I've got a 1901 house, with copper (new) piping. Some of the electrical (old knob and tube circuits) are grounded to the water pipes. The copper pipe goes out to my shutoff valve located outside. so far so good. However, the waterpipe from the shutoff valve to the ground has been replaced with PVC. So, the contact with the ground is actually a PVC waterpipe for at least 1' above ground. My question - Is this considered to be safely grounded? (or do I need to install a ground rod from the cu pipe?).

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    The grounding is accomplished through the metal pipe. Pure water is an effective electrical insulator. It's only conductive when there are dissolved salts in it (not just NaCl, but any metal salts). – Craig Jul 28 '15 at 23:57
  • Thank you @Craig ! (I guess I naturally think of water as a conductor) ~ I'd better fix that right away. Thanks again! – JTinOakland Jul 29 '15 at 0:01
  • @JTinOakland, generally speaking, water is a conductor. You don't really come across pure water in the real world, or at least not in quantities where you'd be worried about its conducting or insulating abilities. However I am by no means saying water is an appropriate path to ground. – mjohns Jul 29 '15 at 1:45
  • While generally speaking, water may be a conductor, in the context of home wiring, it is not. Using this formula, a 1 m piece of 18 gauge wire would have a resistance of 0.02 ohms, but a 3/4" diameter pipe full of sea water would have a resistance of 41 ohms, 2000 times worse. A short to that 41 ohm ground would only carry 3 amps - not enough to trip a breaker. – Johnny Jul 29 '15 at 3:28
  • I found a source for resistivity of tap water, it ranges from 2×10^1 to 2×10^3 Ohm-m, so taking a value in the middle yields 41,000 ohms for that water pipe, higher than most people's skin resistance. – Johnny Jul 29 '15 at 3:37

The grounding is accomplished through the metal pipe, not through the water.

Pure water is an effective electrical insulator. It's only conductive when there are dissolved salts in it (not just NaCl, but any metal salts creating free ions floating around in the water). In that case, water can become a very good conductor. But it isn't really the water doing the conducting.

That PVC pipe is not creating a suitable path to ground.

You need to drive at least one grounding rod (probably an 8' copper or galvanized steel solid rod, which you can buy at your local home improvement warehouse), and run a #6 copper wire from that rod to your main service panel, where you'll bond it to the grounding bus. You also need to leave all of your copper and/or iron pipes bonded to the grounding bus in your main panel.

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    Check your local code requirements, of course, to see how many grounding rods you're actually required to drive, what size wire you're required to use and how you're required to bond the wire to the rods and back to your panel. – Craig Jul 29 '15 at 0:23

Way back when it was allowed to run grounding conductors to any cold water pipe for equipment grounds for circuits. This allowance was removed from the code a LONG time ago due to the practice being potentially very unsafe.

When your water pipe was metallic going out into the yard it was being used as a grounding electrode and should have been bonded to the electrical panel. The grounding electrode and being in contact with the earth had NOTHING to do with the pipe being able to be used as a grounding source for circuits. The function of allowing it to be used to ground circuits came from the fact that the pipe was bonded to the main panel. Problem is, using the pipe as an equipment grounding conductor means that and problems or faults would have to go through every fitting and joint in the path back to the panel.

  • Would you elaborate a little on the pipe (in the old days) only being useful as a grounding electrode because of the bonding to the neutral in the panel? If the neutral to ground path at the transformer was compromised (happens from time to time) you would have no path to local earth ground at all and you could build up substantial voltage potential between the power company neutral and the local earth potential, right? – Craig Jul 29 '15 at 3:54
  • You're totally right about all the fittings and joints in the pipe causing problems, especially over time as old pipe corrodes, and especially since so much of the old pipe was iron or lead, which are much poorer conductors than copper (and steel is worse than raw iron), and iron and common steel are so susceptible to corruption. Lead is a relatively poor conductor, so soldered joints in copper pipe add resistance. Pipe dope or plumber's tape add huge resistance in iron pipe, and of course rust doesn't conduct as well as clean iron, etc. – Craig Jul 29 '15 at 4:15
  • The reason to connect to earth is not for fault clearing or for any kind of equipment ground purposes. The earth is used to stabilize voltage in cases of high voltage spikes or faults, mostly lightning. A metallic water pipe still must be used as an electrode if it exists and is in contact with the earth for 10' or more. My "old days" comment was about using the water piping in a house to connect an equipment ground. – Speedy Petey Jul 29 '15 at 10:46

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