Electricity discharge into water

Question

My question has to do with a piece of equipment in my home, but I thought I would post my question here because I am interested in the behaviour of the electricity rather than anything else.

There is a submersible electric sump pump in my basement. It sits in a pit dug out of soft rock, where water accumulates. The water is fresh water, in the sense that it is rainwater and not seawater, though it is muddy. When the water reaches sufficient depth, a float switch causes the pump to switch on and pump the water out of the house.

Yesterday, I was cleaning sediment out of the pit with my hand. The pit had a substantial amount of water in it. I felt a tingle: Electricity.

I did a quick experiment with my multimeter. I put one probe in the soil and one probe in the water. With the pump plugged in, I measured a small voltage (about 0.5 V AC). When I unplugged the pump, I measured no voltage.

So yes, this is a dangerous situation. The pump is dangerously defective. I have unplugged the pump and am arranging for its replacement. This is not the point of my question here.

My question is what is actually happening with the electricity. I think it’s safe to assume that somewhere in the pump, the 240 VAC mains current is exposed to the water. So:

Why did I feel a small tingle and not the full force of 240 VAC? In the same vein, why do I measure only 0.5 VAC in the water?

Why did I feel anything? Why wouldn’t the exposed wire short directly to the nearest ground point through the water?

Would I have felt a stronger shock if there had been less water in the pit?

What happens in situations like this, in which a whole swimming pool becomes electrified?

Thanks.

Answer 1

Fresh water is a pretty poor conductor of electricity, though muddy water will be a better conductor than clean drinking water.

I'll assume that there really is a problem with the pump, and it's not just some trivial leakage current.

The exposed wire inside the pump is at 240V AC, and the rock around the edge of the pit will be more-or-less 0V (ground). So a current will flow through the water to ground. Since it's fresh water, this won't be a very big current. There will be a voltage gradient between the live wire and the rock.

If you stick a probe into the water at some point (either the multimeter or your hand), then it will be at some voltage between 0V and 240V. Essentially, you're probing at some point in the middle of a resistor.

But since you only measured 0.5V, it suggests that the leakage current was actually very small.

Answer 2

It's more than likely capacitance of a few nano farads between the submerged cable and the surrounding water. This can generate voltages anywhere up to several tens of volts but the amount of current it can deliver is small and will only tingle a bit. However, if you are unsure you did the right thing but you will probably find that if your replaced it with a new one it would do the same BUT call an electrician or somebody who understands this answer to take a 2nd look.

If it were live parts touching water you'd feel more than a tingle. Also, double check that it has an ELCB or MCB breaker in the spur that feeds it. This is law in the UK for submersible pumps - basically if an earth fault appears then it trips the breaker.

You can get the same effect from fluorescent lamps under a power grid cable: -

Also, see this

Please double check everything though to be safe and secure.

Answer 3

Since the pump is in the window well, and the window well ground is close to ground potential, testing the water vs ground in the window well is not a great way to find leakage current. There is very little impedance between the water and ground and it wouldn't tell you how much current is leaking or if there is a dangerous voltage.

The pump needs to be isolated from the ground when you are doing your test. First off, be careful, don't use yourself to do the testing. Put the pump in a plastic 5gal bucket, and measure the potential between the water and mains ground.

Secondly, I'll bet your not feeling electricity but vibration from the pump. I've had a few times when I was sure I felt an electric tingle but it turned out to be a mechanical vibration. After measuring it with a voltmeter there was no potential.

Thirdly, depending on your meter (I'm guessing its a cheaper one), you could probably find 0.5 volts between anything and anything else.