wire equipment grounding terminal. However if they are used to feed tools that have their plug connected to an on board receptacle NO GROUNDING ELECTRODE IS REQUIRED. But if you use the genset to feed a panelboard say for a home or shop etc then a grounding electrode is required.

Gotta love sparky chat

John T

"I realize it wouldn't be practical to ground a portable generator"

If the portable generator is used to power plug and cord connected equipment via an onboard mounted receptacle NO BOND TO A GROUNDING ELECTRODE IS REQUIRED. If its used to feed a panelboard say in a home or shop, its to be configured as a "Separately Derived Source" similar to how a utility transformer is configured in which case a Grounding Electrode is required.

John T

The things that Nickola Tesla invented or discovered a hundred years ago, people are finding to be real.

John, I was working in an auto plant where some of the older bus duct was 480 isolated from ground, I had forgotten about that type of system.
We had hooked up a new motor starter from that bus and one leg was dead to ground. I wanted to check the bus, and there was a bus plug on it with the door hanging open, It didn't look like it was being used, so I figured that would be a good place to check the bus.
I took a power lift up and had to clime out into structural steel to get to that bus plug.
When I got to the plug there were fuses in it. So I figured the best thing to do was to pull the tops of the fuses out before I turned the plug on. What I didn't see was that the plug was already turned on.
I pulled the top of first fuse out, when I took hold of the second fuse, the skin between my thumb and first finger touched the brass shell of the top of the fuse, and I was holding on to the structural steel with my left hand.
That was the worst shock that I got in all the years of working it the trades.
I had gotten bit by 277 a time or two but that 480 hurt.

Dusty

Milan...you stated in the earlier post that if you supplied a home breaker box with a genset you would not want to have the neutral and ground bonded in two places thus the floating neutral in the genny...my question is if I"m supplying my cabin which has a RV type panel (which has an isolated neutral buss) would I want to run a jumper between the neutral and ground to have them bonded in one location? Thanks Dan

Hi Dan. Well, if I recall correctly, your cabin is not connected to the grid.

Right?

If so, that eliminates transfer switches and the associated problems.

I would bond the neutral to ground and run a nice heavy wire from the ground bus to a nice long rod driven into the earth.

Then, I would also leave the generator with it's floating neutral.

There may be others with different opinions, they may say that your cabin is like an RV and leave the neutral floating, but I don't think I would.

Yes...cabin is off grid. So that"s my plan, bond neutral to ground in breaker box and then run ground to driven rod and leave genny alone....thanks for the help. Dan