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PSA An Anatomy of a Tank Crash

I feel your pain, been there.
As Mike points out, whatever leached out of the pump would be the main concern but I noticed on the image that the cord just melted? right at the motor block.
The bad part even if the epoxied motor didn’t suffer any apparent damage, is the expossure to copper from the wires, I’m guessing some arching may have occurred and due to that, some copper at high temperature might have been released into the water column.
 
sfsuphysics said:
See I dunno about that, the resistivity of copper is almost 10 million times less than sea water, so the current is still more likely to go through whole circuit with very little bleed over due to the exposed nature of the wires.

Don't get me wrong the resistivity of sea water is absolutely tiny when compared to air, but when compared to actual conductors it's huge. One of the reasons why you feel a bit of a tingle in the tank, yeah some of it does go through the tank water but it's very small in comparison to what goes through the pump. As a result yeah, the Apex won't detect that.

Personally I'd wonder about the contamination of the water due to all the junk inside the pump more so than I would about electrical current going through the tank.
Click to expand...

The idea would be that both the wires are exposed to water, but not touching each other.
If they are touching, you have a dead short, which should cause even a normal breaker to pop due to overload.
If they are not touching, you have a small gap.
Current will go from hot, through water, to neutral.
As it goes through the water, it spreads out, but not a lot.

Note that conduction through water is actually ionic, not electrons like a wire.
 
Sorry for your losses, Bruce. Thanks for the PSA. Glad it wasn’t a total loss. What pumps were you using?


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rygh said:
The idea would be that both the wires are exposed to water, but not touching each other.
If they are touching, you have a dead short, which should cause even a normal breaker to pop due to overload.
If they are not touching, you have a small gap.
Current will go from hot, through water, to neutral.
As it goes through the water, it spreads out, but not a lot.
Click to expand...
I'm not talking about a short, I'm talking about the wire still physically being connected to the whole circuit (aka the pump), current is more likely to flow through the pump than through water which is why fish don't just get out right electrocuted when a frayed wired shielding gets exposed.
 
That really sucks, sorry about your tank. I would never suspect that these submersible products would leak let alone having the electric cord pretty much fall off.
 
sfsuphysics said:
I'm not talking about a short, I'm talking about the wire still physically being connected to the whole circuit (aka the pump), current is more likely to flow through the pump than through water which is why fish don't just get out right electrocuted when a frayed wired shielding gets exposed.
Click to expand...

At 25C, 35ppt, conductivity of water = 5.302
So 0.19 ohms/meter.
Assuming half a meter between frayed wire and ground, lets call it 0.1 ohms.

Normal wire is lower.
20 feet of 16 guage wire = around 0.08 ohms.

But the device itself is not:
Lets say you have a 500W heater with frayed wires, and 120V supply. (Easier than dealing with motor impedances)
Power = Current * Voltage
Voltage = Current * Resistance.
Do some algebra, and you get 28 ohms for the heater.
Way more than the water.

And actually, the device itself is almost irrelevant to the calculation.
You need to calculate voltage drop in wires from source (house) to both devices (heater + short)
That is really the limiting part.

* As to why fish are not instantly electrocuted:
Fish have a very similar conductivity to salt water.
As current spreads out through the tank, only a small fraction will go through the fish.
 
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