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I have a submersible water pump with a power cable that ends with a three-pronged plug with a protective neutral contact. Having a matching outlet is problematic - there are only unearthed outlets at where the pump will be installed.

Now I don't see why earthing should be needed in the first place. The pump will be fully submerged into a well with drinking water that will have contact with earth. So I assume that it will be automatically earthed at all times.

Is earthing required in such setup?

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migrated from electronics.stackexchange.com Jul 21 '11 at 12:58

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1 Answer

up vote 6 down vote accepted

If earthing is provided, earthing should be used.

And a device-local earth is not sufficient. Imagine if a fault develops inside the device, and the only way for the current to get out and to ground were through the water outlet. As soon as you use the tap - zap.

Also, as with all devices in such an environment, an ELCB (Earth Leakage Circuit Breaker) should be used. This monitors current in the earth wire, and if any current is found on that wire it cuts off the power instantly.

Did you know: Water is a very poor conductor of electricity. Pure water will not conduct at all. It is actually the impurities and minerals in the water that provide the conduction.

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Yes, I know that distilled water is an insulator, however typical drinking water contains salt dissolved and thus is an electrolyte. –  sharptooth Jul 21 '11 at 10:14
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Yes, I know about water's conductivity. But that's purely theoretical knowledge, which could actually deceive people in thinking that only (heavily) polluted water will cause a hazard. In practice all water is a good conductor, except hyper-pure for laboratory use. –  Federico Russo Jul 21 '11 at 10:18
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@Federico Russo: Yeap, I remember an experiment at school: two electrodes are submerged into distilled water and complete a circuit used to power an old boring 60 watts incandescent bulb. At first there's no current since the water is distilled. Then maybe a spoon of salt is dropped into water and mixed with a glass stick - the bulb immediately goes to full power. –  sharptooth Jul 21 '11 at 10:34
    
@sharp Don't you love those free electrons ;) –  Majenko Jul 21 '11 at 10:39
    
Agreed - If earthing is provided it must be used. Earth in the ground is not the same as earth in the electrical system. The big rod that goes into the ground from the electrical system is usually found in areas that have electrical storms and act as the fastest way of discharge. In your case if there is a live voltage leakage it will seep into the earth- but the shortest route would be a human- or less resistive path. Install an earth leakage on that outlet or use a self monitoring device in case of leakage. –  ppumkin Jul 21 '11 at 13:53
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