electricity in a canal

[drewder]

So behind my house there is a irrigation cannel which we and our neighbors and others use to water our plants. Many use flood irrigation but some use electric pumps. In the summer swimming or intertubing in the canal is normal.

Recently I have heard reports of people with “electricity leaks” from pumps which recently killed a family and their dog. This has worried me since its impossible to know if the canal is electrified by looking at it. How far could electricity travel through water in such a situation. Unsure of the voltage but some pumps go up to 440 volts.

[core]

Do you happen to have any links to news stories on the family that was killed? I’d like to read about the circumstances of that one. I’m not having any luck thinking of a circumstance that would kill an entire family out in the middle of the canal.

Not related to the deaths, but: It can be “normal” for the water to be a few volts above ground. This is caused by losses in the cables and gets worse during peak electricity usage (summer). The voltage can rise over time as the power company’s neutral degrades, as with old concentric cables. The poco is not going to dig up cables and replace them. For a hot tub or swimming pool install you’d “fix” the issue by burying a metallic grid (now required by code), but that’s not feasible for an entire canal of course.

Those few volts wouldn’t be enough to kill you, but it could wake you up. And if you were a poor swimmer, and every time you tried to get out of the water you got shocked, well I can see how someone might conceivably drown?

All that to say: I can see where people getting a little bite might cause rumors to be started about “leaking” electricity from pumps. That doesn’t explain the deaths though; I’m thinking they actually came into contact with something. Eager to read the details.

[engineer_comp_geek]

Water doesn’t get “electrified” the way Hollywood shows it. That said, water and electricity aren’t exactly a safe combination either.

Here’s the way it works in the real world.

If you have an “electricity leak” (typically something conductive like a wire or a pump or a light, where the insulation has become degraded somehow), what you have is 120 or 240 volts AC (or maybe up to 440 if you are certain of that voltage) at the fault, and 0 volts at earth ground (meaning literally the ground, as in dirt, mud, rock, etc) at the bottom of the pool or canal or bathtub or whatever. In between there you’ve got water.

You may have heard that electricity takes the shortest path. That’s wrong. Electricity takes all paths. However, more electricity will flow through the shortest path than will flow through longer paths. And actually “shortest” in this context means most conductive, which may or may not be the physically shortest path. More electricity will flow through 100 feet of copper wire than will flow through 3 feet of wet wood, for example.

Since electricity takes all paths, it spreads out through the water. You end up with these voltage gradients all through the water. If the voltage all around you is the same, that’s no biggie. It’s when you have a larger gradient across your body (in other words, a higher voltage on one part of your body than on another) that current will flow through your body.

If you touch a pump with an electrical short in it with one hand and you touch a metal pipe sticking up out of the canal with the other hand, now you’ve got the full 240 volts (or whatever the pump is) straight across your body from one hand to the other. The path of the electricity goes through your chest, and there’s a very good chance it will screw up your heartbeat and kill you.

If you just swim up through the water, you’ll probably be pretty safe as long as you are 20 or 30 feet or more away from the pump. Get in close, like 5 to 10 feet, and you’ll start to feel tingling. If you feel tingling, there is more than enough current there to potentially screw up your heartbeat, so get away from it immediately. At those low current levels though, the chance of death is fairly low. The closer you get to the source of the electricity, the higher the risk of death.

Like core I’m having a bit of difficulty picturing an entire family getting killed, unless they contacted an underwater high voltage line or something like that. Or if they were struck by lightning. Your typical lightning bolt is a few billion volts with a few hundred thousand amps of current flowing. You need a few hundred yards of water between you and the point where the lightning bolt hit before the voltage gradients get down to the safe level.

[bob_2]

@engineer_comp_geek

If you just swim up through the water, you’ll probably be pretty safe as long as you are 20 or 30 feet or more away from the pump. Get in close, like 5 to 10 feet, and you’ll start to feel tingling. If you feel tingling, there is more than enough current there to potentially screw up your heartbeat, so get away from it immediately. At those low current levels though, the chance of death is fairly low. The closer you get to the source of the electricity, the higher the risk of death.

Is that correct? Surely there has to be a potential difference to create a current? I guess that if your feet were close to the source and your head further away it may be enough to feel, but I have my doubts. Of course, if part of your body was grounded, that would be different, but I am picturing someone floating and touching nothing but water. Surely the same as a bird on a HT wire?

[engineer_comp_geek]

The potential difference comes from the voltage gradient through the water.

I’m guessing a bit on the distances, but it is based on where people have told me that they have felt tingling when walking through flooded basements with live 120 volt AC wires in the water or in swimming pools with broken lights that also resulted in live electricity in the water.

At 120 volts you aren’t going to feel anything until you get pretty close, somewhere around 3 to 5 feet or so. Extrapolating on that, if some of these pump motors are indeed 440 volts, then the dangerous range is going to extend a bit further into the water.

ETA: This picture shows a voltage gradient across ground. Even though it shows ground instead of water, I hope it gets the point across.
http://www.lanera.com/stray_voltage/strayvolt/picture/rodgrad111.gif

[bob_2]

In a flooded basement, you are in contact with the ground (unless you are swimming :)). Your picture shows the gradient which I understand. What it doesn’t show is what voltage difference there would be between a point [say] 6 feet from the source and another [say] 12 feet away. Also bearing in mind that the current is being dissipated to ground through the water and a body would have a higher resistance anyway.

I have no intention of trying this, but I do not think that one would feel anything at all while floating, and very little even if you touched ground.

[drewder]

http://blackfootjournal.com/three-die-in-area-canal/

Here is one copy of the story.

[LSLGuy]

@bob++

“Ground” is not a magic point.

Between a point source of voltage at one spot in a pool of water and any points on the bottom / edge of the pool there will be a measurable voltage difference. Likewise between any two points anywhere within the 3D body water there will also be a measurable voltage difference.

If you’re thinking like “The water is completely full of 120V and all the difference between 120V and ground voltage happens only and entirely at the water/dirt interface”, you’re thinking wrongly.

It’s much more “There’s a continuous gradient from 120V above ground voltage at the source and 0V above ground voltage at the water/dirt interface.”

So what you will feel = what you would measure with voltmeter leads attached to your head and feet, is the difference between the voltages present wherever your head and feet are.

As in the picture of cows, what they feel is the difference between the absolute voltage present at their front feet minus the absolute voltage present at their hind feet. And the closer they are to the voltage source, the larger that perceived difference will be.

If you arrange yourself lengthwise to the electric current in water you’ll feel more voltage difference than you will if you arrange yourself crosswise to it. If the cows face directly towards or away from the post they’ll feel much more than if they stand facing tangent to a circle around the post.

And for the case of a point source in a pool with a dispersed ground across the pool’s entire bottom / sides, you’ll in effect disperse the electrical current, so the most concentrated (in terms of space) voltage differences are near the point source and the less concentrated differences are near the much larger area of water / dirt interface.

Which is why e_c_g said the risk is when you get close enough to the source to begin feeling it. And from that point every movement you take towards the source increases the risk at roughly an inverse square law rate.

[core]

According to news stories, the sheriff’s deputies approaching the scene could “feel it in the ground”, and one was knocked down from clear up the bank.

I dug up another story which seems to explain the cause:

Preliminary information from those investigations has revealed an unfortunate sequence of events. Tree contact, exacerbated by windy conditions, damaged both our and a customer’s electrical equipment. This led to a ground wire being energized, sending current from the source along approximately a quarter mile of the customer’s buried conductor to the site of the accidents.

So it was the ground conductor that was energized, and probably at 7.2kV or worse. In that link there is a photo of the pump equipment and the canal. This canal is nothing like what I was picturing. It’s only a few feet wide. If you went in the water you’d be practically on top of the pump, but the pump wasn’t at fault here.

[drewder]

@core

According to news stories, the sheriff’s deputies approaching the scene could “feel it in the ground”, and one was knocked down from clear up the bank.

I dug up another story which seems to explain the cause:

So it was the ground conductor that was energized, and probably at 7.2kV or worse. In that link there is a photo of the pump equipment and the canal. This canal is nothing like what I was picturing. It’s only a few feet wide. If you went in the water you’d be practically on top of the pump, but the pump wasn’t at fault here.

my concern is letting my girls go swimming in the canal (maybe a little wider than that one). How far up and down stream should I inspect to ensure it is safe.

[core]

@drewder

my concern is letting my girls go swimming in the canal (maybe a little wider than that one). How far up and down stream should I inspect to ensure it is safe.

I’m not sure what you would inspect. Even a professional wouldn’t be able to tell anything from just looking at the outside of the equipment. Joe Homeowner could have done any number of nasty things while wiring up his pump. Contractors who should know better take shortcuts and make mistakes too. Telling a couple young girls to stay away from certain things in such a small canal probably isn’t going to work real well.

For the accident in question, it sounds like the power line made contact with another conductor a quarter mile away. Nobody would have seen that walking along the canal.

[bob_2]

@LSLGuy

@bob++

Which is why e_c_g said the risk is when you get close enough to the source to begin feeling it. And from that point every movement you take towards the source increases the risk at roughly an inverse square law rate.

Thank you - ignorance fought.

[Me_Billy]

You would measure for electricity with a voltmeter…

1. One test lead upstream a few feet and the other downstream a few feet.
2. Across the canal - One test lead near one bank of the canal and the other test lead near the other canal bank.
3. And up/down - One test lead near the bottom of the canal and the other near the top of the water.

The absence of any measurement of any electricity would mean there is not electricity in the water in that PARTICULAR section of the canal.

FYI - “Pure water” does not conduct electricity (like distilled). But salt water is a very good conductor as is water with chemicals or minerals in it. Note people have been electrocuted swimming near boat docks where faulty electrical wires were conducting electricity to the salt water.

FYI-2- There is a device called a GFCI or Ground Fault Circuit Interrupter (commonly used as bathroom and kitchen outlets) which will shut off the electricity instantly should there be a fault to ground [canal/sea water]. These are typically required outdoors, but I don’t know if they would require them on water pumps?

[drewder]

Pure water may not be a good conductor but canals don’t generally have pure water.