Shocks from AC leakage from hot water heater : why doesn't grounding stop it?

I heard from a professor who designed an electric hot water heater that the heating elements used can sometimes crack, exposing the wire to the water. This caused 240 V AC to flow through the conductive water and piping, and somehow it can cause painful electric shocks to a person using the shower or washing their hands.

What I immediately asked is why not wire a ground to the metal wall of the hot water heater and forget about the problem? Shouldn’t the ground be a much more favorable path for the electric current, going through nice clean metal all the way back to the breaker box? In order to shock someone using a shower, the current has to pass through all that piping, through poorly conductive water, and through a human body to perhaps the metal of the tub which is presumably connected to ground.

That doesn’t sound like a very good circuit path. Path 1 = heater element->a few inches of water -> copper all the way to the breaker box. Path 2 = heater element ->several feet of water-> metal piping ->water stream from shower -> human body -> tub surface or perhaps person is touching a ground.

Anyways, this particular hot water heater uses an electronic circuit to detect AC leakage and cut off the power when it detects it.

Many heaters don’t have metal walls, rather they are vitreous ceramic over steel. So the walls are insulated. Stainless steel can be used, but is expensive, or going back in time copper, and that is insanely expensive. So commonly they are ceramic coated, and use a sacrificial anode to avoid the inevitable corrosion issues due to imperfect coating.

In general however it is an odd problem. Modern houses now use plastic water pipe, plastic pipe for drains, and there is less and less opportunity to provide a solid ground via water pipe anyway.

The current takes all paths.
When you are wet, you are much more susceptible to electric shock, since your skin resistance is much lower. Even a few volts might be enough to be felt. So, even though a good ground nearby would shunt most of the current, enough can flow down the pipes to shock someone at the other end of a building.

Also, it’s possible that the ground connection at that end of the building is lower resistance, meaning that more current will flow that way.

The vast majority of houses in existence in North America use copper supply pipes. PEX is relatively new (within the past 15 years) for potable water and I think there are a couple states where it’s still not even code.

PVC has indeed been used for sewer pipes for a long time, though.