Fresh Water Conductivity and GFCI

[JoeHam]

So here is my latest display of stupidity:

Last Friday I was up on top of a boat lift inside a metal boathouse. I was running an angle grinder powered by my 48v LiFePO4 through a 3KW inverter.

I know this starts out like a @ghostwriter66 oilfield idiot story and I guess it is!

As I moved the angle grinder the extension cord unplugged itself due to tension (note to self, ALWAYS secure extension cord).

Then I watched the live end of the extension cord drop into the fresh water (Lake Erie) below as I realized I was on top of a steel and aluminum structure which goes into that same water.

I braced myself for a shock, hoping the inverter fuse would blow before it got extremely ugly up there. To my surprise there was no pop at all. I scurried onto the wood ladder and went down for a look being careful not to fall into the water with the live cord in it.

To my surprise the inverter was happily putting out 120v to the extension cord in the water. I shut down the inverter and pulled the cord out of the water and laid it in the sun to dry while I calmed myself and checked my pants.

Lesson 1, never do this again.

Now it's important for me to learn the rest of the story.

Is fresh water that non-conductive that it just wouldn't take any current? Can I test my water conductivity with a multimeter?

From now on all electric in that boathouse will go through a GFCI. Would/should that tripped in this case?? Should the GFCI ground be bonded to the steel superstructure??

I'm hoping for a more elegant understanding before I wire up a GFCI and throw the powered cord back in the water to see if it works or not.

 

[svetz]

Glad that worked out! People do drown in that situation, although it's typically as the electricity paralyzes the muscles so they can't swim.

To get shocked, the power has to flow through you. You're higher resistance than aluminum, so that's a part of it.

...Is fresh water that non-conductive that it just wouldn't take any current? Can I test my water conductivity with a multimeter?

Updated: Seawater is more conductive than fresh water, it's all the ions. Distilled water is very poor. I'd expect lake water to be fairly good though.

Measuring is tricky because how far apart the electrodes are and their size changes the reading. That said, people do it to test water quality in aquariums. See: https://sciencing.com/measure-conductivity-water-multimeter-8523350.html Update: DIY conductivity tester ...From now on all electric in that boathouse will go through a GFCI. Would/should that tripped in this case?? Should the GFCI ground be bonded to the steel superstructure?? I'm hoping for a more elegant understanding before I wire up a GFCI and throw the powered cord back in the water to see if it works or not. It should protect you, a ground is not required for a GFCI. In short they detect an imbalance in the current between hot and neutral...which means current is going through something other than the neutral (e.g., short through the water).

 

[svetz]

...Update: DIY conductivity tester

The 9V battery will probably cause the water to fizz... electrolysis breaking water down into H2 and O2 ... so don't let the electrodes touch and make a spark (even if you did it would probably be a very small poof). ?

 

[JoeHam]

Thanks for the info and clear explanations.

I'm still surprised (fortunately not shocked) that the inverter continued to pump out 120v while the cord receptacle was in the water. It was in the water long enough to short out for sure if the fresh water was conductive enough.

That suggests very little current was flowing in what I assumed would be a short circuit.

Is there any other plausible explanation?

 

[svetz]

...I'm still surprised (fortunately not shocked) that the inverter continued to pump out 120v...

Volts are just potential, electrons (current) is what moves.

But you're right... a short-circuit typically overwhelms a voltage sources ability to stabilize voltage; so it's a good bet the resistance was high enough to not pull the maximum amps from it. Assuming the plug didn't drop near an aluminum stand-off, theoretically the lowest resistance was the 1/2" distance between the hot and neutral in the socket.

... Is there any other plausible explanation? ...

I'm curious too, have to see what others say.
You're probably right it was just the resistance... but if you measure that let us know what the current should have been.

Here are some additional thoughts in decreasing liklihood:

• It could have dropped straight down such that air was trapped inside minimizing the wet surface on the prongs further increasing resistance.
• It just wasn't your time, the big guy has plans for you.
• An electrical eel could have been recently discharged and absorbed... nah....

Update: I once accidentally sprayed water with a hose over a GFCI and it tripped... so fresh water at 0.5" (or distance from hot to ground) ... ≥ 5 mA current.

 

[JoeHam]

Here are some additional thoughts in decreasing liklihood:

• It could have dropped straight down such that air was trapped inside minimizing the wet surface on the prongs further increasing resistance.
• It just wasn't your time, the big guy has plans for you.
• An electrical eel could have been recently discharged and absorbed... nah....

For the first one, it was full of water for sure. I dumped out the water after unplugging it.

The second one? I'm sure the big guy is sick of my stupidity by now . The eel thing is more likely.

 

[ghostwriter66]

So here is my latest display of stupidity:

Last Friday I was up on top of a boat lift inside a metal boathouse. I was running an angle grinder powered by my 48v LiFePO4 through a 3KW inverter.

I know this starts out like a @ghostwriter66 oilfield idiot story and I guess it is!

Actually for it to be one of my oil field "special" folks story it would have had to begin " Last Friday AFTER HEAVY DRINKING they were up on top of a boat lift inside a metal boathouse DRUNK, SHOE-LESS and SINGING LOUDLY SONGS THEY DO NOT KNOW THE WORDS FOR. TheY were running an angle grinder powered by a 48v LiFePO4 through a 30,000W inverter. ONCE THE FIRE WAS UNDER CONTROL ...."

 

[BiduleOhm]

I'm curious too, have to see what others say.

Surface area. The extension cord has a very low surface area at the end so even if the 2 conductives parts are pretty close to each other the current stay relatively low as the surface area is very small.

 

[JoeHam]

Actually for it to be one of my oil field "special" folks story it would have had to begin " Last Friday AFTER HEAVY DRINKING they were up on top of a boat lift inside a metal boathouse DRUNK, SHOE-LESS and SINGING LOUDLY SONGS THEY DO NOT KNOW THE WORDS FOR. TheY were running an angle grinder powered by a 48v LiFePO4 through a 30,000W inverter. ONCE THE FIRE WAS UNDER CONTROL ...."

So I left a few details out ?

 

[Zil]

I always leave those details out. Unless talking about the other guy.

 

[tictag]

Your inverter most likely has a 16A breaker so for 16A+ to flow you'd need a resistance of at most 7Ω between the path of least resistance, which would be the extension lead terminals. Does it matter it was AC? Probably because the water would be acting as a dielectric, though capacitive reactance would be high at such a low frequency.

Dropping a toaster in a bath and killing you is actually an urban myth, unless you were somehow earthed, for example, by grabbing a tap (faucet).

Our favourite Iranian Canadian has done a few videos on this:

 

[offgriddle]

Perfectly pure water would be a perfect electrical insulator but there is no such thing as perfectly pure water, (including distilled).

It is the impurities in water that conduct electricity.

The extension cord's voltage was not high enough to overcome the physical separation between hot and ground and find a path of conduction via the impurities in the water.

The Great OGz has spoken.

 

[Rider]

Actually for it to be one of my oil field "special" folks story it would have had to begin " Last Friday AFTER HEAVY DRINKING they were up on top of a boat lift inside a metal boathouse DRUNK, SHOE-LESS and SINGING LOUDLY SONGS THEY DO NOT KNOW THE WORDS FOR. TheY were running an angle grinder powered by a 48v LiFePO4 through a 30,000W inverter. ONCE THE FIRE WAS UNDER CONTROL ...."

If you live in the south USA, somewhere in there would be the phrase, "Hold muh beer and watch this!"