I'm perplexed

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John Paul

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Norfolk, VA
The answer to the attached slide is yes.

I thought that if you touched a 120V line or energized piece of equipment and you were touching a lower voltage like the earth or another piece of metal then you were toast.

The slides state that the current must return to the transformer/source.

What do yall think?
 
augie47

augie47

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Factual. As drawn it's an ungrounded system. The current needs a path back to the source. With no ground, the person touching the equiomem t is not in a path.
 
wbalsam1

wbalsam1

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Location
Upper Jay, NY
John Paul said:
The answer to the attached slide is yes.

I thought that if you touched a 120V line or energized piece of equipment and you were touching a lower voltage like the earth or another piece of metal then you were toast.

The slides state that the current must return to the transformer/source.

What do yall think?
Click to expand...

In the slide you've presented, I do not see where the man is in jeopardy. The transformer is not grounded; the neutral conductor is not bonded to the service equipment; there does not appear to be a ground-fault path since the metal is not at a potential. In the real world, of course the transformer would be grounded and the neutral would be bonded to the metal enclosure which would provide a fault current path resulting in a serious shock to the man.
 
G

gar

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100129-1432 EST

It appears only part of the slide is displayed.

If less than 1 MA flows thru the person, then probably no problem.

10 MA a good shock.

100 MA for a while may be dead.

If the source and all wiring, except the one short to enclosure, are insulated from the person, and only the one finger touches the enclosure, then no current flows thru the person.

If the transformer neutral is grounded via a ground rod at the pole, and the person is on an insulated pad (boots) and only one finger touches the cabinet, and no other part of the person's body touches anything else, then no current flows thru the person.

If the transformer neutral is grounded, the whole yard is flooded in salt water, the person is standing with bare feet in the water, and solidly grabs the handle of the enclosure, then the person is probably dead.

The point is how much current flows thru the person and their susceptibility to a given current level.

.
 
augie47

augie47

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gar said:
.....................................If the transformer neutral is grounded, the whole yard is flooded in salt water, the person is standing with bare feet in the water, and solidly grabs the handle of the enclosure, then the person is probably dead.

.
Click to expand...

sounds like the logic used to write some of the Code. :)
 
J

John Paul

Member
Location
Norfolk, VA
Thanks. Ungrounded makes sence, but goes against what I've always considered to be unsafe. It's as though you could touch the 120V bare hot wire and not feel a thing if it's ungrounded, even if I'm standing in a pool of salt water with bare feet on top of a metal sewer cap.

The sewer cap is at zero volts but since it's not connected in any way to the source you're saying that no current would flow?
 
J

John Paul

Member
Location
Norfolk, VA
wbalsam1 said:
In the slide you've presented, I do not see where the man is in jeopardy. The transformer is not grounded; the neutral conductor is not bonded to the service equipment; there does not appear to be a ground-fault path since the metal is not at a potential. In the real world, of course the transformer would be grounded and the neutral would be bonded to the metal enclosure which would provide a fault current path resulting in a serious shock to the man.
Click to expand...

Why isn't the metal which shows a fault at a potential? 120V since it's touching the hot wire.
 
G

gar

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Location
Ann Arbor, Michigan
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EE
100129-1505 EST

John Paul:

The sewer cap is zero volts relative to what.

To measure a voltage you need two points. I can choose to call any point I want as my reference point and in turn choose to call it 0 volts. However, note that a voltage gradient can be considered to be at a point. I could have a point A that I call 0 volts, but at that point the voltage gradient is 100 V/ft.

Consider this: You have a normal 120 V circuit. The neutral is grounded at the pole and at entry to your house. You have dry wood floors and plaster walls. You have on normal clothing and footwear. The surface resistance of your hands is about 500,000 ohms. You need to replace a wall switch and you work on it hot with no insulating gloves? You should not do this but you do. You will touch the hot wire in the process. You do not touch the box that is grounded or the ground wire. Will you get a shock? Would you even expect to get a shock?

With a high impedance meter what might be the voltage reading between your body and the hot wire?

In my basement on a cement floor with my urethane sole shoes I read about 52 V on a Fluke 27. So across the 10 megohm input meter there is about 52 V drop and across my shoes about 70 V. The current thru me is about 5 microamperes. Upstairs on a carpet over a wood floor the meter reading is about 30 V.

.
 
charlie b

charlie b

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John Paul said:
The answer to the attached slide is yes.
Click to expand...
Are you sure? :-? The question asked in the slide is whether the man is in jeopardy (i.e., in danger). From the discussion, he is not in danger, so the answer to the slide's question would be "no." Did I miss a step?
 
weressl

weressl

Esteemed Member
Location
Texas by the Gulf, geat crab, great shrimp
John Paul said:
The answer to the attached slide is yes.

I thought that if you touched a 120V line or energized piece of equipment and you were touching a lower voltage like the earth or another piece of metal then you were toast.

The slides state that the current must return to the transformer/source.

What do yall think?
Click to expand...

Can't say.

If the transformer is a grounded system and one of the service wires are grounded, thus becoming a 'neutral', he may be.

The level of danger depends on WHERE the ground fault is occuring.

If the 'neutral' wire is grounded, no danger.

If the load is a resistor, or a wound coil, if the ground fault is close to the 'neutral' end of the resistor or coil, he is moving away from the 'no danger' position. The further the fault is away from the 'neutral' the more potenital that the voltage becomes more dangerous.
 
J

John Paul

Member
Location
Norfolk, VA
charlie b said:
Are you sure? :-? The question asked in the slide is whether the man is in jeopardy (i.e., in danger). From the discussion, he is not in danger, so the answer to the slide's question would be "no." Did I miss a step?
Click to expand...

Oops. You're exactly right, the answer is no according to the presenter. Thanks.
 
J

John Paul

Member
Location
Norfolk, VA
gar said:
Consider this: You have a normal 120 V circuit. The neutral is grounded at the pole and at entry to your house. You have dry wood floors and plaster walls. You have on normal clothing and footwear. The surface resistance of your hands is about 500,000 ohms. You need to replace a wall switch and you work on it hot with no insulating gloves? You should not do this but you do. You will touch the hot wire in the process. You do not touch the box that is grounded or the ground wire. Will you get a shock? Would you even expect to get a shock?

With a high impedance meter what might be the voltage reading between your body and the hot wire? .
Click to expand...

I don't think I'd feel a 'shock' but would have some current running through me. This is a guess based on your post, I'd say the potential difference between me and the hot wire would be 20V.
 
D

__dan

Senior Member
mmm ... braniacs

mmm ... braniacs

ron said:
Here is a good video about electric fences. If you are isolated from the earth, offering no path back to the source, no shock.
http://www.youtube.com/watch?v=-n1pSHzdahc
Click to expand...

I actually did that experiment in high school, freshman year IPS, Introductory Physical Science.

The teacher was out of the room and the guys had found this mini high voltage generator. It looked like a hand held hair clipper, plugged into 120 VAC with a lamp cord, and drew a 1" arc to metal.

I looked over and the guys were zapping the aluminum blackboard rail and touching the rail ten ft. away, they felt nothing.

I knew it was because the circuit path was open, they were not in the load path.

I told them they had to touch the rail and the cold water pipe at the same time, then they would get it.

It took four or five guys to reach, hand to hand, from the rail to the faucet. One guy out of the loop zaps the rail, I was in the middle of the loop. I felt the Zap and threw my arms up in the air to break the connection, woke up on the ground.

If you draw that circuit on paper, I'm pretty sure the secondary, high voltage winding, is not grounded, the blackboard rail was not grounded. The circuit may have been behaving more like a Tesla coil, single wire path with power transmission by resonant coupling between source and the load.

Let's say the guy grabs one leg of oil burner transformer with one hand and touches a 10,000 gal steel water tank with the other hand. Let's say the steel tank is completely insulated from ground on the polystyrene foam boards. The tank would be a capacitive load with single wire connection to source.

If the guys does not get a shock from that you could tune the HV source frequency of oscillation to the resonant frequency of the tank, then he will get it.
 
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