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Often when we think of a lightning transient, the idea is that the lightning strikes a power line and propagates through the hot/neutral or 3 wires of 3-phase system. There is also induced current transients.

However, I wonder to what extent lightning can affect the ground system. Most houses where I live are grounded to the water pipe in the house, so I would guess that if the lightning hit say a bridge with a water main on it, it could affect equipment attached to that water line. One article on lightning surges had the paragraph:

Lightning ground current flow results when a strike that discharges to the earth couples into common ground impedance paths, causing voltage differential across the ground grid and between ground-neutral or ground-line circuits. In short, the reference ground (supposedly zero voltage) is elevated a few milliseconds, therefore creating a large voltage difference between ground and the incoming power and/or data lines.

To what extent, if any, does lightning affect ground systems which use water pipes?

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  • \$\begingroup\$ I feel that the answers so far only deals with the basic theory but OP asks about what happens next. What are the practical consequences for the equipment connected inside the house, if any? \$\endgroup\$ – pipe Oct 3 '18 at 9:06
  • \$\begingroup\$ You are likely to find that your ground is least affected by the lightning compared to the line and neutral wires. If your ground should bounce the power wires will probably bounce more. If there is a direct strike to a grounding conductor (pipe) it should be earthed well enough to divert the bulk of the current into the earth, a metal pipe is a good earth connection and the strike would have to be between the good earth (water main pipe) and your house to couple significant energy to your end. \$\endgroup\$ – KalleMP Oct 3 '18 at 9:17
  • \$\begingroup\$ @pipe It would make sense for the OP to make a model of the described scenario and then ask more pointed questions. At the same time the model could be simulated and results evaluated. \$\endgroup\$ – KalleMP Oct 4 '18 at 16:08
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There is one cardinal mistake. The water pipe may not be used as ground, rather a water pipe has to be grounded. Grounding the pipe provides safety to the user if some device, for example boiler malfunctions so that you don't get electrocuted in bath.

It is forrbiden to attach other devices to the pipe to serve as ground point.

When the lightning strikes a grounded object, the current passes through and the entire grounding potential becomes V=I*R_ground. It is self explanatory that having good ground makes this voltage very small.

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    \$\begingroup\$ The reason it is forbidden to use pipes as ground has to do with maintenance on the pipes. If a worker would cut the pipe, a voltage would occur on each side of the cut, potentially harming the worker. \$\endgroup\$ – Jeroen3 Oct 3 '18 at 7:14
  • \$\begingroup\$ More so in the past but still in some locales the incoming water pipe has been used as the primary ground. Using the water main inside the house as a ground would be unwise, unpredictable and not likely to be recommended. As you say the metal pipes in the house should be grounded to maintain them in connection with the safety earth circuit. \$\endgroup\$ – KalleMP Oct 4 '18 at 16:12
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Ground pipes have a (tiny) resistance, lets say 1m Ohm/meter. Lightning discharge currents can be very high, like 100k amps. So a lightning current flowing trough a GND/earth water pipe can cause a voltage of 1m * 100k = 100V per meter!

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  • \$\begingroup\$ The discharge is trying to get to ground and not into the house. Most of the current would flow into the ground if the pipe has any meaningful length in the ground in a similar way to a regular earthing rod. \$\endgroup\$ – KalleMP Oct 4 '18 at 16:10
  • \$\begingroup\$ No, the lightning discharge is trying to flow through the path that provides the least resistance, and water pipes usually have much lower resistance than earth/ground. If you build a water pipe from top of your roof to earth, the lightning current will definitely flow through the pipe in your house. \$\endgroup\$ – Stefan Wyss Oct 4 '18 at 16:25
  • \$\begingroup\$ Stefan OPs scenario had the lightning induce a surge in the piping external to house and enter via the water pipe, I think he is trying to determine if a water pipe is a reliable earth if it is susceptible to earth bounce from a strike itself. My contention is that it is the best you can do in most cases. \$\endgroup\$ – KalleMP Oct 5 '18 at 7:37
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Lightning does have an effect on the ground. But this is due to an effect called ground potential rise. And not the plumbing in the ground.

When a large current passes through a conductor, there will be a voltage over the resistor. Lightning has a high current, and ground is also a resistor.

Most houses where I live are grounded to the water pipe in the house

This is actually the opposite, the water pipe is grounded to a grounding rod nearby, or a ground provided by your energy supplier.

However, the world is a big old place. In some region the code might have allowed use of the water pipe as ground connection. Electrical code isn't often retroactive.

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  • \$\begingroup\$ In some places the local ground is a metal water main and this may augment or even replace a utility ground if this is poor or missing. Best practice often is for the utility to tie the ground connections together at the point of entry to your premises so you have a known reference. A grounding rod is often used as well in some locales. \$\endgroup\$ – KalleMP Oct 3 '18 at 9:13
  • \$\begingroup\$ @KalleMP That is obviously wrong, lethal hazard. It makes no sense. Of course the pipe is a good ground, because it is grounded, it must be. But this doesn't mean that you may use it as ground. \$\endgroup\$ – Marko Buršič Oct 3 '18 at 9:22
  • \$\begingroup\$ @MarkoBuršič I am not sure what you are objecting to. Metal water mains have been used as grounds in many places and still are. A quick search and read of the first hits confirms this. google.com/search?q=water+main+as+ground+connection \$\endgroup\$ – KalleMP Oct 4 '18 at 16:01
  • \$\begingroup\$ @ Jeroen3 Some of the links in the search results from above indicate that grounding rods are used if the water main has less than 10 feet of pipe in the ground (before it is no longer metal). Grounding of water pipes that have insulating sections to the house ground is also a good idea and discussed, this is to maintain all metal work at ground potential and prevent them becoming electrified. This can happen easily with faulty water heating elements. \$\endgroup\$ – KalleMP Oct 4 '18 at 16:05

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