Theory Jetted Tubs

Status
Not open for further replies.

gary b

Member
Theoretical Thoughts:
For several years now the public has been educated concerning the dangers of using or dropping electrical devices in a bathtub or sink (shock or electrocution). Several products used in bathrooms are so marked.
In a modern bathroom a totally fiberglass and plastic constructed hydromassage bathtub is installed along with accompanying plastic water line system and drain line system. There is absolutely no conductive metal of any kind ever in contact with the tub water. The tub has GFCI protection as required in 680.71.
A two-wire hair dryer or radio (no equipment grounding conductor) plugged into a GFCI protected receptacle outlet is dropped into the jetted tub described above. The primary question here is would a person sitting in the tub receive a shock or be electrocuted? If so, how? Would the GFCI device deactivate the power source and if so how is the alternative current path traced back to the source by-passing the GFCI device?
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Yes, the person could get shocked, even electrocuted. No, the GFCI device will not provide any protection.

Current takes any and all available paths back to its source. The ?normal path? goes into the radio via one wire, passes through the internals, and back to the source via the other wire. With the radio submersed in water, new paths are created. One such path is from an energized component located close to the hot conductor, into the water, around the tub, and to the neutral conductor, and from there back to the source. A person in the tub will form part of that path.

A GFCI works by measuring the difference in current in the two conductors (i.e., phase conductor and grounded conductor). It doesn?t care, and could not tell anyway, where the extra current is going, or how it finds its way back to the source. If the two currents that it measures are not equal, it will trip. But in this case, all the current that leaves the GFCI receptacle on the hot leg will make its way back to the GFCI receptacle on the neutral leg. So the GFCI will see equal currents on both legs, and not be aware that some of that current is passing through a human body.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I agree with Charlie unless the tub is metallic and has a grounded metal waste water piping system connected to it. Then some of the current would flow on the piping. If you could get that current level above 5 ma the GFCI would trip.
 

gary b

Member
Shocking

Shocking

I agree with the statement about the GFCI. I the classroom I took a plastic water jug and filled it with tap water. Next I took a double insulated drill, taped up the trigger so it would run continuously, plugged it into a GFCI protected receptacle outlet and dropped it into the water jug. The GFCI did not trip as I expected. Next I put some meter leads in the water with the drill (pumping water like crazy, yes it is an old drill) an attempted to read some voltage and then some current. The meter stayed at zero. Next I put some gold fish (some I feed my turtle) in the water with the drill. They lived to tell about the water churning experience and did not seemed bothered by the energized drill in the water. Also I tried the goldfish with an energized extension cord end, again them swam happily around the plastic bowl (like two lost souls, in a fish bowl, year after year.) Next (foolish, I know) I put my hand in the water with the drill. I did not feel any shock or electricity at all and I still live to tell about it.

Now I am confused. How is there a path for current flow from the drill (radio, hair dryer, etc.) though the water to complete the circuit with a human in a tub? First I thought of the ?bird on the wire? concept but I don?t think that is quite right. I have thought it though with series and parallel circuits but again I don?t see the current path that would explain how the electricity would enter the body and then return back on the neutral.

Can anyone think of a way to demonstrate how a person would be shocked in a totally plastic tub with actually using a real person, dog, well maybe a cat, no, not even a cat. I tend to think a shock may not happen.

What about voltage gradients? Would that have something to do with this?
Thanks for the help, I know this is a weird obsession but it is really bugging me.
Gary B.
 

wbalsam1

Senior Member
Location
Upper Jay, NY
gary b said:
I tend to think a shock may not happen.

What about voltage gradients?

In my very limited understanding, I would venture this: A radio dropped into a bathtub could have a voltage differential of an extremely hazardous value. Assuming 0 volts at the bottom of the fiberglass tub and 120 volts where the radio enters the tub, say at one end, a 6 feet tall person could see as many as 30 volts diffferential across his body. This could lead to either severe shock immobilization (leading to drowning) or electrocution.
 

dbuckley

Senior Member
But thats the point; there is no voltage gradient across the tub ibecause the tub is entirely insulated. In one respect this is bad; if there were a voltage gradient across the tub in the conventional sense then current would be flowing, presumably to ground (the bonded plughole perhaps) and thus the GFCI should trip.

There does remain a hot-to-neautral shock hazard, I'm sure it's there, despite the experiment with the drill not demonstrating it.
 

mengelman

Member
Location
East Texas
This thread reminded me of something I saw years ago in Las Vegas.
I was waiting to get into a restaraunt and looked down into a fountain. In the bottom was a hardware store varity porcelean lampholder and a standard lamp with wires running across the bottom. I assume this was 12v but I have allways wondered.
I would be curious as to how far 120V will travel in water, like from hot to neutral? I guess the question is what is the dielectric of tap water?
 

hillbilly

Senior Member
I'm going the other way and say that the person in the tub would recieve no shock.
It's the same scenario as a bird sitting on a high voltage wire.

Just a opinion (that I don't plan on testing).

steve
 

wbalsam1

Senior Member
Location
Upper Jay, NY
The shock hazard is much greater when a person is immersed in water. The water makes contact with the entire skin surface, and wounds, cuts and scratches reduce the body's resistance even further. Openings such as ears, nose and mouth even further reduce resistance. A voltage gradient of as little as 1 1/2 volts per foot can cause paralysis. Persons can suffer an immobilizing or a lethal shock. Direct contact is where a person touches the live wire, in this thread a radio in a tub, (hard to avoid touching contact with the radio in such a small area such as a tub), or by indirect contact such as being in the water where voltage gradients are present.
Its hard for me to imagine no gradients present. :) ( But, like I say, I really don't know and would have to defer to those that do.)
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
In my freshman physics class, we had a cute demo. We were given some slightly conductive paper, so conductive silver paint, a voltage source, and a volt meter. We could paint various electrode configurations on the paper, attach the voltage source, and then use the volt meter to plot out equipotential curves on the paper. The current flowing through this two dimensional conductive surface, and the voltages produced, were approximately representative of the way the electric field would distribute itself in three dimensional space, but because there was significant current flow, an ordinary volt meter could detect the potential levels.

Current flow in this conductive paper would be quite a bit like current flow in the insulated water container. The conductive surface (or volume) can be considered a large set of different paths that electricity can follow. Some of these paths are very short, directly from one electrode to the other, and others are very long, leaving one electrode, going around the tub, and returning to the other electrode.

Well, given multiple paths to follow, the rule is that electricity follows _all_ paths available to it, in inverse proportion to the resistivity of that path. In the case of the drill in the bucket, the vast majority of current will flow in the copper wires and normal portions of the drill; a small portion of the current will flow through the water inside the drill between the various energized surfaces, and a still smaller proportion of the current will flow from the energized surfaces, out through the various holes, around the drill, and back to energized surfaces.

Drop a radio into a fully insulated bath tub, and most of the current will be flowing _between_ the electrodes inside the radio. Very little current would take the longer path, out from the radio, around through the water, and back into the radio on the other side. There would thus be very little shock risk.

But again, electricity follows _all_ paths available to it, in inverse proportion to the resistivity. There would be _some_ shock risk.

If the tub had bonded metal surfaces, than there would be significant current flow paths between the radio and the bonded metal. A person in those paths would be shocked, but a GFCI would probably trip. Without the bonded metal surfaces in the water, the current flow path is predominantly inside the appliance, going between hot and neutral, with only a small amount of current flowing outside the appliance. Since the hot and neutral current flow would likely be balanced, it is likely the GFCI would not trip. (With something like a large bathtub, there might be some unexpected current flow path that we didn't account for.)

Drop an electrode connected to 'hot' at one end of the tub, and an electrode connected to 'neutral' at the other end of the tub, and if it were an insulated tub there would both be a very significant shock risk, and very little chance of the GFCI tripping.

-Jon
 

JohnConnolly

Senior Member
Location
Phoenix AZ
My first reaction is that there will be no electrocution.


If the current cant go to ground to trip the GFCI, then it can't go through you.

Can it go through you and back on the neutral?

Beats me.
 
Voltage Gradient

Voltage Gradient

Speaking of voltage gradient reminded me of a theoretical exercise we did in Dr. Bansal's Electromagnetics class. Essentially, there is a cow grazing in the field about 6' from a tree. The tree gets struck by lighting, and the current goes down the tree where it disperses concentrically. If the cow happened to standing with the tree at its side, then the potential difference between its left and right legs were not enough to harm the cow. However, if the unfortunate cow were facing the tree, or had its back to the tree, then the potential difference between the front and back legs was enough to blow the cow up. BBQ anyone?
 

dlhoule

Senior Member
Location
Michigan
JohnConnolly said:
My first reaction is that there will be no electrocution.


If the current cant go to ground to trip the GFCI, then it can't go through you.

Can it go through you and back on the neutral?
Try plugging in one wire to the hot side of GFCI and the other to the neutral. Now grab one with each hand. Do you not think the current will flow through you and back to the grounded conductor without tripping GFCI.:-?

Please do not actually try this.



Beats me.

How many extra spaces do I need?
 

quogueelectric

Senior Member
Location
new york
the metal sphere theory

the metal sphere theory

BigDon said:
Speaking of voltage gradient reminded me of a theoretical exercise we did in Dr. Bansal's Electromagnetics class. Essentially, there is a cow grazing in the field about 6' from a tree. The tree gets struck by lighting, and the current goes down the tree where it disperses concentrically. If the cow happened to standing with the tree at its side, then the potential difference between its left and right legs were not enough to harm the cow. However, if the unfortunate cow were facing the tree, or had its back to the tree, then the potential difference between the front and back legs was enough to blow the cow up. BBQ anyone?
this is directly related to the theory of electrons repel each other. If you believe that this theory is correct then it follows that if you charge a sphere with lots of electrons that they will try to get as far away from each other as possible. on a conductive sphere this means that the electrons will all be on the surface with zero charge in the center of the sphere. What a coincidence!!!!!!!!!!!!!!The earth is a conductive sphere!!!!!!!!!!!!!! Sucks to be a cow facing the tree. MOOOOOOOOOOOOOOOO!
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
I've got no problem with cows. I lived on dairy farms until I was 8. :)

The last several sentences of your post only served to distract from your message. Whatever you were thinking was probably pretty funny in your head, but once it was inserted into the post it looked like incoherent gibberish. Incoherent gibberish usually only finds it's way into posts if booze is involved. :D

That, and there was a kinda "DUH STUPID!" vibe to your post, but that was probably unintentional.
 
Status
Not open for further replies.
Top