Water pipe ionisation voltages
There seems to be considerable antipathy to the idea that earth potential might be higher than zero volts in a circuit. To those who have experienced a small tingle from earthed circuits, the idea that earth can be something other than zero volts is not such a foreign idea but to many it still falls into the nonsense category.

This page doesn't intend to define the exact voltage of earth, as it will vary from circuit to circuit, but will demonstrate by standard electrical calculations the voltage rise above zero in a model single phase electrical installation.
model single phase circuit
For our model we'll nominate the load as a 2kW hot water heating element, with 20 metres between the fuseboard and the street and a further 60 metres along the street to the transformer. We'll also nominate the wire size as 25 sq.mm for each conductor, which is the same as a standard single phase neutral screen underground cable.
So we have 80 metres there and 80 metres back plus the resistance of the water heating coil as the total circuit resistance. We'll ignore the resistances of connections and the fuse.

The resistance of the wire is around 0.0006 ohms per metre (or 0.048 ohms per 80m) and the heater element will be drawing around 9 amps (so will have a resistance of around 26 ohms). The resistances will be:
This means the potential difference between the transformer earth and the meterbox earth will be:

delta V = iR
= 9 x 0.048
= 0.432 volts RMS
= around 400 millivolts RMS

Now, in practice it should be less than this because the ground will be conducting. If we assume a ground resistance of around 3 ohms then the voltage reduction will be negligible as the ground resistance is high compared to the copper wire, so the voltage of the meterboard earth and the bonded water pipe will not reduce to any great extent and certainly not to zero.
Stephen G Butcher  (Posted 10/05/09)
The voltages required to ionise a liquid can be measured in millivolts, so you can see that mains power neutral provides more than enough even if we only count the negative half wave!
a few calculations
The point is that the voltages created by tying mains power to water pipes are so large compared to the voltages required to ionise water molecules that there can be no doubt that ionisation occurs.