Dropping a live power plug on extension cord into a swimming pool is certainly not a great idea. However I wonder if doing so would kill the swimmer in the pool? I've been searching the internet for some sound explanation, not only yes / no without reasoning.

I'm asking because I was doing some work with power tools around the pool over the weekend and the extension lead cable dropped into the pool. Not the plug, just the cable, so nothing happened. And there were no people in the pool anyway. But it got me thinking what would happened if I immersed the plug?

If it's RCD protected (known as GFCI in the US I think) I assume it would immediately trip because from Live roughly 1/2 of the current would go to Ground and 1/2 to Neutral and RCD would kick in. That should protect the swimmers, right?

However what if there was no RCD / GFCI?

I think the electrical current would again go mostly from Live to Ground + Neutral as these are only millimeters away. Some of it may ground through the concrete and plaster pool floor and walls. Also there may be some water grounding through the pump, filter, salinator, etc.

And what about the swimmer? Would he get electrocuted while swimming? He may be more conductive than the water around (blood being more salty than the pool water), that indeed plays against him. On the other hand he's not grounded as long as he keeps floating.

  • Does it matter if he touches the walls?

  • Does it matter how far from the submerged power plug he swims?

I'm not looking for an advice saying that I shouldn't immerse power plug into the pool, I know that :) This is a what if and why that question.

  • Hello, and welcome to Home Improvement. This is interesting, but doesn't have anything to do with Home Improvement. – Daniel Griscom Feb 19 '19 at 22:50
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    I'm voting to close this question as off-topic because it doesn't have anything to do with Home Improvement. – Daniel Griscom Feb 19 '19 at 22:51
  • This looks like a good question for physics.stackexchange.com. That being said, there are too many variables to know if it would be lethal, but it certainly could be. – mrog Feb 19 '19 at 23:22
  • Pure water is actually not very conductive. Most pools today tho use salt water for chlorine generation. Salt water is highly conductive. – Tyson Feb 20 '19 at 0:04
  • While I agree this is off-topic, you should know that your pump(s) are required by code to be grounded directly. So you should see a bare ground wire running off the pump and into the ground. While it might not handle 120v plug being dropped in, it does provide a path to ground that doesn't involve the swimmer – Machavity Feb 20 '19 at 13:30

You mean like this tragic case?

It wouldn't matter, since the worst-case scenario must drive your design decisions.

Current follows all available paths in proportion to their conductance(1/resistance).

Say the path through the pool is 20 ohms/50 millisiemens, but the path through the swimmer is 10 kiloohms/0.1 millisiemens, then only 1/500 of the total current flows through the swimmer. Not so bad, right?

Wait. E=I/C or CE=I, so 0.1x120=I = 12 milliamps. That is enough to stun, which for a swimmer, drowns.


Water and power don't go together. Could it be lethal? The answer is yes.

A similar problem occurs at marinas and boat docks, I believe the last I read at least 5 people a year die each year in the US from stray currents causing involuntary muscle spasms leading to drowning.

Water breaks down the skin resistance and this makes us more susceptible to shock. Water is not a good conductor and distilled water is actually an insulator so while the electrons are trying to find their way back to the source, a body coming in contact with ground becomes a faster way back resulting in a severe shock and possibly electrocution. This problem in the stray currents is not usually line voltage, but a cord end in the water would be line voltage - 120v in the US, 240 on your side of the pond would be worse.

I believe your RCDs are set at 30ma with your feet on the ground this sounds safe to me, but in the water the effects can be worse because muscle spasms. On dry land the slight spasms many times remove you from the danger but in the water you can't get away and when you contact a grounded surface trying to get out the shock gets worse. In the US our GFCI devices trip at less than 6ma, all 120v outdoor outlets including ones around pools are required to be protected for this reason. Hope that helps.

  • And of course it is possible that you could be on a ladder, get a shock and muscle spasm and fall off the ladder (or even kick it away if you can) - breaking the circuit but then risking broken bones. SAFETY FIRST – manassehkatz-Moving 2 Codidact Feb 20 '19 at 0:04

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