Getting shocked in lake or swimming pool

[mull982]

I have read a few articles lately with stories of people getting shocked in lakes and swimming pools and getting either seriously hurt or killed. This led me to a few questions to help me better understand these situations.

I think I saw an experiement done once where a hair dryer was trown in a toilet and due to the fact that the ceramic toilet did not provide a path to ground the circuit breaker did not trip. I assume though that the water in the toilet bowl was energized to the same potential as the circuit. What if a hair dryer or other energized conductor was put into a pool but somehow the pool had no connection to earth (hypothetically speaking). Would all the water in the pool then become energized at the same potential as as the circuit? What if a person was in the pool floating. Would anything happen to them since their entire body would also be at this same potential? What if this person was touching the bottom of the pool would anything happen?

Now lets say that the walls and floor of the pool provided a connection to earth. What would happen to a person floating? Wouldn't they be at the same potential as the circuit and no current would flow through their body? Now if they touched the sides or bottom which was connected to earth then there would be a potential difference across their body and therefore current could flow.

Would the same situations above apply to a lake? I have read cases where people have been swimming (floating) in lakes and been electrocuted. I dont understand this because if they were not in contact with earth and their body was at the same potential as the water then how is current flowing through their body?

 

[BJ Conner]

Mythbusters threw a hair dryer in the bathtub and determine it would kill you.
Swimming pools are a know hazard ( see section 680 of the NEC).
People in lakes ( and the ocean) have been killed by lightning, fallen power lines and faulty shore power systems.
You don't necessarly need a ground. The current is flowing in the water. If your in the water you can be a parallel path of lower resistance and zap!
Only the shock-proof guy can swim in a lightning storm. Me-I'm staying out.

 

[charlie b]

You are assuming (incorrectly) that the entire body of water is at the same potential. If there is current flowing, then there must be a difference in potential that is driving that current. If a person is swimming in a lake, and if something causes current to flow through the water (e.g., shore power failure or electrical fault on board a nearby boat), then the voltage will be highest nearest the source, and there will be voltage dropped all along the current path. That situation is no different than what happens within a wire. So there will be one value of potential in the location of the swimmer?s feet, and a lower value of potential in the location of the swimmer?s head (assuming an attempt to swim away from the source). That difference in potential can cause current to flow through the person?s body. Often, the shock causes the person to lose control of all muscles, and prevents the person from swimming at all. I have heard the term ?drowning by electrocution? used to describe the result.

For my part, I have decided that if I ever find myself in that situation, my actions would be to go vertical, wrap my arms around my chest, and remain as still as I can, treading water with short kicks of my feet or short motions of my hands. My reasoning is that I want to maintain as small a distance as possible from the point on my body that is closest to the source and the point on my body that is farthest from the source. That would mean the smallest possible potential difference across my body, and the smallest possible current through my body. An attempt to swim would more likely be fatal, since it would create a larger potential difference and would result in a higher current.

 

[mull982]

You are assuming (incorrectly) that the entire body of water is at the same potential. If there is current flowing, then there must be a difference in potential that is driving that current. If a person is swimming in a lake, and if something causes current to flow through the water (e.g., shore power failure or electrical fault on board a nearby boat), then the voltage will be highest nearest the source, and there will be voltage dropped all along the current path. That situation is no different than what happens within a wire. So there will be one value of potential in the location of the swimmer?s feet, and a lower value of potential in the location of the swimmer?s head (assuming an attempt to swim away from the source). That difference in potential can cause current to flow through the person?s body. Often, the shock causes the person to lose control of all muscles, and prevents the person from swimming at all. I have heard the term ?drowning by electrocution? used to describe the result.

For my part, I have decided that if I ever find myself in that situation, my actions would be to go vertical, wrap my arms around my chest, and remain as still as I can, treading water with short kicks of my feet or short motions of my hands. My reasoning is that I want to maintain as small a distance as possible from the point on my body that is closest to the source and the point on my body that is farthest from the source. That would mean the smallest possible potential difference across my body, and the smallest possible current through my body. An attempt to swim would more likely be fatal, since it would create a larger potential difference and would result in a higher current.

But wouldn't for the most part the body of water all be at the same potential thus providing an equalpotential plane? Is there that much of a resistance between points in water 5-6ft away to create a signifigant potential. I always thought for the most part that water was conductive.

Lets say that the body of water was somehow isolated from ground. If a circit energized the water then would there be no current flow because there is no connection to ground? Would the water and the person therefore be at the same potential?

 

[wawireguy]

I recommend testing the water with your tick tester before jumping in. Just to be safe..

 

[charlie b]

But wouldn't for the most part the body of water all be at the same potential thus providing an equal potential plane?

No.

Is there that much of a resistance between points in water 5-6ft away to create a significant potential.

Yes there is. And fatalities have resulted from this very phenomenon.

I always thought for the most part that water was conductive.

Pure water is not, but no water is pure. Lake water, for example, is conductive. But its resistance is not zero. As a voltage source drives current through the water, there will be a voltage drop along the current path. The water acts the same way a resistor or a wire would act.

Let?s say that the body of water was somehow isolated from ground. If a circuit energized the water then would there be no current flow because there is no connection to ground? Would the water and the person therefore be at the same potential?

If a body of water were electrically isolated from planet Earth, then you are right. But a lake cannot be isolated in that sense, since the bottom of the lake is dirt. Neither can a bathtub, if the drain piping is metal.

 

[jcole]

Just to add a question to the op scenario.

What if there was no such thing as a grounded system? Would there still be current flow? Current seeks the opposite polarity of the power source that created it, not earth.
Correct?

So if the systems werent grounded to earth then electrocution would not be an issue.

Seems logical to me. What am I missing?

 

[charlie b]

Current seeks the opposite polarity of the power source that created it, not earth. Correct?

Correct.

So if the systems weren?t grounded to earth then electrocution would not be an issue.

Not correct.

Seems logical to me. What am I missing?

What you are missing is capacitive coupling. The electrical systems that I remember from my Navy days were not grounded. But a person could still be electrocuted, if they touched just one side of a live circuit. The circuit wires and the ship?s hull form a capacitor, and a charge will build up between them. So if you touch the wire, your body will complete that circuit, and the charge will discharge through you.

 

[mull982]

You are assuming (incorrectly) that the entire body of water is at the same potential. If there is current flowing, then there must be a difference in potential that is driving that current. If a person is swimming in a lake, and if something causes current to flow through the water (e.g., shore power failure or electrical fault on board a nearby boat), then the voltage will be highest nearest the source, and there will be voltage dropped all along the current path. That situation is no different than what happens within a wire. So there will be one value of potential in the location of the swimmer?s feet, and a lower value of potential in the location of the swimmer?s head (assuming an attempt to swim away from the source). That difference in potential can cause current to flow through the person?s body. Often, the shock causes the person to lose control of all muscles, and prevents the person from swimming at all. I have heard the term ?drowning by electrocution? used to describe the result.

For my part, I have decided that if I ever find myself in that situation, my actions would be to go vertical, wrap my arms around my chest, and remain as still as I can, treading water with short kicks of my feet or short motions of my hands. My reasoning is that I want to maintain as small a distance as possible from the point on my body that is closest to the source and the point on my body that is farthest from the source. That would mean the smallest possible potential difference across my body, and the smallest possible current through my body. An attempt to swim would more likely be fatal, since it would create a larger potential difference and would result in a higher current.

I thought about this one some more

Yes there is a difference in potential across the lake from the voltage source to the bottom of the lake or earth. This voltage potential is what causes the current to flow in the lake. But what if the water in the lake was all at a very low reistance and there was a negligable resistance between any distance 5-6ft away? Then there wouldn't be any potential between any points 5-6ft away in the lake and thus no potential across your body?

How is this different from a substation or ground grid that may have circulating currents. Currents are flowing through the ground grid but when you walk you are not getting shocked?

 

[adelle]

Pure water is not, but no water is pure. Lake water, for example, is conductive. But its resistance is not zero. As a voltage source drives current through the water, there will be a voltage drop along the current path. The water acts the same way a resistor or a wire would act.

Case in point. High power RF tubes like in radar and TV transmitters are often water cooled with the >20kv anode connection immersed in water. Deionized water for this purpose as it's not conductive.

 

[e57]

I recommend testing the water with your tick tester before jumping in. Just to be safe..

I'd would have said it was a bad or ineffective method had I not done it before and clued me into 140V of 208 single phase in a hot tub - due to a motor failure and rotted bonding... People were getting shocked getting in/out of it - and when I got there - swiped the ticker by it in almost a joke - it then went nuts and tweety... :roll: "Well I guess I wont stick my hand in there..." Got my meter out, and made sure not to touch anything or get wet.

 

[kwired]

I thought about this one some more

Yes there is a difference in potential across the lake from the voltage source to the bottom of the lake or earth. This voltage potential is what causes the current to flow in the lake. But what if the water in the lake was all at a very low reistance and there was a negligable resistance between any distance 5-6ft away? Then there wouldn't be any potential between any points 5-6ft away in the lake and thus no potential across your body?

How is this different from a substation or ground grid that may have circulating currents. Currents are flowing through the ground grid but when you walk you are not getting shocked?

The grounding grid is a better conductor than water so there is less voltage potential across the same distance.

 

[nollij]

What you are missing is capacitive coupling. The electrical systems that I remember from my Navy days were not grounded. But a person could still be electrocuted, if they touched just one side of a live circuit. The circuit wires and the ship’s hull form a capacitor, and a charge will build up between them. So if you touch the wire, your body will complete that circuit, and the charge will discharge through you.

Was the ship a DC system? If it is AC I am trying to determine how an ungrounded system can build up charge if all phases are capacitively coupled to the hull.

Intuition tells me that the phase that is grabbed will lower in voltage and the other two phases will have a higher charging current to ground due to the increased voltage from the reference node (hull). The current through the reference node has to sum up to 0 thus the current will have to flow through the person touching the conductor and the capacitive coupling on that phase (in parallel). I guess my question would be, what is the relationship between the voltage difference between the reference node and the touched phase and the capacitance of the system to the reference node. It has been wracking my brain trying to calculate what the location of the reference node would be on the voltage triangle by grabbing a phase.

The maximum possible electrocution current would come from a zero resistance connection to ground in which case the current flowing through that connection would be the charging current flowing through the other two phases to the reference node.

Would this be a symmetrical components ordeal or an iterative calculation with starting values using assumptions. I would figure that the capacitive coupling throughout the system between each of the phase conductors would need to be taken into account as well.

edit: I am just way over thinking this.