Electricity in water

I resently saw a Mythbuster episode where they tested the old toaster in the baththub clichee - and unsurprisingly they concluded that this was indeed lethal for anyone unlucky enough to be in the bathtub at the time.

However, this got me thinking - how big a body of water could you electrocute in this manner? If I was, say, in one end of a largish swimming pool and some evildoer dropped a toaster or other appliance in the other end - what size would the pool have to be to let me stand a reasonable chance of surviving? Water has its own resistance, so certainly parts of the voltage would dissipate given enough distance.

Another scenario - a flooded area, with powerlines falling down and for some reason the current is not cut but continues to flow between the severed ends of the line. How close to this could one swim without getting electrocuted? I’ve seen footage of people swimming in flooded areas with broken powerlines giving of sparks in the water 100 meters away, how lucky were these people that they werent electrocuted?

I’m mainly thinking non-saline water here, but I’d imagine broken waterlines in the ocean are even more hazardous to the surroundings, but how far away?

Now I realise this topic may well have been debated ad naseum before, but since I havent got the ability to search maybe someone could point to another thread if applicable.

One issue here may be that the plumbing is often connected to your electrical system’s ground, so it’s not just that you’re sitting in water in the tub, you’re sitting in water that may be connected to the ground, through the contact between the water and the drain pipe. Usually the grounding is done to the water supply, rather than the drain system, but I can imagine a situation where there would be some connective bridge between the two. Someone more sophisticated about plumbing can perhaps weigh in.

Pointing to earlier threads:

How does one get electrocuted in a lake?

Why didn’t my brother electrocute himself?

What happens if lighting strikes the ocean?

*Practical Sailor * magazine did some articles on boat wiring, charging, and whether or not to ground everything together to protect against lightning damage. There’s also the mysteries of galvanic corrosion; holy cow, that stuff is complicated. Anyway, it was pointed out that boats plugged into shore power at the dock can leak some of that into the water.

They said that a pretty small amount of current can mess with a human’s ability to swim. It won’t be enough to electrocute the victim as it would on land, but he can’t swim, so he drowns. There was a letter from a sailor who paid a marina worker to retrieve something that was dropped overboard. He watched in horror as the kid drowned.

I didn’t see this episode. Wish I had. Maybe it will come on a repeat soon.

Anyway, Mythbusters usually tends to test the “fatality” of an electrical situation by measuring the current flow. If it’s above a certain level, then it is fatal. If not, it’s safe. That’s fair enough to determine if something is dangerous or not, but what really happens is a lot more complex than that.

Electricity kills you in generally one of two ways. It either cooks you to death (literally) or it screws up your heartbeat. It takes a lot of energy to cook you to death, but if you’ve got this much energy, the shock is almost always fatal. The electric chair is a good example of how to cook someone to death.

It takes a surprisingly small amount of energy to screw up your heartbeat, and it is this small current level that Mythbusters generally tests to. But, this is a lot less guarnateed. At these current levels, the electricity might kill you, or it might not. A lot of it depends on exactly what part of your heart’s rhythm it happens to be in when you get shocked. Your heart is more sensitive to being thrown out of whack at some points in your heartbeat than others, which is why it’s a lot more random.

The thing about the low level shock is that the current has to flow through your heart to cause problems. If you are sitting in a tub, and the electricity flows through your legs, it’s no big deal. It might not be very comfortable, but it wont’ kill you. As soon as you reach for the toaster, though, you get your arm involved, and your arm is connected to your chest, and as soon as the current flows through your chest then you really have problems. A shock that barely tingles is more than capable of stopping your heart.

I don’t know what the probabilities are, but people get shocked all the time and don’t die from it. Of course, a few do die, which is why we have safety rules and such.

As long as you kept your arms and chest up out of the water, you’d have a reasonable chance of surviving in a bathtub. If you get your chest involved in the shock, then you’d probably need to be a couple yards or so away from the toaster to have a good chance of survival. For an average sized back yard swimming pool, you’d be pretty safe when the Evil Bad Guy drops the toaster in the other end.

100 meters is pretty far. Power lines are a lot higher voltage than the lines inside your house, but even so the chances of them getting electrocuted at that distance are pretty small.

Power lines are particularly dangerous because they have circuits which automatically shut off the electricity when there is a fault, and other circuits callled reclosers which periodically try to reconnect the line. The reclosers typically try to open the line again a few times quickly, then once after say fifteen seconds, then once after a couple of minutes, and then they give up. You see sparks and wade out into the water, but you don’t feel a shock because at that point the line is dead. You think it’s safe now, so you keep wading out. Then the two minutes goes by and the reclosers connect the line, and a huge current flows through the water (and you). The line shuts back off because the line is still down and shorting out, but you end up dead because of the momentary current.

Thanks for the answers and links… I’m a bit more enlightened now :slight_smile:

They did a reasonable job with this one. The main result was that any modern bathroom appliance has a ground-fault plug, which will trip immediately when the appliance is dunked in a tub. The ground-fault circuit required in the bathroom wiring will also kick in. They ended up removing both those before they could get any results.

After that, they noticed that the person sitting in the water has to be between the appliance and the drain (the only ground) to get any current flow through them. Since the current flow is between the appliance and the drain, to get shocked you must be in that path (unless you provide another path, e.g. by holding onto a grounded piece of metal.

The current flowing through the entire length of the tub ended up pretty small. On top of that, a person sitting in a tub won’t have his heart in the current path. As you said, if a person is sitting in a tub, the current will generally flow through the legs (in parallel with the water around them, which also reduces the current through the legs). Unless the person were laying down in the water, this isn’t going to involve the heart much.

I think they ended up only getting a “dangerous” current (which was the small amount you mentioned they always test for, just a few milliamps) only by putting a toaster very close to the drain. The myth was busted I believe.