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In the past, it was common to connect the earth wire (green/yellow wire) of an appliance to the water pipe. But now we were told to connect it to the earth pin of the power plug, instead. But, why can't we use the water pipe anymore?

Currently, I am installing a new electric heater but have to reuse an old power cord that runs through the wall that has only 2 wires (line and neutral) without the earth wire. So, it is impossible for me to connect the heater's earth wire to the earth pin of the plug. I am thinking of using the old method of using the water pipe to earth my heater. Is it a valid method? Another possibility is to connect my heater's earth to the earth of another appliance nearby. Does it make any difference?

We are following the UK electric wiring standards although strict adherence to the standard is not my concern.

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This statement - "it is impossible for me to connect the heater's earth wire to the earth pin of the plug" is not really true. You can open the walls and replace the wiring with proper up to date type that includes the safety ground conductor. –  Michael Karas Oct 6 '13 at 13:11
    
Water pipe must be metal all the way to where it's bonded to the earth rod... –  Fiasco Labs Oct 6 '13 at 18:38

4 Answers 4

Many homes do not have a waterpipe that would provide an adequate ground. The water supply line is now often plastic. Even if you have copper pipes inside your house they may not be grounded if they are attached to a plastic pipe that runs underground.

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The water inside the pipe cannot function as the conductor? –  JavaMan Oct 7 '13 at 9:04
    
Not reliably. Pipes can empty out. Permanent metal conductors are mandatory. –  The Evil Greebo Nov 5 '13 at 19:15
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@TheEvilGreebo, Javaman: This is not the reason.... even if you did have a good connection, water is just not good conductor. It can have 6 to 15 orders of magnitude worse conductivity than copper, e.g. en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity –  ThePopMachine Nov 5 '13 at 20:00
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The water inside the pipe is a very poor conductor unless it has salt content. It would have to be so salty as to be undrinkable to be a reliable ground. Which means you'd have corrosion problems as well <grin>. Best to run a wire. –  Fiasco Labs Nov 5 '13 at 20:23
    
What?!? You mean all those safety warnings about not dropping toasters in my bathtub with me in it are false?!? (J/K) –  The Evil Greebo Nov 6 '13 at 9:37

If the heater is double insulated (if it is, it will have the "box in a box" symbol on the electrical label on the applicance), it doesn't need an earth connection (and indeed might not have one itself anyway).

If it's not double insulated, I wouldn't take the risk by eathing it through the pipe. Can you guarantee that the pipe has a suitably low resistance to earth? Is it even connected to earth? (As others have said, the water itself will not provide a suitable earth connection, as "pure" water is a poor conductor and sufficient salt content to make it sufficiently conducting might be enough to make it undrinkable).

If the earth connection is poor, a fault on the appliance could potentially make other parts of the house that are connected to the pipe (eg taps, radiators, etc) into exposed live conductors, possibly exposing someone to the risk of a fatal electric shock. Is that really a risk you are willing to take?

The incoming water supply should be bonded to earth to prevent something like that happening, but that does not make the pipework a substitute for a correctly installed earth conductor.

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The first issue that your water pipe must be connected to the earth. That could very well not be the case anymore in new housing. Obviously, if not connected then it is quite useless to use the pipe for earth-wire

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The water inside the pipe cannot function as the conductor? –  JavaMan Oct 7 '13 at 9:03
    
I'd say no: especially in case of plastic pipes, let alone occurrence of air-bubbles etc, there's no way to be sure you'd get a 'connection'. I'd not rely on fluid for a grounding. The reason these pipes were used was because of the material of the pipes themselves as far as I know, not especially because there was water inside. –  Nanne Oct 7 '13 at 10:18
    
The water inside the pipe is a very poor conductor unless it has salt content. It would have to be so salty as to be undrinkable to be a reliable ground. Which means you'd have corrosion problems as well <grin>. Best to run a wire. –  Fiasco Labs Nov 5 '13 at 20:25

If rewiring in the walls is not an option, maybe adding a ground-fault interrupter type outlet is an alternative. In the USA it is, and provides similar safety from electrical shock to people between a damaged device and another path to ground. It just works by sensing an out of balance current flow.

With this option, just leave the earth wire unconnected. Connecting it to some path to ground that is not part of the wiring increases shock risks and other electrical issues, including an increased chance of fire.

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